Orthopedic Basic MCQs 2026 | Board Review & Answers

Question 1

What is the current 5-year survival rate for patients with classic nonmetastatic, high-grade osteosarcoma of the extremity?

Explanation

Multidisciplinary treatment combining systemic chemotherapy and adequate surgical resection has resulted in a 5-year survival rate of 70% in patients with nonmetastatic osteosarcoma of the extremity . The advent of effective chemotherapy has increased the overall survival rate from 20% to 70% in current studies. Arndt CA, Crist WM: Common musculoskeletal tumors of childhood and adolescence. N Engl J Med 1999;341:342-352.

Question 2

A 43-year-old woman has had pain in the left hip for the past 2 months. A radiograph, CT scan, MRI scan, and biopsy specimens are shown in Figures 16a through 16e. What is the most likely diagnosis?

Explanation

16b 16c 16d 16e The imaging studies are consistent with a chondrosarcoma. The radiograph shows a radiolucent lesion in the pelvis, and there are stippled calcifications on the CT scan. The histology shows a low-grade cellular hyaline cartilage neoplasm with stellate, occasionally binucleated chondrocytes. Enchondroma has a more benign histologic appearance.

Question 3

A 40-year-old man with amyloidosis injured his left knee while walking. Figure 17a shows an AP radiograph that was obtained 2 weeks after the injury. The radiograph shown in Figure 17b was obtained after the patient wore a hinged knee brace for 3 months. A clinical photograph is shown in Figure 17c. What is the most likely diagnosis?

Explanation

17b 17c The patient has a Charcot arthropathy of the knee, which is associated with amyloidosis. The rapid joint destruction shown in the radiographs is most consistent with that diagnosis. Drennan D, Fahey J, Maylahn D: Important factors in achieving arthrodesis of the Charcot knee. J Bone Joint Surg Am 1971;53:1180-1193. Soudry M, Binazzi R, Johanson N, et al: Total knee arthroplasty in Charcot and Charcot like joints. Clin Orthop 1986;208:199-204.

Question 4

Following preoperative chemotherapy, the percent of tumor necrosis has been shown to be of prognostic value for which of the following tumors?

Explanation

The grading of response to chemotherapy for osteosarcoma was introduced by Huvos and associates. Patients with tumors that show more than 90% necrosis after neoadjuvant chemotherapy are considered to have had a good response and have better survival rates than those with less than 90% necrosis. However, it should be noted that survival rates for patients with a poor response are still better than in patients who do not receive neoadjuvant chemotherapy. More recently, similar results have been reported in patients with Ewing's sarcoma. Chemotherapy is not typically used for giant cell tumor of bone. Meyers PA, Heller G, Healey J, Huvos A, Lane J, Marcove R, et al: Chemotherapy for nonmetastatic osteogenic sarcoma: The Memorial Sloan- Kettering experience. J Clin Oncol 1992;10:5-15.

Question 5

Which of the following procedures is not part of the routine evaluation of a patient with suspected metastatic disease to bone?

Explanation

The work-up for a patient with an unknown primary lesion that is metastatic to bone includes a search for the primary tumor and other sites of metastasis. This involves conducting a thorough history and physical examination, appropriate laboratory studies, bone scan, chest radiograph, and CT of the chest, abdomen, and pelvis. In women, a mammogram may be indicated. CT of the brain is not necessary in the early stages of a metastatic work-up. Simon MA, Bartucci EJ: The search for the primary tumor in patients with skeletal metastases of unknown origin. Cancer 1986;58:1088-1095.

Question 6

A 43-year-old man has had right groin pain for the past 3 months. A radiograph, CT scan, and biopsy specimen are shown in Figures 18a through 18c. What is the most likely diagnosis?

Explanation

18b 18c The radiographic appearance of the lesion is lytic with calcifications in the supra-acetabular region. The CT scan shows bone destruction and periosteal reaction, which rules out an enchondroma. The histologic appearance is that of cellular cartilage, with a high cytoplasmic:nuclear ratio and basophilic appearance to the cytoplasm; these findings rule out chordoma and metastatic renal cell carcinoma. In addition, chordomas are most frequently found in the sacrum and base of the skull. Because there is no high-grade spindle cell component to suggest dedifferentiation, the most likely diagnosis is chondrosarcoma. Pring M, Weber KL, Unni K, Sim FH: Chondrosarcoma of the pelvis: A review of sixty-four cases. J Bone Joint Surg Am 2001;83:1630-1642.

Question 7

What malignant disease most commonly develops in conjunction with chronic osteomyelitis?

Explanation

The most common malignant disease to arise in conjunction with chronic osteomyelitis is squamous cell carcinoma particularly in patients with a long-standing draining sinus tract. Dell PC: Hand, in Simon MA, Springfield D (eds): Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott-Raven, 1998, pp 405-420.

Question 8

A 77-year-old man has had increasing right knee pain for the past 3 months. A radiograph and coronal T1-weighted MRI scan are shown in Figures 19a and 19b. A biopsy specimen is shown in Figure 19c. What is the most likely diagnosis?

Explanation

19b 19c The radiograph shows a calcified lesion in the medullary canal of the distal femoral diaphysis. The MRI scan shows extensive marrow change distal to the lesion, which is not consistent with an enchondroma. The histology shows a biphasic pattern with low-grade cartilage just apposed to high-grade spindle cell sarcoma. The overall appearance is consistent with dedifferentiated chondrosarcoma. The radiographic appearance is not consistent with enchondroma, and the histologic appearance is not consistent with the other choices. Mitchell AD, Ayoub K, Mangham DC, et al: Experience in the treatment of dedifferentiated chondrosarcoma. J Bone Joint Surg Br 2000;82:55-61.

Question 9

A 10-year-old girl reports activity-related bilateral arm pain. Examination reveals no soft-tissue masses in either arm, and she has full painless range of motion in both shoulders and elbows. The radiograph and bone scan are shown in Figures 20a and 20b, and biopsy specimens are shown in Figures 20c and 20d. What is the most likely diagnosis?

Explanation

20b 20c 20d Based on these findings, the most likely diagnosis is fibrous dysplasia. Twenty percent of patients with fibrous dysplasia have multifocal disease. The lesions show a typical ground glass appearance. Fibrous dysplasia frequently involves the diaphysis of the long bones. There is no associated soft-tissue mass and no periosteal reactions to these lesions, suggesting a benign lesion. The histology shows proliferating fibroblasts in a dense collagen matrix. Trabeculae are arranged in an irregular or "Chinese letter" appearance. Osteogenic sarcoma and Ewing's sarcoma have a much different radiographic appearance of malignant osteoid and small round blue cells. Periosteal chondroma does occur in the proximal humerus but is not typically multifocal. It appears as a surface lesion with saucerization of the underlying bone and a bony buttress adjacent to the lesion. Some patients with multifocal lesions have associated endocrine abnormalities (McCune-Albright syndrome). Wold LA, et al: Atlas of Orthopaedic Pathology. Philadelphia, PA, WB Saunders, 1990, pp 118-119.

Question 10

After excising a mass from the thigh that was thought to be a lipoma, the pathology reveals that the mass is a high-grade sarcoma. Subsequent treatment should include

Explanation

Following excision of a suspected benign soft-tissue tumor that proves to be malignant, repeat excision of the tumor bed is recommended. The initial surgical margins are inadequate after an intralesional or marginal excision, necessitating additional surgery for more definitive local control. While radiation therapy and/or chemotherapy may help to reduce the risk of local recurrence in patients with microscopic residual disease, local control is improved following repeat excision. Radiation therapy alone is inadequate to address poor surgical margins, and would likely be given postoperatively. Bisphosphonates have no current role in the treatment of soft-tissue sarcoma. Noria S, Davis A, Kandel R, et al: Residual disease following unplanned excision of soft-tissue sarcoma of an extremity. J Bone Joint Surg Am 1996;78:650-655.

Question 11

Chemotherapy is routinely included in the treatment of which of the following soft-tissue sarcomas?

Explanation

Most soft-tissue sarcomas are treated with a combination of radiation therapy and wide resection. Rhabdomyosarcomas are an exception, where chemotherapy is included in all treatment plans. Chemotherapy for other soft-tissue sarcomas is controversial. Enzinger FM, Weiss SW: Rhabdomyosarcoma, in Soft Tissue Tumors, ed 3. St Louis, MO, CV Mosby, 1995, p 539.

Question 12

A 10-year-old child reports acute leg pain after wrestling with his brother. AP and lateral radiographs are shown in Figures 21a and 21b. What is the best course of action?

Explanation

21b The radiographs show an eccentric metaphyseal lesion with a well-defined reactive rim of bone that is consistent with a nonossifying fibroma. Pathologic fractures through benign lesions should be treated as appropriate for the fracture, allowing the fracture to heal. Biopsy is not needed when the radiographic diagnosis is benign. MRI, in the presence of a fracture, is not particularly helpful because of the hematoma. If radiographic findings reveal that the lesion appears aggressive, a biopsy should be performed, obtaining tissue away from the fracture site. Marks KE, Bauer TW: Fibrous tumors of bone. Orthop Clin North Am 1989;20:377.

Question 13

An 83-year-old man has a painful mass of the great toe. Radiographs and a biopsy specimen are seen in Figures 22a and 22b. What is the most likely diagnosis?

Explanation

22b Gouty arthritis, pseudogout, and infection can all present with inflammatory arthritis and periarticular erosions. Strongly negative birefringent crystals are seen in gout. The histologic image shows elongated "needle-like" crystals of gout. Epidermal inclusion cysts are rarely painful and usually have a history of localized penetrating trauma. Hamilton W, Breedman KB, Haupt HM, Lackman R: Knee pain in a 40-year-old man. Clin Orthop 2001;383:282-285,290-292.

Question 14

Eosinophilic granuloma frequently occurs as a solitary lesion in the tubular long bones. After biopsy, what is the best course of action?

Explanation

Most lesions of eosinophilic granuloma are simply observed, but larger aggressive lesions may require curettage and bone grafting. Frequently, biopsy is required to rule out a malignant diagnosis. The differential diagnosis of eosinophilic granuloma is osteomyelitis, Ewing's sarcoma of bone, or osteogenic sarcoma. The biopsy alone can be followed by spontaneous resolution. In some patients, low-dose radiation therapy is used. Chemotherapy or amputation is not indicated for these benign lesions.

Question 15

A 52-year-old man has had back pain radiating to the left leg for the past 5 weeks. A radiograph, MRI scans, and biopsy specimens are shown in Figures 23a through 23f. What is the most likely diagnosis?

Explanation

23b 23c 23d 23e 23f The histology shows cells with bubbly, abundant clear cytoplasm typical of physaliphorous cells; therefore, the most likely diagnosis is chordoma. These tumors arise from notocord rests in the upper and lower spine.

Question 16

A 15-year-old boy has had pain in the right knee for the past 3 months. He denies any history of trauma. Examination reveals a firm mass in the distal thigh; the remainder of the examination is unremarkable. A radiograph is shown in Figure 24. What further work-up should be completed prior to biopsy?

Explanation

The radiograph shows an aggressive destructive lesion. In this age group, and based on the anatomic location, a primary malignant tumor (osteosarcoma) is likely. Additional staging studies to identify metastatic disease are imperative prior to any biopsy. MRI of the femur helps to reveal skip metastasis and provides information regarding the anatomic location of the lesion. CT of the chest and a bone scan evaluate for distant metastatic spread. A bone scan is also useful in evaluating the extent of local bone activity about the lesion. Simon MA, Finn HA: Diagnostic strategy for bone and soft tissue tumors. J Bone Joint Surg Am 1993;75:622-631.

Question 17

What is the most common primary malignant tumor of bone in childhood?

Explanation

Osteosarcoma is the most common primary malignant tumor of bone in childhood, followed by Ewing's sarcoma. Rhabdomyosarcoma is a soft-tissue sarcoma of childhood. Chondrosarcoma rarely occurs in childhood. Osteochondromas are benign tumors of the bone. Simon M, Springfield D, et al: Osteogenic Sarcoma: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 226.

Question 18

A patient with a 5-cm synovial sarcoma located in the distal portion of the rectus femoris muscle undergoes excision of the mass. The procedure is performed through a 10-cm longitudinal incision. Only a portion of the rectus femoris is removed; the vast majority of the muscle is preserved. The plane of dissection is beyond the reactive zone, and the pathology reveals that the margins are negative. This procedure is classified as

Explanation

The patient underwent a wide resection, which involves excision of the tumor along with a cuff of normal tissue that completely surrounds the tumor. The plane of resection is beyond the reactive zone. A radical resection involves removal of the entire affected muscle from origin to insertion. In a marginal excision, the plane of dissection is through the reactive zone of the tumor. A marginal excision is generally considered inadequate surgery for high-grade sarcomas. In an intralesional resection, the plane of dissection is through the tumor. Excision within the reactive zone but beyond the tumor is the same as a marginal excision. Enneking WF: Staging of musculoskeletal neoplasms, in Current Concepts of Diagnosis and Treatment of Bone and Soft Tissue Tumors. Heidelberg, Germany, Springer-Verlag, 1984.

Question 19

Which of the following tumors is most likely to present with a pathologic fracture in a child?

Explanation

In nearly 50% of patients with a unicameral bone cyst, the lesion remains asymptomatic until a fracture occurs, usually as the result of relatively minor trauma. If the lesion expands, the bone is weakened and may cause pain. Fibrous cortical defects are usually an incidental finding and typically asymptomatic. Malignant bone tumors such as osteosarcoma and Ewing's sarcoma most commonly cause pain, and pathologic fracture occurs in less than 10% of patients. Giant cell tumors are uncommon in children and usually are painful. Wilkins RM: Unicameral bone cysts. J Am Acad Orthop Surg 2000;8:217-224. Dormans JP, Pill SG: Fractures through bone cysts: Unicameral bone cysts, aneurysmal bone cysts, fibrous cortical defects, and nonossifying fibromas. Instr Course Lect 2002;51:457-467.

Question 20

A 60-year-old woman has a mass in the right scapula. Figures 25a and 25b show a CT scan and a biopsy specimen. The cells are lymphocyte common antigen positive, Ewing's specific antigen (CD99) negative, and keratin negative. What is the next step in management?

Explanation

25b The clinical history, CT scan, and histology are most consistent with a lymphoma of bone. An important part of the staging is bone marrow aspiration and biopsy. The other studies listed are not indicated. Lymphoma of bone, when localized, is usually treated with chemotherapy and radiation therapy and has excellent survival rates. Widespread lymphoma has a worse prognosis. Finiewicz K, van Biesen K: Non-Hodgkins lymphoma, in Golomb H, Vokes E (eds): Oncologic Therapies, ed 2. Berlin, Germany, Springer, 2003, pp 295-318.

Question 21

The use of multiagent adjuvant chemotherapy is associated with a clear survival benefit in which of the following diseases?

Explanation

The use of multiagent chemotherapy has been shown to be associated with a survival benefit in patients with osteosarcoma. The use of chemotherapy in adults with soft-tissue sarcoma remains somewhat controversial. It has not been associated with improved survival rates in patients with renal carcinoma, dedifferentiated chondrosarcoma, or melanoma. Menendez LR (ed): Orthopaedic Knowledge Update: Musculoskeletal Tumors. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, p 53.

Question 22

Which of the following definitions best describes Batson's vertebral vein system?

Explanation

The venous plexus was described by Batson and helps to explain the common distribution of metastatic cells to the vertebrae, skull, ribs, and proximal long bones. Batson studied the vertebral vein system extensively by using contrast agents in human cadavers and live monkeys. Batson's plexus is a valveless system that allows retrograde embolism from the major organs such as the breast, prostate, lung, kidney, and thyroid. It is located within the thoracoabdominal cavity and has connections to the proximal long bones and an intercommunicating network of thin-walled veins with a low intraluminal pressure. Batson OV: Function of vertebral veins and their role in spread of metastases. Ann Surg 1940;112:138-149.

Question 23

Figure 26 shows the radiograph of a 48-year-old woman who has right arm pain and hematuria. A bone scan reveals increased uptake in the left ribs and thoracic spine. A needle biopsy specimen shows that the lesion is highly keratin positive and composed primarily of clear cells. What is the best course of action?

Explanation

The lesion has the typical "blown out" lytic radiographic appearance that is most commonly found in thyroid or renal cell metastases. Given the history of hematuria and histology findings, the most likely diagnosis is metastatic renal cell carcinoma. This tumor is relatively resistant to chemotherapy. Radiation therapy is used as a postoperative adjuvant treatment with varying response rates. Surgery should be performed after preoperative embolization to decrease the risk of intraoperative bleeding, as no tourniquet can be used in this location. Patients with metastatic renal cell carcinomas may survive for years, resulting in a higher likelihood of local tumor progression with ineffective adjuvant therapy. Intramedullary fixation combined with curettage and cementation will provide the best chance of local control while maintaining the patient's native shoulder and elbow joints. A total humeral resection is an extensive surgery with considerable morbidity and is not indicated for this patient because less extensive surgery is likely to be effective. Harrington KD, Sim FH, Enis JE, Johnston JO, Diok HM, Gristina AG: Methylmethacrylate as an adjunct in internal fixation of pathological fractures: Experience with three hundred and seventy-five cases. J Bone Joint Surg Am 1976;58:1047-1054. Sun S, Lang EV: Bone metastases from renal cell carcinoma: Preoperative embolization. J Vasc Interv Radiol 1998;9:263-269.

Question 24

A 15-year-old boy has had pain in the right shoulder for the past 3 months. He denies any history of trauma and has no constitutional symptoms. Examination reveals a large firm mass in the proximal arm. A radiograph and MRI scan are shown in Figures 27a and 27b. Biopsy specimens are shown in Figures 27c and 27d. Management should consist of

Explanation

27b 27c 27d The patient has an aneurysmal bone cyst. The fluid-fluid levels seen on the MRI scan are typical for aneurysmal bone cyst, and the histology is consistent with a cystic lining. Vascular lakes, multinucleated giant cells, reactive bone, fibrovascular tissue, and an absence of atypical cells or numerous mitoses are seen histologically. Aneurysmal bone cysts will typically continue to grow and cause further bone destruction; therefore, observation is not recommended. Steroid injections are not effective. A thorough curettage of the cyst lining and bone grafting are required. Wide resection and chemotherapy are reserved for more aggressive tumors. There is no evidence of infection radiographically or histologically. Telangiectatic osteosarcoma should also be considered in the differential diagnosis; therefore, biopsy is an important part of the work-up. Wold LA, et al: Atlas of Orthopaedic Pathology. Philadelphia, PA, WB Saunders, 1990, pp 232-233.

Question 25

A 38-year-old woman with metastatic thyroid carcinoma has had increasing pain in the left hip for the past 3 months. An AP radiograph and coronal T1-weighted MRI scan are shown in Figures 28a and 28b. Management should consist of

Explanation

28b The radiograph and MRI scan reveal a lytic lesion in the left femoral neck region that extends to the lesser trochanter. Although external beam radiation and radioactive iodine infusion may be helpful in controlling the local disease, the patient is at high risk for femoral neck fracture given the location of the lesion. Prophylactic surgery is indicated; therefore, the treatment of choice is a cemented bipolar hemiarthroplasty. The use of a compression hip screw and side plate or an intramedullary nail has a high likelihood of failure with disease progression. Postoperative treatment with radiation therapy and bisphosphonates is also indicated. Mirels H: Metastatic disease in long bones: A proposed scoring system for diagnosing impending pathologic fractures. Clin Orthop 1989;249:256-264. Swanson KC, Pritchard DJ, Sim FH: Surgical treatment of metastatic disease of the femur. J Am Acad Orthop Surg 2000;8:56-65.

Question 26

What is the primary mechanism of corrosion at the modular taper junction (head-neck interface) of a total hip arthroplasty?

Explanation

Mechanically assisted crevice corrosion (MACC), also known as fretting corrosion, is the primary mode of failure at modular head-neck taper junctions in total hip arthroplasty. The cyclic loading causes mechanical disruption of the protective oxide layer (fretting), and the fluid in the confined modular space becomes acidic and depleted of oxygen (crevice), leading to synergistic corrosion.

Question 27

A 14-year-old boy presents with a 3-month history of thigh pain and swelling. Radiographs reveal a permeative diaphyseal lesion in the femur with a 'periosteal onion-skin' reaction. Biopsy shows uniform small round blue cells. Which of the following chromosomal translocations is most characteristic of this condition?

Explanation

Ewing sarcoma is characterized by the t(11;22) chromosomal translocation, which results in the fusion of the EWS gene on chromosome 22 with the FLI1 gene on chromosome 11. t(9;22) is seen in some chondrosarcomas and chronic myelogenous leukemia. t(X;18) is pathognomonic for synovial sarcoma. t(12;16) is seen in myxoid liposarcoma, and t(2;13) is seen in alveolar rhabdomyosarcoma.

Question 28

A 68-year-old man presents with progressive bowing of his right tibia and dull, aching pain. Laboratory studies reveal an isolated elevation in serum alkaline phosphatase. Which of the following is the primary cellular defect in the early pathogenesis of this disease?

Explanation

The patient's presentation is classic for Paget's disease of bone. The primary pathogenetic defect lies in the osteoclast. The osteoclasts are increased in number, abnormally large, and possess many more nuclei than normal osteoclasts. This leads to an initial phase of intense bone resorption, which is followed by a robust, haphazard compensatory bone formation by osteoblasts, resulting in structurally weak woven bone.

Question 29

Which of the following modifications to a cortical screw design will most significantly increase its pullout strength in diaphyseal bone?

Explanation

Screw pullout strength is primarily determined by the outer diameter of the screw, thread pitch, thread depth, and the quality of the bone. Pullout strength is directly proportional to the outer diameter and thread depth, and inversely proportional to the thread pitch. Decreasing the thread pitch (meaning more threads per unit of length) increases the amount of bone engaged by the screw threads, thereby increasing pullout strength.

Question 30

Which of the following statements best describes the process of intramembranous ossification during fracture healing?

Explanation

Intramembranous ossification involves the direct differentiation of mesenchymal stem cells into osteoblasts without a cartilaginous intermediate. It is the primary mechanism of primary bone healing (contact healing) in rigidly fixed fractures (e.g., with compression plating) where strain is minimized to absolute stability. Secondary bone healing (callus formation) relies on endochondral ossification, which requires a cartilaginous template.

Question 31

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy confirms a giant cell tumor of bone. If medical management is initiated, denosumab works by targeting which of the following?

Explanation

In Giant Cell Tumor (GCT) of bone, the actual neoplastic cells are the mononuclear spindle-shaped stromal cells, which express high levels of RANK ligand (RANKL). This RANKL binds to RANK receptors on the surface of normal osteoclast precursors, leading to their differentiation into reactive, bone-resorbing multinucleated giant cells. Denosumab is a monoclonal antibody that binds directly to RANKL on the stromal cells, preventing its interaction with RANK and inhibiting osteoclast-like giant cell formation.

Question 32

Tranexamic acid (TXA) is frequently used in orthopedic surgery to reduce perioperative blood loss. Which of the following correctly describes its primary mechanism of action?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It acts as an antifibrinolytic agent by competitively inhibiting the activation of plasminogen to plasmin. It achieves this by binding to the lysine-binding sites on plasminogen, thereby blocking the binding of plasminogen to fibrin. This prevents the degradation of fibrin clots, effectively reducing surgical bleeding.

Question 33

A 70-year-old man develops a chronic prosthetic joint infection 2 years following a total knee arthroplasty. Which of the following represents the primary component of the biofilm matrix that protects the bacteria from host immune clearance and antibiotics?

Explanation

Biofilms are structured communities of bacteria encased in a self-produced polymeric matrix adhered to an implant surface. This glycocalyx matrix is primarily composed of extracellular polymeric substances (EPS), which include heavily hydrated polysaccharides, proteins, and extracellular DNA (eDNA). This matrix acts as a physical and chemical barrier against host immune cells and prevents the penetration of many antimicrobial agents.

Question 34

A 5-year-old child with a history of multiple low-energy fractures, blue sclerae, and hearing loss is diagnosed with Osteogenesis Imperfecta. Which of the following molecular defects is most commonly responsible for this condition?

Explanation

Osteogenesis imperfecta (OI) is primarily caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes, which encode type I collagen. The most common pathophysiologic mutation results in the substitution of the critical amino acid glycine (the smallest amino acid) with a bulkier amino acid within the collagen triple helix. Because glycine normally occupies the crowded center of the triple helix, this substitution disrupts the tight helical structure, leading to fragile, poorly formed bone.

Question 35

A 55-year-old man presents with progressive pelvic pain. Imaging reveals a large, destructive lesion in the ilium with 'popcorn' intralesional calcifications. Biopsy demonstrates a hypercellular cartilage matrix with pleomorphic binucleated chondrocytes. Which of the following is the most appropriate definitive management for this lesion?

Explanation

The clinical, radiographic, and histopathologic findings describe a conventional intermediate to high-grade chondrosarcoma. Conventional chondrosarcomas are notably resistant to both chemotherapy and radiation therapy due to their poor vascularity, slow growth, and dense extracellular matrix. Therefore, the cornerstone of treatment for intermediate and high-grade chondrosarcomas (or those located in the pelvis) is wide surgical resection with negative margins alone. Curettage is generally reserved for benign or low-grade cartilaginous lesions in the extremities.

Question 36

A 55-year-old man undergoes an anterior lumbar interbody fusion. Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized to augment the fusion. What is the primary intracellular signaling pathway through which rhBMP-2 exerts its osteoinductive effect?

Explanation

BMPs, including rhBMP-2, bind to serine/threonine kinase receptors on the cell surface. Upon activation, they primarily signal through the phosphorylation of receptor-regulated Smad proteins (Smads 1, 5, and 8). These phosphorylated Smads form a complex with the common-partner Smad 4, which then translocates to the nucleus to regulate transcription of osteogenic target genes, including Runx2, thereby promoting osteoblast differentiation.

Question 37

A 30-year-old athlete sustains a suspected Achilles tendon injury. During the physiological loading of the tendon, the initial 'toe region' of the stress-strain curve represents which specific microscopic change?

Explanation

The stress-strain curve for ligaments and tendons begins with a non-linear 'toe region.' This region corresponds to the straightening out (uncrimping) of the naturally crimped collagen fibers as initial tension is applied. Once uncrimped, the curve becomes linear (the elastic region), where the fibers are stretched until yielding occurs.

Question 38

Articular cartilage relies on its unique biochemical and biomechanical properties for load bearing. Which specific zone of articular cartilage contains the highest concentration of proteoglycans and is primarily responsible for resisting compressive forces?

Explanation

The deep (radial) zone of articular cartilage possesses the highest concentration of proteoglycans (aggrecan) and the lowest water content. The collagen fibers (primarily type II) in this zone are oriented perpendicularly to the articular surface. This composition makes the deep zone primarily responsible for resisting significant compressive forces. The superficial zone has the highest water and collagen content, which functions primarily to resist shear forces.

Question 39

A 45-year-old patient who smokes 1 pack per day sustained a closed tibial shaft fracture that was treated with intramedullary nailing. Six months postoperatively, radiographs demonstrate a hypertrophic nonunion with abundant callus formation but no bridging bone. What is the most appropriate next step in management?

Explanation

Hypertrophic nonunions are characterized by abundant callus formation (elephant's foot) without bridging bone. This occurs due to inadequate mechanical stability, not a lack of biology (as seen in atrophic nonunions). The biological potential for healing is intact. Therefore, treatment focuses on increasing mechanical stability. In the setting of a previously nailed tibial shaft, this is most effectively achieved with exchange intramedullary nailing to a larger diameter nail, which increases biomechanical stability and allows the biologically active fracture to heal.

Question 40

A 65-year-old man presents with an acutely swollen, erythematous, and severely painful right knee. Joint aspiration yields synovial fluid with 55,000 WBC/mm³, 85% polymorphonuclear leukocytes, and strongly negatively birefringent needle-shaped crystals under polarized light microscopy. Which of the following best describes the pathogenesis of the crystal formation in this patient?

Explanation

The patient has an acute gout flare, characterized by monosodium urate crystals that are needle-shaped and strongly negatively birefringent under polarized light. Gout results from hyperuricemia, which in the vast majority of cases (approximately 90%) is due to the underexcretion of uric acid by the kidneys, rather than overproduction. Pseudogout (CPPD) involves a defect in inorganic pyrophosphate metabolism, resulting in rhomboid, weakly positively birefringent calcium pyrophosphate crystals.

Question 41

A 16-year-old boy presents with a 6-month history of dull, aching back pain that is significantly worse at night and rapidly relieved by NSAIDs. Imaging reveals a 1 cm radiolucent nidus surrounded by reactive sclerosis in the posterior elements of T11. Histologically, the lesion consists of interconnecting woven bone trabeculae with prominent osteoblastic rimming. What is the most common potential clinical consequence of this specific lesion location?

Explanation

The patient has an osteoid osteoma, classically presenting with night pain relieved by NSAIDs and a radiolucent nidus <1.5 cm with reactive sclerosis. When located in the spine (typically the posterior elements), osteoid osteomas frequently cause a painful scoliosis. The concavity of the curve is characteristically directed toward the side of the lesion due to asymmetric paraspinal muscle spasm. Neurological deficits are rare because of the small size of the nidus, unlike osteoblastomas. They do not undergo malignant transformation.

Question 42

A 72-year-old man undergoes an elective total hip arthroplasty. Postoperatively, he is prescribed rivaroxaban for deep vein thrombosis prophylaxis. Rivaroxaban exerts its anticoagulant effect by directly inhibiting which of the following components of the coagulation cascade?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that function by selectively and reversibly inhibiting free and clot-bound Factor Xa in the coagulation cascade. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin inhibits vitamin K epoxide reductase. Clopidogrel and ticagrelor inhibit the platelet P2Y12 ADP receptor.

Question 43

During the endochondral ossification process of fracture healing, mesenchymal stem cells differentiate into various lineages. Which transcription factor is the master regulator essential for the differentiation of these stem cells into chondrocytes?

Explanation

Sox9 is the critical master transcription factor required for the differentiation of multipotent mesenchymal stem cells into the chondrogenic lineage during both embryonic development and endochondral fracture healing. Runx2 (Cbfa1) and Osterix (Sp7) are the essential transcription factors required for osteoblast differentiation. Beta-catenin is a key component of the Wnt signaling pathway, promoting osteoblastogenesis over chondrogenesis.

Question 44

A 60-year-old woman presents with groin pain and a palpable mass over her total hip arthroplasty. Her implant features a modular femoral neck-stem junction with a titanium stem and cobalt-chromium neck. Aspiration shows aseptic, metal-stained fluid. She is diagnosed with an adverse local tissue reaction (ALTR). The primary mode of corrosion occurring at this specific modular junction is:

Explanation

Mechanically assisted crevice corrosion (MACC) is the primary mechanism of corrosion and subsequent adverse local tissue reaction (ALTR) or pseudotumor formation at the modular junctions (such as head-neck or neck-stem tapers) of total hip arthroplasties. MACC involves both fretting (micromotion that mechanically disrupts the protective oxide passivation layer) and crevice corrosion (electrochemical changes due to restricted fluid exchange, leading to a localized drop in pH and oxygen depletion, which accelerates metal ion release).

Question 45

A 25-year-old elite track sprinter undergoes a skeletal muscle biopsy. The predominant muscle fibers identified are fast-twitch, easily fatigable, white fibers that contain low concentrations of myoglobin and mitochondria. Which primary biochemical pathway do these specific fibers rely on for ATP production?

Explanation

The description perfectly matches Type IIb (or IIx in humans) muscle fibers. These are fast-twitch, highly fatigable, 'white' fibers due to low myoglobin and mitochondrial content. They rely primarily on anaerobic glycolysis for rapid, short-duration energy (ATP) production. In contrast, Type I fibers are slow-twitch, fatigue-resistant 'red' fibers (high myoglobin) that rely heavily on aerobic pathways, including oxidative phosphorylation and beta-oxidation.

Question 46

A 65-year-old woman with postmenopausal osteoporosis is started on denosumab therapy to reduce her fracture risk. Which of the following best describes the molecular mechanism of action of this medication?

Explanation

Denosumab is a fully human monoclonal antibody that binds directly to RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand). By binding to RANKL, it prevents RANKL from interacting with the RANK receptor on the surface of osteoclasts and their precursors. This inhibition halts osteoclast formation, function, and survival, leading to significantly decreased bone resorption. Option A is incorrect as denosumab binds the ligand (RANKL), not the receptor. Option C describes the experimental drug odanacatib. Option D describes the mechanism of bisphosphonates. Option E describes romosozumab.

Question 47

During the manufacturing of ultra-high molecular weight polyethylene (UHMWPE) for total joint arthroplasty, highly cross-linked polyethylene is typically subjected to gamma irradiation. What is the primary purpose of the subsequent thermal treatment (melting or annealing) of the polyethylene?

Explanation

Gamma irradiation is used to cross-link UHMWPE, which significantly improves wear resistance. However, irradiation breaks carbon-carbon bonds, generating free radicals. If these free radicals are left in the material, they can react with oxygen in vivo or on the shelf, leading to oxidative degradation, chain scission, embrittlement, and increased wear. Thermal treatments, such as remelting or annealing, are applied after irradiation to increase the mobility of the polymer chains, allowing the free radicals to recombine and thus reducing the risk of oxidation. Remelting actually reduces crystallinity and ultimate tensile strength, making Options A and C incorrect.

Question 48

A 45-year-old man requires a unilateral uniplanar external fixator for an open tibial shaft fracture. You are selecting half-pins for the construct. According to biomechanical principles, increasing the core diameter of the half-pin by a factor of two will increase its bending stiffness by what factor?

Explanation

The bending stiffness of a solid cylinder, such as a half-pin or a screw, is proportional to the area moment of inertia, which is proportional to the radius (or diameter) to the fourth power (r^4). Therefore, doubling the core diameter increases the bending stiffness by a factor of 2^4 = 16. This is a crucial concept in external and internal fixation, as the core diameter of the pin/screw is the most significant geometric factor in determining its resistance to bending forces.

Question 49

A 60-year-old man is scheduled for a total hip arthroplasty. His medical history includes a previous deep vein thrombosis, and he is currently taking rivaroxaban. What is the precise pharmacological target of this anticoagulant medication?

Explanation

Rivaroxaban is an oral, direct, reversible, competitive inhibitor of Factor Xa. It binds directly to the active site of Factor Xa, interrupting both the intrinsic and extrinsic pathways of the blood coagulation cascade, thereby inhibiting thrombin generation and thrombus development. Option A describes direct thrombin inhibitors like dabigatran. Option C describes the mechanism of low-molecular-weight heparins (LMWH) and fondaparinux. Option D describes warfarin. Option E describes antiplatelet agents like clopidogrel.

Question 50

The molecular mechanism of distraction osteogenesis relies on mechanical tension applied to healing tissue to stimulate bone formation. When the biological environment is stable and the distraction rate is optimal, which of the following types of ossification is predominantly observed?

Explanation

In distraction osteogenesis (Ilizarov technique), when the biological environment is mechanically stable and the distraction rate is optimal (classically 1 mm/day divided into 4 increments), the primary mechanism of bone formation is intramembranous ossification. This process involves the direct differentiation of mesenchymal stem cells into osteoblasts, laying down bone without a cartilaginous intermediate. Endochondral ossification (Option A) involves a cartilage intermediate and is typically seen in secondary fracture healing where there is relative micromotion or instability.

Question 51

A 14-year-old boy presents with a painful mass in the diaphysis of his left femur. A biopsy reveals a small round blue cell tumor. Cytogenetic testing demonstrates a t(11;22)(q24;q12) translocation. This chromosomal translocation results in the fusion of which of the following gene pairs?

Explanation

The t(11;22)(q24;q12) translocation is the hallmark genetic aberration of Ewing sarcoma, present in approximately 85% to 90% of cases. It fuses the EWSR1 (Ewing sarcoma breakpoint region 1) gene on chromosome 22 with the FLI1 (Friend leukemia integration 1) gene on chromosome 11, resulting in a chimeric transcription factor (EWS-FLI1) that drives tumorigenesis. SYT-SSX is associated with synovial sarcoma [t(X;18)]. PAX3-FOXO1 is associated with alveolar rhabdomyosarcoma [t(2;13)]. TLS-CHOP is seen in myxoid liposarcoma [t(12;16)]. COL1A1-PDGFB is associated with dermatofibrosarcoma protuberans.

Question 52

A 35-year-old man presents with severe axial back pain and radiculopathy. An MRI confirms a herniated nucleus pulposus at L4-L5. Which type of collagen predominates in the normal adult nucleus pulposus, making it biomechanically suited to resist compressive loads?

Explanation

The intervertebral disc is composed of the outer annulus fibrosus and the inner nucleus pulposus. The normal adult nucleus pulposus is rich in proteoglycans (predominantly aggrecan) and Type II collagen, which gives it the ability to resist high compressive loads by maintaining a high water content. In contrast, the annulus fibrosus, which resists tensile forces, is predominantly composed of Type I collagen. Type X collagen is classically found in the hypertrophic zone of the physis.

Question 53

Following a closed humerus fracture, a patient develops a complete radial nerve palsy. Electromyography (EMG) performed 4 weeks later reveals fibrillation potentials and no voluntary motor unit action potentials. Assuming this is a Sunderland third-degree nerve injury, which structural components of the nerve have been disrupted?

Explanation

The Sunderland classification describes five degrees of nerve injury. 1st degree (neuropraxia): Local conduction block, intact axon and myelin sheath disruption. 2nd degree (axonotmesis): Disruption of the axon, intact endoneurium. 3rd degree: Disruption of the axon and endoneurium, intact perineurium. 4th degree: Disruption of the axon, endoneurium, and perineurium, intact epineurium. 5th degree (neurotmesis): Complete nerve transection. A third-degree injury involves scarring within the endoneurial tubes, which impedes axonal regeneration, often resulting in variable and incomplete recovery.

Question 54

In the pathogenesis of periprosthetic joint infection, Staphylococcus aureus often forms a biofilm on the implant surface. During the attachment and maturation phases of biofilm formation, which matrix component provides the essential three-dimensional structural integrity, rendering the bacteria highly resistant to systemic antibiotics?

Explanation

Biofilm formation on orthopedic implants begins with bacterial adherence, followed by proliferation and accumulation into a mature biofilm. The hallmark of a mature biofilm is the production of an extracellular polymeric substance (EPS), predominantly composed of polysaccharide intercellular adhesin (PIA), also known as poly-N-acetylglucosamine (PNAG), along with extracellular DNA and proteins. This EPS matrix encases the bacteria, protecting them from host immune responses and significantly reducing the penetration and efficacy of systemic antibiotics.

Question 55

A 28-year-old man undergoes an open reduction and internal fixation of a distal radius fracture under regional anesthesia. During the administration of a supraclavicular brachial plexus block, the patient suddenly develops perioral numbness, tinnitus, and subsequent generalized seizures followed by ventricular arrhythmias. Which of the following local anesthetics was most likely used, given its high potential for irreversible cardiac toxicity?

Explanation

The clinical scenario describes Local Anesthetic Systemic Toxicity (LAST). Bupivacaine is a long-acting amide local anesthetic that is notoriously lipophilic and has a high affinity for cardiac voltage-gated sodium channels. If inadvertently injected intravascularly, it can cause severe, refractory cardiac arrhythmias (such as ventricular fibrillation) and cardiovascular collapse. Its cardiac toxicity is much more difficult to resuscitate (often requiring lipid emulsion therapy) compared to lidocaine. Ropivacaine was developed specifically as an alternative to bupivacaine because it is less lipophilic and has a significantly improved cardiac safety profile.

Question 56

A 35-year-old woman presents with a local recurrence of a giant cell tumor of the distal femur. She is deemed a poor surgical candidate and is started on systemic therapy with a specific monoclonal antibody. What is the primary mechanism of action of this medication in the treatment of her condition?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand). In giant cell tumor (GCT) of bone, the neoplastic mononuclear stromal cells express RANKL, which strongly recruits and activates reactive multinucleated giant cells (osteoclast-like cells) responsible for massive osteolysis. By binding and inhibiting RANKL, denosumab halts osteoclastic bone resorption, leading to tumor calcification and disease control.

Question 57

A 15-year-old boy completes a course of neoadjuvant chemotherapy and subsequently undergoes a wide resection for a classic, nonmetastatic, high-grade osteosarcoma of the distal femur. Which of the following is considered the single most important prognostic factor for his overall survival?

Explanation

The histologic response to neoadjuvant chemotherapy, determined by the percentage of tumor necrosis in the resected specimen, is the single most reliable prognostic factor for overall survival in patients with classic high-grade osteosarcoma. Greater than 90% necrosis indicates a favorable response and correlates with significantly higher survival rates compared to a poor response (less than 90% necrosis).

Question 58

Bone morphogenetic proteins (BMPs) play a crucial role in osteoinduction by promoting the differentiation of mesenchymal stem cells into osteoblasts. Which intracellular signaling pathway is primarily activated upon the binding of BMPs to their respective serine/threonine kinase receptors?

Explanation

Bone morphogenetic proteins (BMPs) are members of the TGF-beta superfamily. They initiate intracellular signaling by binding to type I and type II serine/threonine kinase receptors, which subsequently phosphorylate receptor-regulated Smad proteins (primarily Smad1, Smad5, and Smad8). These then form a complex with Smad4 and translocate to the nucleus to regulate the transcription of osteogenic genes.

Question 59

A 28-year-old man presents with a slow-growing, deep-seated soft-tissue mass in his left foot. MRI demonstrates a well-circumscribed, lobulated mass adjacent to the plantar fascia. Biopsy reveals a biphasic histologic pattern consisting of spindle cells and epithelial cells. What is the characteristic chromosomal translocation associated with this neoplasm?

Explanation

The clinical and histologic findings are diagnostic of synovial sarcoma. This tumor is characterized by the reciprocal chromosomal translocation t(X;18)(p11;q11), which fuses the SYT gene on chromosome 18 with either the SSX1, SSX2, or SSX4 gene on the X chromosome. t(11;22) is seen in Ewing sarcoma. t(12;16) is seen in myxoid liposarcoma. t(9;22) is seen in extraskeletal myxoid chondrosarcoma. t(2;13) is characteristic of alveolar rhabdomyosarcoma.

Question 60

A 68-year-old man undergoes revision total hip arthroplasty for aseptic loosening. Intraoperatively, extensive granulomatous tissue and periprosthetic osteolysis are noted around the uncemented acetabular component. The primary wear mechanism responsible for generating the ultra-high molecular weight polyethylene (UHMWPE) particles that lead to this cascade is best described as:

Explanation

Adhesive wear is the dominant mode of wear in metal-on-polyethylene and ceramic-on-polyethylene bearing surfaces. It occurs when microscopic asperities on the harder surface adhere to the softer polyethylene surface during articulation. As motion continues, the microscopic bonds are sheared off, generating millions of submicron-sized polyethylene particles that incite a macrophage-mediated inflammatory response leading to osteolysis.

Question 61

A 45-year-old woman with a history of end-stage renal disease presents with diffuse, dull bone pain and proximal muscle weakness. Laboratory studies show hypocalcemia, hyperphosphatemia, and elevated intact parathyroid hormone (PTH) levels. Radiographs demonstrate focal radiolucencies traversing the femoral necks bilaterally. The primary histologic defect in this patient's skeletal pathology is characterized by:

Explanation

The patient is presenting with signs of osteomalacia secondary to renal osteodystrophy (caused by decreased 1,25-dihydroxyvitamin D production and consequent hypocalcemia). The focal radiolucencies are Looser zones (pseudofractures). The fundamental histologic hallmark of osteomalacia (and rickets in children) is an accumulation of unmineralized osteoid seams due to defective mineralization of newly formed bone matrix.

Question 62

The biomechanical properties of articular cartilage are determined by its complex ultrastructural organization. Which zone of articular cartilage contains the highest concentration of proteoglycans, the lowest concentration of water, and thick collagen fibrils oriented perpendicular to the articular surface?

Explanation

The deep (radial) zone of articular cartilage is specifically adapted to resist compressive loads. It is characterized by having the highest proteoglycan content, the lowest water content (approximately 65%), and thick Type II collagen fibrils arranged radially (perpendicular to the joint surface). The superficial zone, by contrast, has the highest water content and collagen fibers oriented parallel to the surface to resist shear forces.

Question 63

Following a primary surgical repair of a complete flexor digitorum profundus (FDP) laceration in Zone II of the hand, a specific post-operative rehabilitation protocol is initiated to prevent adhesions while promoting tendon healing. Within this digital synovial sheath, the majority of the nutritional supply for the healing tendon is provided by:

Explanation

Flexor tendons within the digital synovial sheath (Zone II) receive nutrition via a dual mechanism: vascular perfusion (through the vincula system) and synovial diffusion. Experimental studies have firmly established that synovial fluid diffusion is the predominant source of nutrition for these tendons, providing the majority of the essential nutrients. This diffusion is significantly enhanced by the physical pumping action of tendon gliding, reinforcing the physiologic rationale for early motion rehabilitation.

Question 64

An 8-year-old boy presents with severe limb pain, bleeding gums, and extensive petechiae. Radiographs of the lower extremities reveal generalized osteopenia with a characteristic dense provisional zone of calcification at the metaphyses. The underlying biochemical defect responsible for this clinical presentation involves the failure of which specific process?

Explanation

The presentation is classic for scurvy (Vitamin C deficiency). Ascorbic acid is an essential cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase, which mediate the hydroxylation of proline and lysine residues on nascent procollagen chains. This hydroxylation is critical for stabilizing the collagen triple helix via hydrogen bonding. A deficiency leads to fragile connective tissues, weakened blood vessels (bleeding), and defective bone matrix synthesis.

Question 65

A 65-year-old woman is prescribed rivaroxaban for venous thromboembolism (VTE) prophylaxis following an elective primary total knee arthroplasty. By which specific mechanism does this pharmacological agent disrupt the coagulation cascade?

Explanation

Rivaroxaban is a novel oral anticoagulant (NOAC) that exerts its effect through the direct, selective, and reversible inhibition of Factor Xa. This interrupts the intrinsic and extrinsic pathways of the blood coagulation cascade at their point of convergence, effectively reducing the generation of thrombin and subsequent thrombus formation. Dabigatran inhibits thrombin (Factor IIa). Heparin potentiates antithrombin III. Warfarin inhibits vitamin K epoxide reductase. Aspirin inhibits COX-1.

Question 66

Nitrogen-containing bisphosphonates, such as alendronate and zoledronic acid, are highly effective antiresorptive agents used in the management of osteoporosis and metastatic bone disease. What is their primary intracellular mechanism of action within the osteoclast?

Explanation

Nitrogen-containing bisphosphonates exert their antiresorptive effects by inhibiting farnesyl pyrophosphate (FPP) synthase, a key enzyme in the mevalonate pathway. This disruption prevents the prenylation of small GTP-binding proteins (such as Ras, Rho, and Rac) essential for osteoclast function, cytoskeletal organization, and the maintenance of the ruffled border, ultimately leading to osteoclast apoptosis. Non-nitrogenous bisphosphonates, in contrast, are metabolized into non-hydrolyzable ATP analogs that induce cell death.

Question 67

Normal articular cartilage is highly organized into distinct histologic zones, each tailored to specific biomechanical demands. Which of the following best describes the structural and biochemical characteristics of the deep (radial) zone of articular cartilage?

Explanation

The deep (radial) zone of articular cartilage is characterized by the lowest concentration of water and the highest concentration of proteoglycans. Its collagen fibrils (predominantly Type II) are arranged perpendicular to the joint surface and anchor into the underlying tidemark. In contrast, the superficial (tangential) zone has the highest water content, the lowest proteoglycan content, and collagen fibers oriented parallel to the joint surface to resist shear forces.

Question 68

Recombinant human bone morphogenetic proteins (rhBMPs) are utilized in orthopaedic surgery for their profound osteoinductive properties. Which of the following BMPs is currently FDA-approved for use as an adjunct in the treatment of acute, open tibial shaft fractures stabilized with an intramedullary nail?

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved as an adjunct for the treatment of acute, open tibial shaft fractures stabilized with an intramedullary nail, as well as for anterior cervical discectomy and fusion using an approved interbody fusion device. While rhBMP-7 (Osteogenic Protein-1 or OP-1) was previously available under a Humanitarian Device Exemption for recalcitrant tibial nonunions, BMP-2 remains the primary approved agent for the acute open tibia indication. BMP-3 is unique in that it actually acts as a negative regulator (inhibitor) of bone formation.

Question 69

A 28-year-old man sustains a closed midshaft humerus fracture and subsequently develops a complete radial nerve palsy. According to the Sunderland classification of peripheral nerve injuries, which of the following accurately defines a third-degree nerve injury?

Explanation

The Sunderland classification expands upon Seddon's system. A first-degree injury is neuropraxia (myelin damage). A second-degree injury is axonotmesis (axon disrupted, endoneurium intact). A third-degree injury involves disruption of both the axons and the endoneurium, but the perineurium remains intact. Healing in third-degree injuries is unpredictable due to intrafascicular fibrosis, potentially leading to incomplete recovery. A fourth-degree injury involves disruption of the perineurium with an intact epineurium (neuroma-in-continuity), and a fifth-degree injury is complete transection (neurotmesis).

Question 70

A 65-year-old man presents with an acutely painful and swollen knee 3 days after undergoing an inguinal hernia repair. Joint aspiration yields synovial fluid with a white blood cell count of 35,000 cells/mm³. Polarized light microscopy reveals weakly positively birefringent, rhomboid-shaped crystals. What is the most likely composition of these crystals?

Explanation

The presence of weakly positively birefringent, rhomboid-shaped crystals is pathognomonic for calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, commonly referred to as pseudogout. Pseudogout attacks are frequently precipitated by acute medical illnesses or surgical procedures. Monosodium urate crystals (seen in gout) are strongly negatively birefringent and needle-shaped. A synovial WBC count of 35,000 cells/mm³ is highly consistent with an inflammatory crystal arthropathy, though septic arthritis must always be considered in the differential.

Question 71

Corrosion of metallic orthopaedic implants can lead to catastrophic implant failure and adverse local tissue reactions. Which of the following scenarios best describes the mechanism underlying galvanic corrosion?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials (e.g., a stainless steel wire around a titanium rod) are placed in direct contact within an electrolytic solution (such as human body fluid). The less noble metal acts as an anode and undergoes accelerated corrosion. Fretting corrosion, in contrast, results from micromotion at modular interfaces (such as a head-neck taper) that repetitively strips the protective oxide (passivation) layer. Corrosion fatigue combines cyclic mechanical loading with a corrosive environment.

Question 72

During the active cross-bridge cycle of skeletal muscle contraction, specific molecular interactions dictate the binding and unbinding of actin and myosin. What is the direct biophysical consequence of a new molecule of adenosine triphosphate (ATP) binding to the myosin head?

Explanation

In the sliding filament mechanism of muscle contraction, the binding of a new ATP molecule to the myosin head decreases myosin's affinity for actin, causing it to detach from the actin filament. Subsequent hydrolysis of ATP into ADP and inorganic phosphate (Pi) provides the energy to 'cock' the myosin head into a high-energy state. The actual 'power stroke' is triggered later when the inorganic phosphate is released from the myosin head. Calcium binding to troponin C is responsible for exposing the myosin-binding sites on actin.

Question 73

Secondary fracture healing occurs primarily through endochondral ossification and progresses through distinct biological phases. During the formation of the soft callus (cartilaginous phase), which collagen type is predominantly synthesized by the local cellular infiltrate?

Explanation

Secondary bone healing relies on endochondral ossification, which features an intermediate cartilaginous soft callus. During this phase, proliferating chondrocytes predominantly synthesize Type II collagen, the major structural protein of hyaline cartilage. As the process progresses to the hard callus phase, the cartilage is resorbed and replaced by woven bone, which is predominantly composed of Type I collagen. Type X collagen is uniquely secreted by hypertrophic chondrocytes during the late stages of endochondral ossification, facilitating mineralization.

Question 74

A 15-year-old boy presents with progressive knee pain and swelling. Radiographs reveal an aggressive, permeative, diaphyseal lesion in the distal femur with an associated 'onion-skin' periosteal reaction. A biopsy reveals sheets of uniform, small, round, blue cells. Which of the following specific chromosomal translocations is most strongly associated with this diagnosis?

Explanation

The clinical presentation, radiographic findings (diaphyseal permeative lesion, 'onion-skin' periosteal reaction), and histology (small round blue cells) are classic for Ewing sarcoma. Ewing sarcoma is heavily characterized by the t(11;22)(q24;q12) chromosomal translocation in approximately 85% of cases, which creates the EWS-FLI1 fusion protein. Other notable translocations include t(X;18) for synovial sarcoma, t(12;16) for myxoid liposarcoma, and t(2;13) for alveolar rhabdomyosarcoma.

Question 75

Rivaroxaban is frequently prescribed for extended venous thromboembolism (VTE) prophylaxis following total joint arthroplasty. What is the specific molecular mechanism of action by which rivaroxaban exerts its anticoagulant effect?

Explanation

Rivaroxaban, along with apixaban, belongs to the class of direct oral anticoagulants (DOACs) that function by directly and selectively inhibiting Factor Xa, preventing the conversion of prothrombin to thrombin. Dabigatran is an example of a direct thrombin (Factor IIa) inhibitor. Warfarin acts as a Vitamin K antagonist by inhibiting epoxide reductase. Heparins (including low-molecular-weight heparins like enoxaparin) exert their effect by binding to and potentiating antithrombin III. Aspirin irreversibly inhibits COX-1, affecting platelet aggregation.

Question 76

What is the primary biomechanical tradeoff of heating highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) above its melting point (remelting) during processing to quench residual free radicals?

Explanation

Highly cross-linked polyethylene is irradiated to improve wear resistance, but this creates free radicals that can lead to oxidation and degradation in vivo. To eliminate these free radicals, the material is either annealed (heated below the melting point) or remelted (heated above the melting point). Remelting effectively eliminates all free radicals, preventing oxidation, but the structural changes associated with melting significantly decrease the ultimate tensile strength, yield strength, and fatigue resistance of the material. Annealing preserves mechanical strength but leaves residual free radicals.

Question 77

A 14-year-old boy presents with a painful mass in his diaphyseal femur. Biopsy reveals a uniform population of small round blue cells. Immunohistochemistry is positive for CD99. Which of the following chromosomal translocations is most characteristic of this specific malignancy?

Explanation

The patient's presentation and histology are classic for Ewing sarcoma. Ewing sarcoma is characterized by a specific balanced chromosomal translocation, t(11;22)(q24;q12), which fuses the EWS gene on chromosome 22 with the FLI1 gene on chromosome 11, forming the EWS-FLI1 fusion protein. Option B t(X;18) is associated with synovial sarcoma. Option C t(12;16) is seen in myxoid liposarcoma. Option D t(2;13) is characteristic of alveolar rhabdomyosarcoma. Option E t(9;22) is the Philadelphia chromosome seen in CML, and also associated with extraskeletal myxoid chondrosarcoma t(9;22)(q22;q12).

Question 78

In the microscopic architecture of normal articular cartilage, which zone is characterized by the highest concentration of proteoglycans, the lowest water content, and type II collagen fibers oriented primarily perpendicular to the joint surface?

Explanation

The deep (radial) zone of articular cartilage provides the greatest resistance to compressive loads. It contains the highest concentration of proteoglycans and the lowest concentration of water. The collagen fibers (primarily type II) in this zone are thickest and are arranged vertically (perpendicular to the articular surface), anchoring the cartilage to the underlying calcified zone. The superficial zone has the highest water content and collagen fibers parallel to the joint surface to resist shear stress.

Question 79

According to Perren's strain theory of fracture healing, the formation of primary lamellar bone requires a tissue strain environment of less than what percentage?

Explanation

Perren's strain theory dictates the type of tissue that can form in a fracture gap based on the relative deformation (strain) of that tissue. Granulation tissue can tolerate up to 100% strain. Fibrocartilage and woven bone can form in environments with 2% to 10% strain. Primary lamellar bone formation, however, is highly sensitive to deformation and can only occur when the strain across the fracture gap is less than 2%.

Question 80

Which of the following best describes the primary phenotypic change in bacteria during the maturation phase of a biofilm in a periprosthetic joint infection, making eradication with standard systemic antibiotics exceedingly difficult?

Explanation

Biofilm formation involves initial attachment, maturation, and detachment (dispersion). During the maturation phase, bacteria encase themselves in a protective extracellular polymeric substance (EPS) matrix and undergo a phenotypic shift from a highly metabolic, free-floating (planktonic) state to a metabolically dormant (sessile) state. Standard antibiotics target active cellular processes (like cell wall synthesis and protein translation); therefore, these metabolically inactive sessile bacteria are highly recalcitrant to standard antimicrobial therapy.

Question 81

The 'windlass mechanism' of the foot provides dynamic stability to the longitudinal arch and facilitates propulsion. This mechanism is primarily driven by the dynamic tensioning of the plantar fascia during which specific phase of the gait cycle?

Explanation

The windlass mechanism is described by J.H. Hicks as the dynamic tensioning of the plantar fascia as it wraps around the metatarsal heads during passive extension (dorsiflexion) of the metatarsophalangeal (MTP) joints. This tensioning occurs primarily during terminal stance and preswing (heel-off to toe-off), drawing the calcaneus and metatarsals closer together, thereby elevating the longitudinal arch and locking the midtarsal joints to create a rigid lever for forward propulsion.

Question 82

A 65-year-old man presents with progressive bowing of his right tibia, a localized increase in skin temperature over the leg, and elevated serum alkaline phosphatase levels with normal calcium and phosphorus. Which of the following best describes the primary cellular abnormality in this condition?

Explanation

The patient's presentation is classic for Paget disease of bone (osteitis deformans). The primary cellular abnormality in Paget disease is an intense, focal increase in bone resorption by massive, hyperactive, multinucleated osteoclasts. These abnormal osteoclasts frequently contain viral-like inclusion bodies, and the disease process is often linked to SQSTM1 gene mutations and potential paramyxovirus infection. The intense osteoclastic phase is followed by disorganized osteoblastic woven bone formation.

Question 83

The use of the reamer-irrigator-aspirator (RIA) system for autologous bone graft harvesting from the femoral canal is most associated with which of the following advantages when compared to traditional iliac crest bone grafting (ICBG)?

Explanation

The RIA system was developed to manage intramedullary canal pressures during reaming but has become a valuable tool for bone graft harvest. Its primary advantage over ICBG is a massive reduction in donor site morbidity (pain, nerve injury, infection) while yielding large volumes (often 40-80 cc) of bone graft. The RIA effluent contains high concentrations of mesenchymal stem cells (MSCs) and growth factors comparable to or exceeding those found in ICBG, though it does not provide structural mechanical support.

Question 84

During skeletal muscle contraction via the sliding filament theory, calcium ions released from the sarcoplasmic reticulum bind directly to which of the following proteins to initiate the exposure of myosin-binding sites on actin?

Explanation

Muscle contraction is initiated when an action potential triggers the release of calcium ions from the sarcoplasmic reticulum. These calcium ions directly bind to Troponin C on the thin actin filaments. This binding causes a conformational change in the troponin complex (which includes Troponin C, I, and T), causing tropomyosin to shift away from the myosin-binding sites on actin, allowing the myosin heads to bind and cross-bridge cycling to begin.

Question 85

A 21-year-old man presents with nocturnal thigh pain that is completely relieved by NSAIDs. Radiographs demonstrate a radiolucent nidus with surrounding dense reactive sclerosis in the femoral diaphysis.

Which of the following molecules is secreted in abnormally high concentrations by the nidus of this lesion, accounting for the characteristic pain pattern?

Explanation

The clinical and radiographic presentation describes an osteoid osteoma. The hallmark symptom is intense nocturnal pain that typically responds dramatically to nonsteroidal anti-inflammatory drugs (NSAIDs) or aspirin. This distinct clinical feature is due to the nidus of the osteoid osteoma producing high levels of prostaglandins, particularly Prostaglandin E2 (PGE2), up to 100 to 1,000 times normal tissue levels. NSAIDs block cyclooxygenase, inhibiting PGE2 synthesis and relieving the pain.

Question 86

A 32-year-old woman is diagnosed with an unresectable giant cell tumor of the sacrum. Her multidisciplinary tumor board decides to initiate systemic medical therapy to help consolidate the lesion before attempting any possible surgical intervention. Which of the following best describes the mechanism of action of the most appropriate medication prescribed for this patient?

Explanation

Denosumab is a human monoclonal antibody that binds directly to RANKL, preventing it from binding to the RANK receptor on the surface of osteoclasts and their precursors. This effectively inhibits osteoclast formation, function, and survival, leading to decreased bone resorption. It is FDA-approved for the treatment of giant cell tumor of bone (GCTB) that is unresectable or where surgical resection is likely to result in severe morbidity. The neoplastic stromal cells in GCTB express high levels of RANKL, which recruit and activate the reactive multinucleated giant osteoclast-like cells responsible for aggressive bone destruction.

Question 87

A 65-year-old woman is evaluated for severe osteoporosis and started on an oral nitrogen-containing bisphosphonate. At the cellular level, these medications primarily inhibit osteoclastic bone resorption through which of the following mechanisms?

Explanation

Nitrogen-containing bisphosphonates (e.g., alendronate, risedronate, zoledronic acid) act by inhibiting the enzyme farnesyl pyrophosphate (FPP) synthase within the mevalonate pathway. This inhibition prevents the prenylation of small GTPase proteins (such as Ras, Rho, and Rac) that are essential for osteoclast ruffled border formation, function, and survival, ultimately leading to osteoclast apoptosis and significantly decreased bone resorption. Non-nitrogenous bisphosphonates act via a different mechanism, becoming incorporated into non-hydrolyzable ATP analogues.

Question 88

Bone Morphogenetic Proteins (BMPs) play a crucial role in osteoinduction and bone healing. Following the binding of a BMP to its specific cell surface serine/threonine kinase receptor, which of the following intracellular signaling molecules is directly phosphorylated to propagate the signal to the nucleus?

Explanation

BMPs signal through highly specific cell surface serine/threonine kinase receptors. Upon ligand binding, the type II receptor phosphorylates the type I receptor, which subsequently phosphorylates receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These phosphorylated R-Smads then form a complex with the common-partner Smad (Co-Smad), Smad 4, and translocate to the nucleus to regulate the transcription of target osteogenic genes. In contrast, Smad 2 and 3 are typically activated by TGF-beta and Activin signaling.

Question 89

Articular cartilage is structurally organized into distinct zones, each with unique biomechanical properties. Which zone is characterized by the highest concentration of proteoglycans, the lowest concentration of water, and collagen fibers that are oriented perpendicular to the articular surface?

Explanation

The deep (radial) zone of normal articular cartilage contains the highest concentration of proteoglycans and the lowest water content. The collagen fibers (primarily type II) in this zone are oriented perpendicular to the articular surface to provide high compressive strength and securely anchor the uncalcified cartilage to the underlying calcified zone via the tidemark. Conversely, the superficial zone has the highest water content, the lowest proteoglycan content, and collagen fibers arranged parallel to the joint surface to resist shear stress.

Question 90

A 55-year-old man presents with an acutely painful, erythematous, and swollen right knee. A diagnostic joint aspiration is performed, yielding opaque synovial fluid. Analysis reveals a white blood cell count of 85,000 cells/mm³ with 90% polymorphonuclear leukocytes, and a fluid glucose level of 25 mg/dL (serum glucose is 100 mg/dL). Which of the following is the most appropriate next step in management?

Explanation

The synovial fluid analysis strongly suggests a septic joint. Fluid that is opaque, has >50,000 WBCs/mm³ (often >80,000) with a predominance of polymorphonuclear leukocytes (>75%), and significantly decreased glucose compared to serum is highly diagnostic of bacterial septic arthritis. The definitive standard of care is urgent surgical debridement and irrigation (arthroscopic or open) combined with empiric intravenous antibiotics, which should be tailored once culture and sensitivity results are available.

Question 91

Perren's strain theory dictates the type of bone healing that occurs based on the interfragmentary strain present at the fracture site. According to this biomechanical theory, primary (direct) bone healing without the formation of an external callus occurs when the interfragmentary strain is maintained below what threshold?

Explanation

According to Perren's strain theory, primary (direct or osteonal) bone healing occurs when absolute stability is achieved, resulting in an interfragmentary strain of less than 2%. Under these rigid conditions, osteoclasts create cutting cones across the fracture, followed by osteoblasts depositing lamellar bone, without intermediate callus formation. Secondary bone healing with robust callus formation occurs under conditions of relative stability, where interfragmentary strain is between 2% and 10%. If strain exceeds 10%, bone cannot form, often resulting in fibrous tissue or nonunion.

Question 92

In metal-on-polyethylene total hip arthroplasty, the most common mechanism of wear leading to the generation of clinically significant submicron polyethylene particles, osteolysis, and subsequent aseptic loosening is characterized by a harder surface scratching and plowing into a softer surface. What is the specific tribological term for this type of wear?

Explanation

Abrasive wear occurs when a harder surface (the metal or ceramic femoral head) plows into and scratches a softer surface (the polyethylene liner), cutting away material and releasing wear particles. Adhesive wear involves micro-welding and tearing off of the softer material onto the harder material. Third-body wear happens when an independent hard particle (e.g., bone, cement debris) becomes trapped between the two bearing surfaces. Fatigue wear results from repeated cyclical compressive loading, leading to subsurface cracking and delamination.

Question 93

Rivaroxaban and apixaban are frequently used for effective venous thromboembolism (VTE) prophylaxis following total joint arthroplasty. At which specific point in the coagulation cascade do these oral medications exert their primary mechanism of action?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that specifically and directly inhibit Factor Xa (FXa) in the common pathway of the coagulation cascade. Dabigatran directly inhibits thrombin (Factor IIa). Fondaparinux and low-molecular-weight heparins (LMWH) indirectly inhibit FXa by potentiating the action of antithrombin III. Warfarin acts by inhibiting vitamin K epoxide reductase, preventing the synthesis of vitamin K-dependent factors (II, VII, IX, X). Aspirin irreversibly inhibits COX-1, preventing platelet aggregation.

Question 94

During the normal gait cycle, maximum dorsiflexion of the ankle occurs just prior to heel off. Which muscle group is primarily responsible for eccentrically controlling the forward progression of the tibia over the talus during the midstance phase?

Explanation

During midstance, the body's center of mass moves forward over the planted foot, causing the tibia to naturally translate anteriorly over the fixed talus into dorsiflexion. The gastrocnemius-soleus complex (plantar flexors) fires eccentrically during this phase to decelerate and smoothly control this forward tibial progression, preventing the knee from buckling into excess flexion. The tibialis anterior acts eccentrically during initial contact/loading response to prevent 'foot slap'. The quadriceps act eccentrically during loading response to control knee flexion for shock absorption.

Question 95

Among the following solid metallic materials commonly used in orthopedic implants, which possesses the lowest modulus of elasticity (Young's modulus), thereby theoretically minimizing the risk of stress shielding when utilized for diaphyseal fixation?

Explanation

The modulus of elasticity (Young's modulus) represents the intrinsic stiffness of a material. Cortical bone has a modulus of approximately 15-20 GPa. Titanium alloys (e.g., Ti-6Al-4V) have a modulus of about 100-110 GPa, which is roughly half the stiffness of stainless steel (approx. 200 GPa) and cobalt-chromium alloys (approx. 210-230 GPa). Although titanium is still significantly stiffer than bone, its relatively lower elastic modulus compared to other common solid surgical metals translates to improved load-sharing and theoretically less severe stress shielding around intramedullary stems and plates.

Question 96

A 12-year-old boy presents with a painful mass in his diaphyseal femur. Radiographs demonstrate a permeative lytic lesion with an 'onion-skin' periosteal reaction. Histological evaluation of a core needle biopsy reveals uniform, small round blue cells that stain positive for CD99. Cytogenetic analysis is pending. Which of the following is the most common chromosomal translocation associated with this diagnosis?

Explanation

The clinical and radiographic presentation, along with the finding of CD99-positive small round blue cells, is highly characteristic of Ewing sarcoma. The most common chromosomal translocation found in Ewing sarcoma is t(11;22)(q24;q12), which results in the EWS-FLI1 fusion protein. t(X;18) is associated with synovial sarcoma, t(12;16) with myxoid liposarcoma, t(2;13) with alveolar rhabdomyosarcoma, and t(9;22) (the Philadelphia chromosome) with chronic myelogenous leukemia.

Question 97

In an orthopedic biomechanics laboratory, a researcher is testing the viscoelastic properties of a human anterior cruciate ligament (ACL) graft. A constant, unchanging tensile load is applied to the graft, and the researcher observes a gradual, time-dependent increase in the deformation (length) of the tissue. Which of the following biomechanical phenomena does this observation describe?

Explanation

Creep is the progressive deformation of a viscoelastic material when subjected to a constant load over time. Stress relaxation, another viscoelastic property, is the decrease in stress (force) within a material over time when it is held at a constant deformation or length. Hysteresis represents the energy lost (usually as heat) during the loading and unloading cycles of a material. Anisotropy refers to a material possessing different mechanical properties depending on the direction of the applied load.

Question 98

Recombinant human bone morphogenetic proteins (rhBMP-2 and rhBMP-7) are potent osteoinductive agents frequently utilized to augment bone healing and spinal fusion. BMPs belong to the transforming growth factor-beta (TGF-beta) superfamily. Through which of the following intracellular signaling pathways do BMPs predominantly exert their osteoinductive effects?

Explanation

Bone morphogenetic proteins (BMPs) initiate intracellular signaling primarily through the Smad pathway. Upon binding to specific serine/threonine kinase surface receptors, BMPs cause the phosphorylation of receptor-regulated Smads (R-Smads: 1, 5, and 8). These phosphorylated R-Smads then complex with a common-partner Smad (Co-Smad 4) and translocate into the nucleus, where they act as transcription factors to upregulate osteogenic genes such as Runx2.

Question 99

A 55-year-old man presents with a progressively enlarging, painful mass in his right proximal humerus over the last 6 months. Radiographs demonstrate a large radiolucent lesion with intralesional 'popcorn' calcifications and deep endosteal scalloping greater than two-thirds of the cortical thickness. Biopsy confirms a Grade II chondrosarcoma. What is the most appropriate definitive surgical management?

Explanation

Conventional chondrosarcomas (intermediate Grade II and high Grade III) are famously resistant to standard chemotherapy and radiation therapy due to their poor vascularity, slow growth, and abundant extracellular matrix. Therefore, the mainstay of treatment for Grade II and III chondrosarcomas is wide surgical resection with negative margins. Intralesional curettage is only appropriate for benign cartilaginous lesions (like enchondromas) or carefully selected Grade I atypical cartilaginous tumors in the appendicular skeleton.

Question 100

During the pathogenesis of periprosthetic joint infection (PJI), Staphylococcus aureus adheres to the orthopedic implant surface and forms a biofilm, making the infection highly recalcitrant to host immune responses and systemic antibiotics. Which of the following phases of biofilm formation is specifically characterized by the production of a complex extracellular polymeric substance (EPS) matrix and active quorum sensing?

Explanation

Biofilm formation occurs in sequential stages: (1) Initial reversible attachment of planktonic bacteria to the implant surface; (2) Irreversible attachment mediated by specific adhesins like MSCRAMMs; (3) Maturation, characterized by cellular multiplication, complex architectural development, synthesis of the extracellular polymeric substance (EPS) matrix, and quorum sensing (cell-to-cell communication); and (4) Dispersion or detachment, where bacteria escape the biofilm to colonize new sites.

General Orthopedics 2026 Practice Questions: Set 13 (Solved)

Orthopedic Basic 2026 MCQs: Board Review Questions & Answers (Part 2)

Key Takeaway

Orthopedic Basic 2026 MCQs: Board Review Questions & Answers (Part 2)

Chapter Index