Orthopedic Basic MCQs 2026 | Board Review & Answers

Question 1

Figure 26a shows the radiograph of a 55-year-old woman who has pain in her right leg after falling. Laboratory studies reveal an elevated alkaline phosphatase level. A biopsy specimen from the proximal tibia is shown in Figure 26b. What is the most likely diagnosis?

Explanation

26b Paget's disease of bone is a metabolic disorder of bone remodeling. The normally coupled process of bone resorption and deposition is lost, resulting in excessive localized bone resorption and compensatory increased bone formation. Pagetic bone tends to be more brittle; therefore, it is susceptible to pathologic fractures and subsequent deformities. Lander PH, Hadjipavlou AG: A dynamic classification of Paget's disease. J Bone Joint Surg Br 1986;68:431-438.

Question 2

Radiographs of a pediatric patient reveal a suspected osteosarcoma of the distal femur. Additional staging studies should consist of

Explanation

CT of the abdomen and pelvis is not part of the staging of osteosarcoma. Staging studies should consist of CT of the chest, radiographs of the chest and primary tumor, MRI of the primary tumor, and a bone scan. The MRI should be obtained prior to the biopsy.

Question 3

Which of the following factors are considered prognostic of survival in patients with soft-tissue sarcomas?

Explanation

The factors that are independently prognostic of patient survival are tumor size, tumor grade, and tumor depth (ie, subfascial versus superficial). These factors are the basis for the American Joint Committee on Cancer staging criteria. Patient age and neurovascular invasion are not prognostic. Surgical margin is prognostic for local recurrence but not conclusively for patient survival or metastasis. Metastatic disease is also predictive of survival. Cheng EY, Thompson RC Jr: New developments in the staging and imaging of soft-tissue sarcomas. Instr Course Lect 2000;49:443-451.

Question 4

Figures 27a through 27c show the AP radiograph, MRI scan, and biopsy specimen of an otherwise healthy man who has a painful wrist. Serum chemistry studies are normal. What is the most likely diagnosis?

Explanation

27b 27c The osseous sites most frequently involved by giant cell tumor of bone are the distal femur, proximal tibia, and distal radius with approximately 10% of giant cell tumors involving the distal radius. The goals of treatment are to remove the tumor completely and to preserve maximum function of the extremity.

Question 5

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. Osteochondroma is a benign tumor of bone. Simon MA, Springfield DS, et al: Osteogenic Sarcoma: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 266.

Question 6

A 44-year-old woman has bilateral knee pain, and history reveals bilateral hip replacements. Radiographs are seen in Figure 28a, and histopathologic specimens from the total hip replacement are shown in Figures 28b and 28c. Laboratory studies reveal anemia. What is the most likely diagnosis?

Explanation

28b 28c Rheumatoid arthritis is an inflammatory arthritis that usually involves multiple joints. Radiologic findings of periarticular erosion, osteopenia, and minimal osteophyte formation favor rheumatoid arthritis over osteoarthritis. Pigmented villonodular synovitis and Charcot arthropathy are more often considered monoarticular diseases. There are no radiographic findings of Paget's disease.

Question 7

In addition to pain, which of the following factors are considered most predictive of the risk of pathologic fracture?

Explanation

While guidelines for predicting fracture risk are at best imprecise, the scoring system by Mirels (pain, anatomic location, and pattern of bony destruction) has been shown to be most predictive of fracture risk. Functional pain, peritrochanteric location, and lytic bone destruction are the greatest risk factors for pathologic fracture. The factors of patient weight, age, soft-tissue mass, and location within bone are all of lesser importance. Frassica FJ, Frassica DA, McCarthy EF, Riley LH III: Metastatic bone disease: Evaluation, clinicopathologic features, biopsy, fracture risk, nonsurgical treatment, and supportive management. Instr Course Lect 2000;49:453-459.

Question 8

Patients with which of the following primary carcinomas have the shortest overall survival rate after a solitary metastasis to bone?

Explanation

The median survival of patients after discovery of bone metastasis from primary lung carcinoma is shorter compared with other primary sites.

Question 9

A 63-year-old woman has a femoral neck fracture. A biopsy specimen obtained from the fracture site at the time of her hemiarthroplasty reveals metastatic carcinoma. Seven days after surgery, she becomes confused and lethargic. Which of the following laboratory values is most likely implicated in the patient's symptoms at this time?

Explanation

Although many hematologic and electrolyte abnormalities may be present in a patient with advanced metastatic cancer, an elevated serum calcium level is most commonly associated with confusion. Treatment with hydration, diuretics, and bisphosphonates is recommended. Clohishy D: Management of skeletal metastasis in clinical orthopaedics, in Craig E (ed): Operative Orthopaedics. Philadelphia, PA, Lippincott Williams & Wilkins, 1999, pp 994-997.

Question 10

Figure 29a shows the clinical photograph of a 26-year-old woman who has had the leg deformity since birth. She reports difficulty with walking and weight bearing and notes increased discomfort and swelling when the leg is dependent. She denies any history of trauma or family history of a similar disorder. Examination reveals a fixed equinovarus deformity of the foot but no evidence of a limb-length discrepancy. No other cutaneous findings or soft-tissue masses are noted. Sagittal and axial T1- and T2-weighted MRI scans are shown in Figures 29b and 29c. What is the most likely diagnosis?

Explanation

29b 29c Because the MRI scans show marked dilation and proliferation of lymphatic channels that completely involve all the leg muscles and the clinical photograph shows the severe swelling associated with this disease, the most likely diagnosis is lymphangiomatosis. Poliomyelitis affects the anterior horn cells and manifests as muscle atrophy. Neurofibromatosis can have a similar clinical appearance but usually is associated with other systemic and cutaneous findings. Congenital band syndrome results in amputated or shortened extremities. Chronic venous stasis disease usually is not associated with joint contractures, and typically it affects older individuals. Surgical excision is the only known treatment; this patient underwent an above-knee amputation. Berquist TH (ed): MRI of the Musculoskeletal System, ed 3. Philadelphia, PA, Lippincott Raven, 1997, p 771.

Question 11

Figures 30a and 30b show the MRI scans of a 51-year-old woman who has had an enlarging soft-tissue mass in her right thigh for the past 18 months. Examination reveals no inguinal adenopathy. Results of a biopsy show a high-grade sarcoma. A bone scan is unremarkable, and a CT scan of the chest shows no evidence of pulmonary metastasis. According to the Musculoskeletal Tumor Society (MSTS) staging system, the tumor should be classified as what stage?

Explanation

30b According to the MSTS staging system, soft-tissue sarcomas are staged according to the following factors: grade, site, and metastasis. Roman numerals are used to designate malignant tumors, and Arabic numerals are used for benign tumors. Low-grade malignant tumors are staged as I while intermediate- and high-grade tumors are staged as II. Site is defined as intracompartmental (A) or extracompartmental (B). Any metastasis is staged as III. This patient has a high-grade tumor (II), and the MRI scans show that it is confined to a single compartment; therefore, it is staged as IIA. There is no evidence of metastatic disease. Enneking WF, Spanier SS, Goodman MA: A system for the surgical staging of musculoskeletal sarcoma. Clin Orthop 1980;153:106-120. Peabody TD, Monson D, Montag A, Schell MJ, Finn H, Simon MA: A comparison of the prognoses for deep and subcutaneous sarcomas of the extremities. J Bone Joint Surg Am 1994;76:1167-1173. Pisters PW, Leung DH, Woodruff J, Shi W, Brennan MF: Analysis of prognostic factors in 1,041 patients with localized soft tissue sarcomas of the extremities. J Clin Oncol 1996;14:1679-1689.

Question 12

An otherwise healthy 45-year-old man has a 3-cm subcutaneous mass on his anterior thigh that is presumed to be a lipoma. Removal of the mass is performed in the office of his primary care physician. Pathologic evaluation shows a high-grade malignant fibrous histiocytoma. Staging reveals no evidence of metastatic disease. Management at this time should consist of

Explanation

Resection of a previously excised soft-tissue sarcoma is recommended. Reoperation is recommended after removal of malignant tumors previously believed to be benign, as approximately one half of the patients will have residual tumor in the re-excised specimen. Observation is not indicated in most patients because local recurrence is likely. Radiation therapy alone may result in long-term local control but is felt to be less effective than reoperation. Chemotherapy and radiation therapy alone are not recommended. Wide excision is the most important factor for local disease control. Radiation therapy after surgical re-excision may also decrease the risk of local recurrence. Peabody TD, Monson D, Montag A, Schell MJ, Finn H, Simon MA: A comparison of the prognoses for deep and subcutaneous sarcomas of the extremities. J Bone Joint Surg Am 1994;76:1167-1173. Giuliano AE, Eilber FR: The rationale for planned reoperation after unplanned total excision of soft-tissue sarcomas. J Clin Oncol 1985;3:1344-1348. Lewis JJ, Leung D, Espat J, Woodruff JM, Brennan MF: Effect of resection in extremity soft tissue sarcoma. Ann Surg 2000;231:655-663.

Question 13

A 13-year-old girl has had a firm mass and pain in her right shoulder for the past several weeks. She denies any history of trauma. A radiograph and MRI scan are shown in Figures 31a and 31b. Biopsy specimens are shown in Figures 31c and 31d. What is the most likely diagnosis?

Explanation

31b 31c 31d The patient has osteosarcoma. The radiograph suggests an aggressive primary tumor of bone, and the histology shows malignant cells surrounded by osteoid, classic for osteosarcoma. Ewing's sarcoma histologically consists of small round blue cells. Osteochondroma and periosteal chondroma occur in the shoulder but have a different histologic pattern and a less aggressive radiographic appearance. Chondrosarcomas rarely occur in children. Simon MA, Springfield DS, et al: Osteogenic Sarcoma: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 266.

Question 14

The use of radiation therapy is most effective in metastatic bone disease from which of the following tumors?

Explanation

Both myeloma and lymphoma are more responsive to radiation therapy. The other types of tumors are relatively more resistant to radiation. Doses of 25 to 50 Gy (2,500 to 5,000 cGy) are usually sufficient for myeloma and lymphoma, while carcinomas frequently require a higher dosage. Large cell lymphoma of bone is usually of B-cell origin and is treated with chemotherapy and radiation therapy. Simon MA, Springfield DS, et al: Surgery for Bone and Soft Tissue Tumors. Philadelphia, PA, Lippincott Raven, 1998, p 683.

Question 15

Figures 32a and 32b show the radiographs of an active 13-year-old boy who has persistent left thigh pain and a limp despite a trial of protected weight bearing. Management should consist of

Explanation

32b The plain radiographs show an eccentric metaphyseal lesion involving a long bone in a skeletally immature patient. The lesion is longer than it is wide, with distinctly lobular outer edges that are sclerotic. These findings are characteristic of a nonossifying fibroma. Small asymptomatic lesions may be followed clinically. Larger lesions that occupy greater than two thirds of the width of the shaft and are located in areas of high mechanical stress such as the femur are more prone to fracture than smaller lesions. Pain is often a sign of impending fracture or the presence of a small fracture that may not be apparent on radiographs. The natural history of the lesion is to resolve over a period of years. The procedure that would allow the patient to return to contact sports is curettage and bone grafting. Intralesional steroid injection has been advocated in the treatment of unicameral bone cysts and eosinophilic granuloma but not nonossifying fibromas. En block resection is not indicated for a benign lesion. Low-dose radiation therapy has been used for eosinophilic granuloma but not for nonossifying fibromas. Walker RN, Green NE, Spindler KP: Stress fractures in skeletally immature patients. J Pediatr Orthop 1996;16:578-584.

Question 16

The spread of malignant cells to the vertebrae is often through

Explanation

In 1940, Batson described a valveless plexus of veins that extend from the dural venous sinuses of the skull to the sacrum. This system permits retrograde blood flow and enables tumor cells to enter vertebral bodies at multiple levels. Increased intra-abdominal pressure will enhance this retrograde blood flow.

Question 17

A 10-year-boy has had thigh pain for the past several months. He denies any history of trauma. Examination reveals no soft-tissue mass, and mild tenderness. Figures 33a and 33b show the plain radiograph and MRI scan, and the biopsy specimens are shown in Figures 33c and 33d. What is the most likely diagnosis?

Explanation

33b 33c 33d The diagnosis is eosinophilic granuloma. The plain radiograph and MRI scan show a lesion in the midshaft of the femur. There is no soft-tissue mass. There is reactive bone about the lesion that suggests a less aggressive tumor. The histology reveals eosinophils in an otherwise bland cellular background with no evidence of mitotic figures or malignant cells to suggest sarcoma. The diagnostic elements are the amphophilic (ie, pale purple) histiocytes with cigar-shaped nuclei, some of which have linear longitudinal grooves. There is no histologic evidence of infection. Lymphoma of bone would be an unusual occurrence in this age group, and the histology is not consistent with that diagnosis.

Question 18

A 35-year-old patient has right hip pain. Figures 34a and 34b show the coronal MRI scan and the biopsy specimen. What is the most likely diagnosis?

Explanation

34b Alcohol abuse, steroids, Caisson disease, sickle cell disease, and radiation therapy are the leading causes of nontraumatic osteonecrosis of the femoral head. Arlet J: Nontraumatic avascular necrosis of the femoral head: Past, present, and future. Clin Orthop 1992;277:12-21.

Question 19

A 16-year-old girl has a painful foot mass. A radiograph, MRI scan, and biopsy specimens are shown in Figures 35a through 35d. What is the most likely diagnosis?

Explanation

35b 35c 35d Synovial sarcoma should always be considered in the differential diagnosis of a foot mass; however, the histopathology shows a typical example of PVNS, with hemosiderin, giant cells, and synovium. Synovial chondromatosis would have metaplastic cartilage in the synovium. The radiograph shows subtle erosion of the lateral cortex of the cuboid, and the MRI scan shows a soft-tissue mass. Ghert MA, Scully SP, Harrelson JM: Pigmented villonodular synovitis of the foot and ankle: A review of six cases. Foot Ankle Int 1999;20:326-330. Jones BC, Sundaram M, Kransdorf MJ: Synovial sarcoma: MR imaging findings in 34 patients. Am J Roentgenol 1993;161:827-830. Sartoris DJ, Resnick D: Magnetic resonance imaging of pediatric foot and ankle disorders. J Foot Surg 1990;29:489-494.

Question 20

What is the most common diagnosis in a patient older than age 40 years with a destructive bony lesion?

Explanation

The most common destructive lesions in a patient older than age 40 years are, in decreasing incidence, metastatic carcinoma, multiple myeloma, lymphoma, and chondrosarcoma. Osteosarcoma is found primarily in younger patients.

Question 21

A 78-year-old man being seen for routine follow-up studies reports mild to moderate pain in his left arm. The patient has a history of lung cancer and severe heart disease. A radiograph and a bone scan are shown in Figures 36a and 36b. Treatment should consist of

Explanation

36b The patient has lung cancer metastatic to the left humerus that is mildly to moderately symptomatic. Radiographically, the lesion is small and is not associated with significant cortical destruction. Metastatic lesions in the upper extremity have a lower incidence of pathologic fracture than lesions in the lower extremity.

Question 22

Which of the following benign bone lesions can develop lung metastases?

Explanation

Although considered benign bone lesions, lung metastases can develop in giant cell tumors and chondroblastomas. These often can be treated with multiple thoracotomies, resulting in long-term survival. Roberts PF, Taylor JG: Multifocal benign chondroblastomas: Report of a case. Hum Pathol 1980;11:296-298.

Question 23

The radiographic feature seen in Figure 37 that best indicates a slow-growing process is the

Explanation

For the fibula (or any bone) to bow, a long-standing process needs to be present. Pressure from a rapid process would cause erosion, not allowing the bone to remodel. The other features are helpful confirmatory findings but also may be associated with aggressive processes. In this patient, the fibular deformity is caused by a sessile osteochondroma of the tibia. Lodwick GS, Wilson AJ, Farrell C, Virtama P, Dittrich F: Determining growth rates of focal lesions of bone from radiographs. Radiology 1980;134:577-583.

Question 24

A 52-year-old man has a stage IIB malignant fibrous histiocytoma of the distal femur. Local treatment should consist of

Explanation

Local control of malignant fibrous histiocytoma of bone typically consists of wide excision. Curettage and bone grafting is a procedure with an intralesional surgical margin, with an unacceptable rate of local recurrence. Prophylactic fixation is considered for patients with metastatic disease. Radiation therapy alone is not adequate for local control of this tumor. Neoadjuvant chemotherapy is often used primarily for systemic tumor control.

Question 25

A patient undergoes excision of a 3- x 3-cm soft-tissue sarcoma. The final histology results show tumor at the surgical margin. The surgical procedure performed is classified as

Explanation

Surgical margins are classified according to the plane of dissection in relation to the tumor. An intralesional procedure is where the plane of dissection passes through the tumor. A marginal resection involves a resection where the plane of dissection is through the reactive zone of the tumor. A wide resection is through completely normal tissue outside of the reactive zone. A radical resection removes all of the muscles and/or bone of the compartment that are involved by the tumor.

Question 26

A 14-year-old boy presents with an 8-week history of thigh pain and swelling. Radiographs reveal a permeative, diaphyseal lesion with a periosteal 'onion-skin' reaction in the femur. A core needle biopsy demonstrates sheets of small round blue cells. Which of the following genetic translocations is most commonly associated with this patient's diagnosis?

Explanation

The clinical and radiographic presentation is classic for Ewing sarcoma. The most common genetic translocation in Ewing sarcoma is t(11;22)(q24;q12), which results in the EWS-FLI1 fusion protein and is found in approximately 85-90% of cases. t(X;18) is associated with synovial sarcoma; t(2;13) is associated with alveolar rhabdomyosarcoma; t(12;16) is associated with myxoid liposarcoma; and t(9;22) is seen in chronic myelogenous leukemia (Philadelphia chromosome) as well as extraskeletal myxoid chondrosarcoma.

Question 27

A 32-year-old woman presents with worsening knee pain over several months. Radiographs demonstrate an eccentric, expansile, lytic epiphyseal lesion in the distal femur. A biopsy confirms a giant cell tumor of bone. She is treated with a targeted monoclonal antibody prior to surgical curettage to downstage the tumor. What is the precise mechanism of action of the medication most likely administered to this patient?

Explanation

The patient is being treated with denosumab, a human monoclonal antibody that specifically binds to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). In giant cell tumors of bone, the neoplastic mononuclear stromal cells express RANKL, which recruits and activates the reactive, multinucleated giant cells (osteoclast-like cells) expressing RANK. By binding RANKL, denosumab prevents RANK activation, thereby potently inhibiting bone resorption, facilitating tumor ossification, and decreasing both the size and vascularity of the tumor prior to surgery.

Question 28

A 55-year-old man undergoes wide excision of a deep soft-tissue mass in his anterior thigh. Histopathology reveals an undifferentiated pleomorphic sarcoma. When counseling the patient regarding his long-term prognosis, which of the following is considered the single most important prognostic factor for overall survival and the development of distant metastasis?

Explanation

In adult soft tissue sarcomas, the histologic grade (which incorporates differentiation, mitotic count, and percentage of necrosis) is the single most important prognostic factor for overall survival and the development of distant metastasis. While tumor size (> 5 cm) and tumor depth are important staging criteria that influence prognosis, grade remains the most significant independent predictor. Surgical margin status is the most important predictor of local recurrence but has less impact on overall survival than histologic grade.

Question 29

A 65-year-old man presents with progressive aching pain in his right shoulder. Radiographs show a large, ill-defined lytic lesion in the proximal humerus with intralesional 'popcorn' and 'ring-and-arc' calcifications. There is endosteal scalloping greater than two-thirds of the cortical thickness. Biopsy confirms an intermediate-grade conventional chondrosarcoma. What is the most appropriate primary treatment for this patient?

Explanation

The clinical and radiographic presentation indicates a primary conventional chondrosarcoma. Conventional chondrosarcomas are notoriously resistant to both chemotherapy and radiation therapy due to their poor vascularity, slow cellular division, and dense extracellular matrix. The absolute mainstay of treatment for intermediate- and high-grade conventional chondrosarcoma is wide surgical resection. Intralesional curettage may sometimes be utilized for low-grade (grade 1) chondrosarcomas in the long bones, but it is contraindicated for intermediate- or high-grade lesions.

Question 30

A 60-year-old woman with a history of lung cancer complains of moderate pain in her hip, particularly when weight-bearing. Radiographs reveal a purely lytic metastatic lesion in the peritrochanteric region of the proximal femur. The lesion occupies approximately 25% of the total cortical diameter. According to the Mirels scoring system, what is her calculated score, and what is the recommended management regarding surgical intervention?

Explanation

Mirels' criteria assess the risk of pathologic fracture using four categories, each scored from 1 to 3: Site (Upper extremity=1, Lower extremity=2, Peritrochanteric=3); Pain (Mild=1, Moderate=2, Severe/Functional=3); Lesion type (Blastic=1, Mixed=2, Lytic=3); and Size (<1/3=1, 1/3-2/3=2, >2/3=3). This patient scores: Site (peritrochanteric) = 3; Pain (moderate) = 2; Lesion (lytic) = 3; Size (<1/3) = 1. The total score is 3+2+3+1 = 9. A Mirels score of 9 or greater indicates a high risk for pathologic fracture (typically >15%), making prophylactic internal fixation highly recommended prior to beginning radiation therapy.

Question 31

A 16-year-old girl is diagnosed with a conventional high-grade osteosarcoma of the distal femur. She completes a 10-week course of neoadjuvant multidrug chemotherapy and subsequently undergoes limb-salvage surgery. Histopathologic analysis of the resected tumor is performed to assess tumor necrosis. Which of the following thresholds of tumor necrosis is the most widely accepted indicator of a 'good' histologic response to neoadjuvant chemotherapy?

Explanation

In the treatment of conventional high-grade osteosarcoma, the degree of tumor necrosis following neoadjuvant chemotherapy is one of the most important prognostic factors for long-term survival. The most widely accepted threshold for a 'good response' is > 90% tumor necrosis (classified as Grade III or IV in the Huvos grading system). Patients demonstrating > 90% necrosis have significantly higher 5-year survival rates compared to poor responders (< 90% necrosis).

Question 32

A 12-year-old boy presents with mild knee pain and swelling after a minor twist. Radiographs reveal an eccentric, expansile, lytic metaphyseal lesion in the proximal tibia. MRI demonstrates multiple fluid-fluid levels within the lesion. A biopsy confirms the diagnosis of a primary aneurysmal bone cyst (ABC). Which of the following genetic alterations is most specific to the pathogenesis of this lesion?

Explanation

Primary aneurysmal bone cysts (ABCs) are true neoplastic processes driven by specific genetic rearrangements, most commonly involving the USP6 gene on chromosome 17p13. This translocation leads to the upregulation of USP6, a ubiquitin-specific protease, driving the formation of the cystic lesion. GNAS mutations are associated with fibrous dysplasia. EXT1 and EXT2 mutations cause hereditary multiple exostoses. BRAF V600E mutations are commonly associated with Langerhans cell histiocytosis and occasionally non-ossifying fibromas.

Question 33

A 68-year-old man presents with generalized fatigue, severe lower back pain, and a recent laboratory finding of hypercalcemia. Radiographs show multiple punched-out lytic lesions in his skull and pelvis. Routine laboratory tests reveal a mild normocytic anemia and elevated serum creatinine. Which of the following tests is the most appropriate initial diagnostic step to confirm the suspected underlying malignancy?

Explanation

The patient's presentation of hypercalcemia, renal dysfunction (elevated creatinine), anemia, and lytic bone lesions comprises the classic 'CRAB' criteria for multiple myeloma. To confirm the diagnosis, identifying and quantifying the monoclonal protein (M-spike) is essential. Serum protein electrophoresis (SPEP) with immunofixation, along with urine protein electrophoresis (UPEP), is the most appropriate diagnostic step. Technetium-99m bone scans rely on osteoblastic activity and often yield false-negative results in multiple myeloma, which is characterized by purely osteoclastic bone destruction. Low-dose whole-body CT, MRI, or PET-CT are the preferred imaging modalities.

Question 34

A 19-year-old college student presents with right thigh pain that has been present for 4 months. The pain is described as a deep, dull ache that is significantly worse at night and is completely relieved within 30 minutes of taking ibuprofen. A CT scan shows a 7-mm radiolucent nidus surrounded by dense sclerotic bone in the femoral diaphysis. The dramatic relief of pain with nonsteroidal anti-inflammatory drugs (NSAIDs) is primarily due to the high concentration of which of the following substances within the tumor nidus?

Explanation

The clinical presentation is classic for an osteoid osteoma, a benign bone-forming tumor characterized by nocturnal pain relieved by NSAIDs. The tumor nidus produces extremely high levels of prostaglandins, particularly Prostaglandin E2 (PGE2), sometimes at concentrations 100 to 1000 times higher than in normal tissue. PGE2 causes intense local vasodilation and lowers the pain threshold of local sensory nerve fibers. NSAIDs inhibit cyclooxygenase (COX), effectively halting prostaglandin synthesis and providing rapid, profound pain relief.

Question 35

A 62-year-old man with a history of a radical nephrectomy 5 years ago presents with an impending pathologic fracture of his right humerus. Radiographs show a large, expansile, purely lytic lesion in the humeral diaphysis. Surgical stabilization with an intramedullary nail and lesional curettage is planned. Given the most likely primary malignancy, which of the following complications must the orthopedic surgeon be specifically prepared for during the surgical intervention?

Explanation

The patient has a metastatic lesion from renal cell carcinoma (RCC). Both RCC and thyroid carcinoma are known to produce highly vascular, osteolytic bone metastases. These metastases can lead to massive, potentially life-threatening intraoperative hemorrhage during surgical stabilization or resection. Due to this significant risk, preoperative selective arterial embolization of the tumor is strongly recommended 24 to 48 hours prior to orthopedic surgical intervention.

Question 36

A 45-year-old man presents with a painless, deep, 8-cm soft-tissue mass in his thigh. Core needle biopsy confirms a high-grade pleomorphic sarcoma. Which of the following is the most appropriate imaging study to evaluate for the most common site of metastasis in this patient?

Explanation

The most common site of distant metastasis for the majority of soft-tissue sarcomas of the extremities is the lungs. Therefore, a non-contrast CT of the chest is the most appropriate and sensitive imaging study for initial staging to rule out pulmonary metastases.

Question 37

A 32-year-old woman presents with worsening knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femur extending to the subchondral bone without a sclerotic margin. Biopsy confirms a giant cell tumor of bone. In cases where the tumor is deemed unresectable, which of the following targeted therapies acts by binding to RANKL to inhibit osteoclast-like giant cells?

Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL, preventing the activation of the RANK receptor on the surface of osteoclasts and the multinucleated giant cells characteristic of giant cell tumor (GCT) of bone. It is FDA-approved for adults and skeletally mature adolescents with GCT of bone that is unresectable or where surgical resection is likely to result in severe morbidity.

Question 38

Nitrogen-containing bisphosphonates (e.g., alendronate, zoledronic acid) are frequently used in the management of metastatic bone disease and osteoporosis. They primarily inhibit osteoclastic bone resorption through which of the following intracellular mechanisms?

Explanation

Nitrogen-containing bisphosphonates accumulate in osteoclasts and inhibit farnesyl pyrophosphate synthase, a key enzyme in the mevalonate pathway. This inhibition prevents the prenylation of small GTPase proteins (like Ras, Rho, and Rab) that are essential for osteoclast function, ruffled border formation, and cell survival, ultimately leading to osteoclast apoptosis.

Question 39

A 14-year-old boy presents with a painful, swollen thigh. A representative radiograph is shown below.

It demonstrates a permeative, diaphyseal lesion in the femur with an 'onion-skin' periosteal reaction. A core needle biopsy is performed. Which of the following chromosomal translocations is most characteristic of this patient's likely diagnosis?

Explanation

The clinical and radiographic findings describe Ewing sarcoma. Ewing sarcoma is classically associated with the t(11;22)(q24;q12) chromosomal translocation, which results in the EWS-FLI1 fusion protein. This occurs in approximately 85% to 90% of cases. t(X;18) is seen in synovial sarcoma, t(12;16) in myxoid liposarcoma, and t(2;13) in alveolar rhabdomyosarcoma.

Question 40

A 55-year-old man presents with progressive aching pain in his right shoulder. Radiographs show a large, ill-defined lytic lesion in the proximal humerus with intralesional 'popcorn' calcifications and endosteal scalloping greater than two-thirds of the cortical thickness. Biopsy confirms a grade 2 conventional chondrosarcoma. What is the most appropriate definitive management?

Explanation

Conventional chondrosarcomas (especially grades 2 and 3) are typically resistant to both chemotherapy and radiation therapy due to their poor vascularity and slow growth rate. Therefore, the mainstay of treatment for a grade 2 chondrosarcoma is wide surgical resection alone to achieve negative margins.

Question 41

A 19-year-old man presents with a 6-month history of right thigh pain that is worse at night and relieved significantly by ibuprofen. A CT scan of the femur is shown below.

It reveals a 7-mm radiolucent nidus surrounded by dense sclerotic bone in the femoral diaphysis. If a molecular analysis of the resected tissue were performed, the lesion would most likely demonstrate high levels of which of the following?

Explanation

The presentation is classic for an osteoid osteoma. Osteoid osteomas characteristically secrete high levels of prostaglandins, mediated by robust expression of cyclooxygenase-2 (COX-2) within the nidus. This accounts for the intense, night-predominant pain that typically responds dramatically to NSAIDs or salicylates.

Question 42

A 65-year-old man presents with generalized bone pain and a newly discovered lytic lesion in the proximal femur. Laboratory studies show anemia, hypercalcemia, and renal insufficiency. A diagnosis of multiple myeloma is suspected. Which of the following imaging modalities is the most sensitive and appropriate for assessing the full extent of skeletal tumor burden in this patient?

Explanation

The patient's presentation fits the CRAB criteria (hyperCalcemia, Renal failure, Anemia, Bone lesions) highly suggestive of multiple myeloma. Whole-body MRI or low-dose whole-body CT are currently the modalities of choice for assessing skeletal involvement, as they are highly sensitive for detecting marrow replacing lesions. Technetium-99m bone scans are notoriously insensitive for myeloma because the lesions are purely lytic and lack the osteoblastic response required for tracer uptake.

Question 43

During the process of secondary bone healing (enchondral ossification), which of the following local microenvironmental factors is primarily responsible for directing the differentiation of mesenchymal stem cells into chondrocytes during the formation of the soft callus?

Explanation

Secondary bone healing (healing with callus formation) requires relative stability (moderate mechanical strain) and occurs in an environment that initially has lower oxygen tension (hypoxia) due to the disruption of local blood supply. The combination of hypoxia and micro-motion promotes the differentiation of mesenchymal stem cells into chondrocytes, leading to the formation of a cartilaginous soft callus that is later calcified.

Question 44

A 68-year-old man with a long-standing history of Paget disease of bone presents with acutely worsening, severe pain in his left thigh and a newly palpable mass. Radiographs reveal a destructive lytic lesion with cortical breakthrough in the proximal femur, located within an area of thickened, sclerotic Pagetoid bone. What is the most likely diagnosis of this new lesion?

Explanation

In an older patient with known Paget disease of bone who presents with an acute onset of severe pain, a new mass, and aggressive bone destruction, secondary malignant transformation must be highly suspected. Osteosarcoma is the most common sarcoma to arise in Paget disease (Pagetoid osteosarcoma), occurring in roughly 1% of affected patients. It carries a notoriously poor prognosis.

Question 45

In the evaluation of a peripheral nerve injury, electromyography (EMG) is often utilized to assess the functional status of the motor unit. Fibrillation potentials and positive sharp waves observed on needle EMG of a resting muscle typically indicate which of the following?

Explanation

On electromyography (EMG), fibrillation potentials and positive sharp waves are classic signs of active muscle denervation. They arise from the spontaneous, unstable depolarizations of individual muscle fibers that have lost their nerve supply. These findings typically become evident 2 to 3 weeks following an acute peripheral nerve injury. In contrast, reinnervation is characterized by the appearance of polyphasic motor unit action potentials (MUAPs).

Question 46

A 15-year-old boy presents with a painful mass in his distal femur. Biopsy confirms high-grade osteosarcoma. His family history is significant for a sister who developed breast cancer at age 30 and a mother who died of a soft-tissue sarcoma. Which of the following gene mutations is most likely responsible for this underlying familial syndrome?

Explanation

This patient has a family history highly characteristic of Li-Fraumeni syndrome, which is caused by a germline mutation in the TP53 tumor suppressor gene. Patients with Li-Fraumeni syndrome have a significantly increased risk of developing osteosarcoma, soft-tissue sarcomas, breast cancer, leukemia, and brain tumors. APC is associated with familial adenomatous polyposis. EXT1 is associated with multiple hereditary exostoses. NF1 causes neurofibromatosis type 1. GNAS mutations are associated with McCune-Albright syndrome and fibrous dysplasia.

Question 47

A 32-year-old woman presents with worsening right knee pain. Radiographs show an eccentric, lytic epiphyseal lesion in the proximal tibia extending directly to the subchondral bone without a sclerotic rim. Biopsy reveals mononuclear cells interspersed with multinucleated giant cells. Because the lesion is large and close to the joint surface, she is prescribed a targeted biologic agent to downstage the tumor prior to surgical curettage. What is the primary mechanism of action of this medication?

Explanation

The clinical and radiographic presentation is classic for a giant cell tumor (GCT) of bone. Denosumab is a monoclonal antibody often used to treat large or unresectable GCTs. It works by binding to RANK-ligand (RANKL), which is secreted by the neoplastic mononuclear stromal cells. This prevents RANKL from binding to the RANK receptor on osteoclast precursors, thereby inhibiting their recruitment, differentiation, and bone-resorbing activity.

Question 48

A 12-year-old boy presents with a 2-month history of a painful diaphyseal mass in his right femur. Radiographs reveal a permeative destructive lesion with an aggressive 'onion-skin' periosteal reaction. Histologic examination demonstrates a uniform population of small round blue cells. Cytogenetic analysis of this tumor is most likely to reveal which of the following chromosomal translocations?

Explanation

The patient's presentation and histologic findings are diagnostic of Ewing sarcoma. Ewing sarcoma is classically associated with the t(11;22)(q24;q12) translocation, which fuses the EWS gene on chromosome 22 with the FLI1 gene on chromosome 11. t(X;18) is associated with synovial sarcoma. t(12;16) is found in myxoid liposarcoma. t(2;13) is characteristic of alveolar rhabdomyosarcoma. t(9;22) is the Philadelphia chromosome seen in chronic myelogenous leukemia (CML).

Question 49

A 65-year-old man presents with severe back pain and fatigue. Radiographs of the spine and skull demonstrate multiple 'punched-out' lytic lesions without reactive sclerosis. Laboratory studies show hypercalcemia, anemia, and an elevated serum creatinine level. Serum protein electrophoresis reveals an M-spike. Radiographs of the right femur reveal a large 4 cm lytic lesion in the subtrochanteric region with 60% cortical destruction. What is the most appropriate initial management for the femur?

Explanation

The patient's presentation meets the CRAB criteria (hyperCalcemia, Renal failure, Anemia, Bone lesions) for multiple myeloma. The subtrochanteric femur lesion has a high risk of pathologic fracture (Mirels' criteria). The standard of care for an impending or actual pathologic fracture from multiple myeloma or metastatic disease in the diaphyseal/subtrochanteric femur is surgical stabilization, typically with an intramedullary nail, followed by local radiation therapy and systemic treatment for the myeloma. Wide en bloc resection is generally reserved for isolated solitary plasmacytomas or primary bone sarcomas.

Question 50

A 28-year-old man presents with a slow-growing, painful mass on the plantar aspect of his foot. Magnetic resonance imaging demonstrates a soft-tissue mass adjacent to the plantar fascia. A biopsy reveals a biphasic pattern consisting of both spindle cells and epithelial cells. This specific tumor is most strongly associated with which of the following genetic abnormalities?

Explanation

The tumor described is a synovial sarcoma, which frequently presents in young adults as a deep-seated mass in the extremities, classically near joints or in the foot. Histologically, it can have a biphasic (spindle and epithelial components) or monophasic (spindle cells only) appearance. It is characterized by the t(X;18)(p11;q11) translocation, resulting in the fusion of the SYT gene on chromosome 18 with one of the SSX genes on the X chromosome.

Question 51

A 55-year-old man presents with a slowly expanding mass in his right pelvis. Radiographs show a lytic lesion with characteristic stippled and 'popcorn' calcifications in the ilium. Core needle biopsy confirms a grade II (intermediate-grade) conventional chondrosarcoma. Staging imaging shows no evidence of metastatic disease. Which of the following is the most appropriate definitive management?

Explanation

Conventional chondrosarcomas are notably resistant to both standard chemotherapy and radiation therapy due to their low mitotic fraction, poor vascularity, and abundant extracellular matrix. Therefore, the mainstay of treatment for intermediate (grade II) and high-grade (grade III) chondrosarcomas is wide surgical resection with negative margins. Intralesional curettage may be considered only for select benign or very low-grade (grade I/atypical cartilaginous tumor) lesions in the appendicular skeleton.

Question 52

A 16-year-old boy complains of nagging right thigh pain that is distinctly worse at night and dramatically relieved within 30 minutes of taking ibuprofen. Plain radiographs reveal a thickened diaphyseal cortex with a central 8-mm radiolucent nidus. The intense localized pain experienced by the patient is primarily mediated by local tumor production of which of the following substances?

Explanation

The clinical picture is pathognomonic for an osteoid osteoma. These benign bone-forming tumors secrete high levels of prostaglandins, specifically prostaglandin E2 (PGE2), which mediate the characteristic severe, nocturnal pain. The prompt relief of symptoms following the administration of nonsteroidal anti-inflammatory drugs (NSAIDs) or aspirin is due to the inhibition of cyclooxygenase, which decreases PGE2 production.

Question 53

A 45-year-old woman is evaluated for a deep, painless mass in her posterior thigh. MRI shows a large, well-circumscribed, multilobulated mass that is markedly hyperintense on T2-weighted images, resembling a cyst but with solid enhancement. Biopsy demonstrates a myxoid stroma with a rich, delicate, 'chicken-wire' capillary network and scattered lipoblasts. This tumor's pathogenesis is driven by a chromosomal translocation that leads to which of the following fusion genes?

Explanation

The clinical presentation, MRI findings (pseudocystic appearance on T2), and histology (myxoid stroma, chicken-wire vascularity, lipoblasts) are classic for myxoid liposarcoma. This soft-tissue sarcoma is characterized by the t(12;16)(q13;p11) translocation, which produces the FUS-DDIT3 (also known as TLS-CHOP) fusion transcript. EWS-FLI1 is seen in Ewing sarcoma; SYT-SSX in synovial sarcoma; PAX3-FOXO1 in alveolar rhabdomyosarcoma; and BCR-ABL in CML.

Question 54

A 9-year-old girl is brought to the clinic due to a limb-length discrepancy and a noticeable limp. Radiographs demonstrate a 'shepherd's crook' deformity of her proximal femur with a well-defined medullary lesion showing a 'ground-glass' appearance. On examination, she has large café-au-lait macules with irregular, 'coast of Maine' borders. She also has a history of early onset menarche at age 7. The underlying cellular pathophysiology of her musculoskeletal condition involves a mutation resulting directly in:

Explanation

This patient has McCune-Albright syndrome, a triad of polyostotic fibrous dysplasia, café-au-lait spots (with irregular borders), and precocious puberty (or other endocrinopathies). It is caused by an activating post-zygotic somatic mutation in the GNAS gene, which encodes the alpha subunit of the stimulatory G protein (Gs-alpha). This mutation leads to constitutive activation of adenylate cyclase, resulting in high intracellular levels of cyclic AMP (cAMP). This aberrant signaling impairs the differentiation of bone-marrow stromal cells into mature osteoblasts, leading to the formation of structurally weak fibrous tissue and woven bone.

Question 55

According to the Musculoskeletal Tumor Society (MSTS) / Enneking staging system for malignant bone tumors, which of the following characteristics determines the difference between a Stage IIA and a Stage IIB lesion?

Explanation

The MSTS/Enneking staging system for musculoskeletal sarcomas classifies tumors based on histologic grade (I = low grade, II = high grade), compartmental containment (A = intracompartmental, B = extracompartmental), and metastasis (III = any regional or distant metastasis). Therefore, a Stage II lesion is high grade. The distinction between Stage IIA and Stage IIB is strictly based on whether the tumor is confined within its anatomic compartment (IIA) or extends beyond the compartment boundaries (IIB).

Question 56

A 45-year-old male undergoes a posterolateral lumbar fusion using recombinant human bone morphogenetic protein-2 (rhBMP-2). Once rhBMP-2 binds to its target cell surface receptor, which of the following intracellular signaling molecules is primarily responsible for translocating to the nucleus to upregulate osteogenic gene expression?

Explanation

BMPs act via serine/threonine kinase receptors that, upon activation, phosphorylate intracellular SMAD proteins (specifically SMAD 1, 5, and 8). These receptor-regulated SMADs form a complex with the co-SMAD (SMAD 4), translocate to the nucleus, and regulate the transcription of osteogenic genes such as Runx2. Beta-catenin is the primary intracellular mediator for the Wnt signaling pathway, while NF-kappa B is involved in RANK/RANKL signaling in osteoclasts.

Question 57

A 65-year-old woman presents with groin pain 5 years after a total hip arthroplasty utilizing a cobalt-chromium modular head on a titanium alloy stem. Aspiration yields fluid with elevated cobalt levels but no bacterial growth. What is the predominant mechanism of corrosion at the modular head-neck junction?

Explanation

Trunnionosis at the modular head-neck junction is primarily driven by mechanically assisted crevice corrosion (MACC), also known as fretting corrosion. Micro-motion at the interface disrupts the passive oxide layer (fretting), and the confined space of the modular junction leads to oxygen depletion and a drop in pH (crevice corrosion), accelerating material loss. While mixing dissimilar metals theoretically risks pure galvanic corrosion, the mechanical disruption of the passivation layer in a confined crevice is the primary driver in modern modular orthopedic implants.

Question 58

A 12-year-old boy presents with a painful, swollen thigh. Radiographs show a permeative diaphyseal lesion with an 'onion-skin' periosteal reaction. Biopsy reveals sheets of uniform small round blue cells. Which of the following chromosomal translocations is most characteristic of this diagnosis?

Explanation

The clinical, radiographic, and histologic presentation is classic for Ewing sarcoma. The hallmark genetic abnormality is a balanced translocation between chromosomes 11 and 22, t(11;22)(q24;q12), which fuses the EWSR1 gene with the FLI1 gene. t(9;22) is seen in extraskeletal myxoid chondrosarcoma (and CML). t(X;18) is associated with synovial sarcoma. t(12;16) is seen in myxoid liposarcoma. t(2;13) is characteristic of alveolar rhabdomyosarcoma.

Question 59

An infant presents with multiple fractures and is diagnosed with malignant infantile osteopetrosis. Radiographs reveal diffuse osteosclerosis and an 'erlenmeyer flask' deformity of the distal femurs. The pathogenesis of this specific severe autosomal recessive disorder is most commonly related to a mutation affecting which of the following?

Explanation

Osteopetrosis is characterized by defective osteoclast function leading to impaired bone resorption and overly dense but brittle bone. The most common mutation in malignant infantile (autosomal recessive) osteopetrosis affects the TCIRG1 gene, which encodes the a3 subunit of the vacuolar H+-ATPase pump essential for creating the acidic environment at the osteoclast ruffled border required for bone resorption. Type I collagen defects cause osteogenesis imperfecta. FGFR3 mutations cause achondroplasia. Runx2 mutations cause cleidocranial dysplasia.

Question 60

Ligaments and tendons exhibit viscoelastic properties. During a continuous traction experiment on a cadaveric ACL, a constant load is applied over a prolonged period. The observation that the ligament continues to slowly elongate over time under this steady load is termed:

Explanation

Creep is the progressive deformation (elongation) of a viscoelastic material when subjected to a constant load over time. Stress relaxation, conversely, is the decrease in internal stress over time when a material is held at a constant strain (constant length). Hysteresis represents the energy lost (as heat) during the loading and unloading cycles of a viscoelastic material.

Question 61

A 4-year-old boy is evaluated for severe bowing of the lower extremities. Laboratory results demonstrate low serum phosphorus, normal serum calcium, normal parathyroid hormone (PTH), and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. Which of the following best describes the underlying pathophysiology?

Explanation

The patient has X-linked hypophosphatemic (XLH) rickets, the most common heritable form of rickets. It is caused by a mutation in the PHEX gene. This leads to an overproduction or reduced degradation of Fibroblast Growth Factor 23 (FGF-23). High levels of circulating FGF-23 inhibit sodium-phosphate cotransporters (NaPi-IIa and NaPi-IIc) in the proximal renal tubule, causing severe renal phosphate wasting. FGF-23 also inhibits 1-alpha-hydroxylase, preventing the compensatory rise in 1,25-dihydroxyvitamin D.

Question 62

Articular cartilage relies on its unique extracellular matrix to provide a nearly frictionless surface and distribute joint loads. Which of the following combinations best describes the composition and structural arrangement of articular cartilage in the superficial zone compared to the deep zone?

Explanation

Articular cartilage is divided into distinct zones. The superficial (tangential) zone has the highest water content (approx 80%) and lowest proteoglycan concentration. Its Type II collagen fibers are oriented parallel to the joint surface to resist shear forces. In contrast, the deep zone has the lowest water content (approx 65%), the highest proteoglycan concentration, and collagen fibers oriented perpendicular to the joint surface to resist compressive loads.

Question 63

A 70-year-old patient is scheduled for a total knee arthroplasty. The patient has a history of atrial fibrillation and takes rivaroxaban daily. What is the precise mechanism of action of this anticoagulant?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that exert their effect by directly binding to and inhibiting Factor Xa, blocking the conversion of prothrombin to thrombin. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin is a vitamin K antagonist. Heparin and low molecular weight heparins (like enoxaparin) act by enhancing the activity of antithrombin III.

Question 64

Following total joint arthroplasty, periprosthetic joint infections are notoriously difficult to eradicate due to bacterial biofilm formation. Which of the following represents the critical first step in the formation of a biofilm on an orthopedic implant?

Explanation

The formation of a biofilm on an implant occurs in a stepwise fashion: 1) Within minutes of implantation, a 'conditioning film' of host proteins (fibronectin, fibrinogen) coats the implant; 2) Planktonic bacteria undergo reversible adherence to this film via non-specific physical forces; 3) Adherence becomes irreversible via specific bacterial adhesins (e.g., MSCRAMMs); 4) Bacteria proliferate and secrete an extracellular polymeric substance (EPS) to form the mature biofilm matrix; 5) The biofilm matures and later uses quorum sensing to coordinate dispersion.

Question 65

During the repair phase following an acute skeletal muscle laceration, which of the following cell types is primarily responsible for regenerating true muscle fibers by proliferating and fusing to existing myofibers?

Explanation

Satellite cells are the resident stem cells of skeletal muscle, located between the basal lamina and the sarcolemma of mature muscle fibers. Following acute muscle injury, these cells become activated, proliferate as myoblasts, and subsequently differentiate and fuse to damaged myofibers or to each other to mediate structural repair and regeneration. Fibroblasts and myofibroblasts deposit collagen and contribute to scar tissue formation, but do not regenerate functional contractile muscle tissue.

Question 66

A 45-year-old man with a history of multiple enchondromas (Ollier disease) presents with a rapidly enlarging, painful mass in his proximal humerus. Radiographs demonstrate a destructive bone lesion with intralesional 'ring and arc' calcifications and cortical breakthrough. A core needle biopsy confirms a high-grade conventional chondrosarcoma. Which of the following genetic mutations is most characteristic of the pathogenesis of this patient's underlying condition and subsequent malignant transformation?

Explanation

Mutations in the isocitrate dehydrogenase genes (IDH1 and IDH2) are highly characteristic of solitary enchondromas, Ollier disease, Maffucci syndrome, and conventional chondrosarcomas. EXT1 and EXT2 mutations are associated with multiple hereditary exostoses (osteochondromas). GNAS1 mutations are seen in fibrous dysplasia (McCune-Albright syndrome). The t(11;22) translocation is characteristic of Ewing sarcoma.

Question 67

Bone morphogenetic proteins (BMPs) are potent osteoinductive growth factors critical for normal fracture healing and spinal fusion. Following the binding of BMP-2 to its transmembrane serine/threonine kinase receptor on a mesenchymal stem cell, which of the following intracellular signaling molecules is directly phosphorylated to translocate into the nucleus and initiate osteogenic gene transcription?

Explanation

BMPs belong to the transforming growth factor-beta (TGF-beta) superfamily. They bind to heterodimeric serine/threonine kinase receptors, which then phosphorylate receptor-regulated Smads, specifically Smad 1, 5, and 8. These phosphorylated Smads form a complex with the common-mediator Smad 4 and translocate to the nucleus to regulate transcription of osteogenic genes like Runx2. Beta-catenin is involved in the Wnt pathway. NF-kB is a key effector in the RANKL/osteoclastogenesis pathway.

Question 68

A 32-year-old woman is diagnosed with a massive, unresectable giant cell tumor (GCT) of the sacrum causing severe pain and radiculopathy. The multidisciplinary tumor board recommends initiation of denosumab therapy. Which of the following best describes the precise mechanism of action of this pharmacological agent?

Explanation

Denosumab is a fully human monoclonal antibody that specifically targets and binds to RANK ligand (RANKL). In Giant Cell Tumor of bone, the neoplastic mononuclear stromal cells secrete excessive amounts of RANKL, which recruits and activates normal osteoclast precursors via the RANK receptor, leading to the formation of the destructive multinucleated giant cells. By neutralizing RANKL, denosumab halts the osteoclast-mediated bone destruction.

Question 69

Orthopedic biomechanics heavily relies on understanding material properties. When evaluating a new titanium alloy for femoral stems, researchers plot a standardized stress-strain curve under tensile loading. Which of the following material properties is derived by calculating the slope of the linear (elastic) portion of this curve?

Explanation

The slope of the linear, elastic portion of the stress-strain curve represents the Modulus of Elasticity (Young's modulus), which defines the material's stiffness. Toughness is represented by the total area under the entire stress-strain curve. Yield strength is the stress level at which the material ceases to behave entirely elastically and begins plastic (permanent) deformation. Ultimate tensile strength is the maximum stress the material can withstand before failure.

Question 70

A 65-year-old man presents with intractable lower back pain, anemia, and hypercalcemia. Radiographs demonstrate multiple 'punched-out' lytic lesions in the skull and a compression fracture of L3 without a sclerotic rim. Serum protein electrophoresis reveals an elevated monoclonal immunoglobulin spike. The profound osteoclast activation and resultant bone destruction seen in this patient's disease are predominantly driven by tumor cell secretion of which of the following cytokines?

Explanation

The patient has Multiple Myeloma. Myeloma cells interact with bone marrow stromal cells to secrete potent osteoclast activating factors, most notably Interleukin-6 (IL-6), macrophage inflammatory protein-1 alpha (MIP-1a), and RANKL. IL-6 is a critical cytokine for both the survival/proliferation of myeloma cells and the profound stimulation of osteoclastogenesis that causes the characteristic 'punched-out' lytic lesions lacking a reactive sclerotic rim.

Question 71

Normal adult articular cartilage exhibits an anisotropic and depth-dependent structural organization essential for its biomechanical function. Compared to the deep zone, the superficial (tangential) zone of articular cartilage is uniquely characterized by which of the following structural properties?

Explanation

The superficial zone of articular cartilage has the highest water content (approx. 80%), the highest concentration of collagen, and the lowest concentration of proteoglycans. The collagen fibers (mostly Type II) are oriented parallel to the joint surface to resist sheer stress. In contrast, the deep zone has the lowest water content (approx. 65%), the highest proteoglycan content, and collagen fibers oriented perpendicular to the joint surface to resist compressive loads.

Question 72

A 28-year-old male presents with a slowly enlarging, painless mass about the knee. Radiographs reveal a soft-tissue mass with stippled calcifications. MRI confirms a well-circumscribed soft-tissue lesion adjacent to the joint capsule but without intra-articular extension. A biopsy demonstrates a biphasic tumor containing both epithelioid and spindle cells. What is the characteristic chromosomal translocation diagnostic for this sarcoma?

Explanation

The patient's clinical presentation, biphasic histology, and calcifications on radiograph (present in 30% of cases) are characteristic of Synovial Sarcoma. Synovial sarcoma is defined by the t(X;18) translocation, which creates the SYT-SSX fusion gene. t(11;22) is Ewing sarcoma; t(12;16) is myxoid liposarcoma; t(9;22) is extraskeletal myxoid chondrosarcoma; t(2;13) is alveolar rhabdomyosarcoma.

Question 73

An orthopedic research team conducts a randomized controlled trial comparing a novel DVT prophylaxis regimen to aspirin after total hip arthroplasty. The study concludes there is no statistically significant difference in VTE events between the two groups (p = 0.15). However, post-market surveillance later proves the novel regimen is genuinely superior. The initial trial's failure to detect this true difference is an example of a Type II error. Which of the following adjustments during the trial's design phase would have most effectively reduced the probability of this specific error?

Explanation

A Type II error (beta) occurs when a study fails to reject a false null hypothesis (i.e., missing a true difference). The probability of avoiding a Type II error is the statistical 'power' of the study, calculated as 1 - beta. The most direct and critical method to increase the power of a study and minimize the risk of a Type II error is to increase the sample size. Decreasing the alpha level would actually increase the chance of a Type II error while protecting against a Type I error.

Question 74

A 55-year-old woman is scheduled for limb-salvage surgery for a 10 cm high-grade pleomorphic sarcoma of the anterior thigh. The multidisciplinary tumor board discusses the timing of radiation therapy (RT). If the patient receives preoperative (neoadjuvant) RT instead of postoperative RT, she is at highest risk for which of the following?

Explanation

The seminal randomized trial by O'Sullivan et al. comparing preoperative versus postoperative radiation for extremity soft tissue sarcomas demonstrated that preoperative RT has a significantly higher risk of major acute wound complications (approx. 35% vs 17%). However, preoperative RT utilizes a lower total dose and a smaller radiation field, leading to significantly less long-term morbidity such as limb fibrosis, severe edema, and joint stiffness compared to postoperative RT. Overall survival rates are equivalent.

Question 75

A 48-year-old man with end-stage renal disease presents with bilateral hip pain and proximal muscle weakness. Laboratory testing reveals hypocalcemia, hyperphosphatemia, and markedly elevated parathyroid hormone (PTH) levels. Radiographs show subperiosteal resorption in the phalanges. The underlying pathophysiology of this patient's metabolic bone disease is most directly linked to the failing kidneys' inability to produce adequate amounts of which of the following?

Explanation

This patient is suffering from renal osteodystrophy (secondary hyperparathyroidism). The kidney is the primary site of 1-alpha-hydroxylase, the critical enzyme that converts inactive 25-hydroxyvitamin D into the active form, 1,25-dihydroxyvitamin D (calcitriol). In end-stage renal disease, 1-alpha-hydroxylase deficiency leads to decreased gut absorption of calcium, causing hypocalcemia which subsequently triggers severe secondary hyperparathyroidism. 25-hydroxylase is located in the liver.

Question 76

A 70-year-old woman is prescribed a nitrogen-containing bisphosphonate for the treatment of severe osteoporosis. At the cellular level, what is the primary mechanism of action of this medication?

Explanation

Nitrogen-containing bisphosphonates (e.g., alendronate, zoledronic acid) bind to hydroxyapatite crystals in bone. They are internalized by osteoclasts during bone resorption and inhibit the enzyme farnesyl pyrophosphate synthase within the mevalonate pathway. This disruption prevents the prenylation of small GTP-binding proteins essential for osteoclast function and survival, ultimately leading to osteoclast apoptosis.

Question 77

When testing the viscoelastic properties of an anterior cruciate ligament graft, the structural testing apparatus holds the graft at a constant deformation (strain) and measures the load over time. Which of the following viscoelastic phenomena is best described by the observed decrease in peak load over time?

Explanation

Stress relaxation is the hallmark viscoelastic property where the stress (or load) decreases over time when the material is held at a constant deformation (strain). Creep, conversely, is the progressive deformation (strain) over time when a material is subjected to a constant load. Hysteresis represents the energy lost (usually as heat) during the loading and unloading cycle of a viscoelastic material.

Question 78

Bone morphogenetic proteins (BMPs) are often utilized to promote spinal fusion and fracture healing. Following the binding of a BMP to its corresponding serine/threonine kinase cell-surface receptor, which intracellular signaling pathway is primarily activated to regulate the transcription of osteogenic genes like Runx2?

Explanation

BMPs signal primarily through the canonical Smad pathway. They bind to serine/threonine kinase receptors on the cell surface, leading to the phosphorylation of intracellular receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These phosphorylated R-Smads form a complex with the common-partner Smad (Co-Smad), Smad 4, which then translocates to the nucleus to regulate the transcription of target osteogenic genes.

Question 79

A 65-year-old man presents with an acutely swollen, erythematous, and exquisitely painful right knee. Aspiration of the knee yields cloudy synovial fluid with 45,000 WBCs/mm^3 (85% polymorphonuclear leukocytes). Polarized light microscopy reveals negatively birefringent, needle-shaped crystals. What is the composition of the crystals?

Explanation

The clinical presentation and synovial fluid analysis are diagnostic for an acute gout attack, which is characterized by the deposition of monosodium urate crystals. These crystals appear needle-shaped and exhibit strong negative birefringence under polarized light microscopy. In contrast, calcium pyrophosphate dihydrate (CPPD) crystals, seen in pseudogout, are typically rhomboid-shaped and weakly positively birefringent.

Question 80

Articular cartilage is structurally organized into distinct zones. Which zone is characterized by the highest concentration of proteoglycans, the lowest concentration of water, and collagen fibers oriented perpendicularly to the articular surface?

Explanation

The deep (radial) zone of articular cartilage contains the highest concentration of proteoglycans and the lowest water content. Its collagen fibers are arranged perpendicularly to the joint surface to resist compressive forces. The superficial zone has the highest water content and collagen fibers oriented parallel to the joint surface to resist shear forces.

Question 81

During the initiation of a skeletal muscle contraction, calcium ions are released from the sarcoplasmic reticulum. To which of the following proteins does calcium directly bind to expose the myosin-binding sites on the actin filament?

Explanation

Skeletal muscle contraction is regulated by the troponin-tropomyosin complex. Calcium ions bind to Troponin C, causing a conformational change in the troponin complex. This change pulls tropomyosin away from the myosin-binding sites on the actin filament, allowing the myosin heads to attach and initiate the cross-bridge cycle. Troponin I is the inhibitory subunit, and Troponin T binds the complex to tropomyosin.

Question 82

During revision surgery for a failed total hip arthroplasty, evidence of extensive third-body wear is noted on the polyethylene liner, caused by retained bone cement fragments. This type of wear is characterized by microscopic gouges and scratches. What is the specific term for this wear mechanism?

Explanation

Abrasive wear occurs when a harder surface, or hard third-body particles (such as bone fragments, polymethylmethacrylate cement, or metal debris), plows into and scratches a softer bearing surface (like polyethylene), removing material in the process. Adhesive wear involves two surfaces bonding and then tearing apart. Fatigue wear is caused by cyclic subsurface stresses leading to microcracks and delamination.

Question 83

A 35-year-old male sustains a closed midshaft femur fracture that is treated with a statically locked intramedullary nail. Which biologic process predominantly bridges the fracture gap during the formation of the soft callus in this scenario?

Explanation

Intramedullary nailing provides relative stability to a fracture, which promotes secondary bone healing through callus formation. The predominant biologic process that bridges the fracture gap in the central, relatively hypoxic environment of the soft callus is endochondral ossification, where a cartilage template is formed and subsequently replaced by bone. Primary bone healing (cutting cones) requires absolute stability (e.g., compression plating).

Question 84

A detailed biomechanical analysis of a normal gait cycle is being performed on a healthy adult. At which specific phase of the gait cycle does the maximum compressive joint reaction force across the hip occur?

Explanation

The maximum compressive joint reaction force across the hip joint occurs during the mid-stance phase of the gait cycle. During mid-stance, the entire body weight is supported on a single limb. To keep the pelvis level and prevent a Trendelenburg drop, the hip abductor muscles must contract powerfully. The combined force of the body weight and the strong abductor muscle contraction results in a joint reaction force that can reach 2.5 to 3 times body weight.

Question 85

Denosumab is a potent medication utilized in the management of osteoporosis, giant cell tumor of bone, and skeletal metastases. What is the exact molecular target of this fully human monoclonal antibody?

Explanation

Denosumab specifically targets and binds to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL), a protein predominantly produced by osteoblasts and stromal cells. By neutralizing RANKL, denosumab prevents it from interacting with the RANK receptor on osteoclasts and their precursors, thereby profoundly inhibiting osteoclastogenesis, activation, and survival. It mimics the natural function of osteoprotegerin (OPG).

Question 86

A 35-year-old male presents with a slow-growing mass in the plantar aspect of his foot. Biopsy reveals a biphasic tumor with both epithelial and spindle cell components, establishing a diagnosis of synovial sarcoma. Which of the following chromosomal translocations is most characteristic of this diagnosis?

Explanation

Synovial sarcoma is classically characterized by the t(X;18)(p11;q11) translocation, which results in the SYT-SSX fusion gene. This translocation is found in over 90% of synovial sarcomas. t(11;22) is characteristic of Ewing sarcoma. t(12;16) is seen in myxoid liposarcoma. t(9;22) is associated with extraskeletal myxoid chondrosarcoma. t(2;13) is characteristic of alveolar rhabdomyosarcoma.

Question 87

A 15-year-old boy presents with knee pain that is worse at night. Radiographs reveal a mixed lytic and sclerotic lesion with a sunburst periosteal reaction in the distal femur. A biopsy confirms osteosarcoma. Mutations in which of the following tumor suppressor genes are most frequently associated with the development of this malignancy?

Explanation

Osteosarcoma is highly associated with mutations in the RB1 (retinoblastoma) and TP53 (Li-Fraumeni syndrome) tumor suppressor genes. Patients with hereditary retinoblastoma have a significantly increased risk of developing osteosarcoma. APC mutations are linked to familial adenomatous polyposis, NF1/NF2 to neurofibromatosis, and EXT1/EXT2 are associated with hereditary multiple exostoses (osteochondromas).

Question 88

A 45-year-old female with advanced chronic kidney disease presents with diffuse bone pain and muscle weakness. Laboratory studies show hypocalcemia, hyperphosphatemia, and significantly elevated parathyroid hormone (PTH) levels. Which of the following is the primary pathophysiologic mechanism leading to her elevated PTH?

Explanation

This patient has secondary hyperparathyroidism due to chronic kidney disease (CKD), a condition referred to as renal osteodystrophy. In CKD, the kidneys fail to adequately express 1-alpha-hydroxylase, leading to decreased conversion of 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol). This deficiency reduces gastrointestinal calcium absorption, resulting in hypocalcemia. The hypocalcemia, along with hyperphosphatemia, directly stimulates the parathyroid glands to increase PTH secretion.

Question 89

A biomechanics researcher is testing the viscoelastic properties of a human Achilles tendon. When a constant tensile load is applied to the tendon over an extended period, the tendon continues to deform slowly over time. This viscoelastic phenomenon is best described as:

Explanation

Creep is defined as the progressive, time-dependent deformation of a viscoelastic material when it is subjected to a constant load (or stress). Stress relaxation, by contrast, is the decrease in internal stress over time when a material is held at a constant deformation (or strain). Hysteresis represents the energy lost (usually dissipated as heat) during the loading and unloading cycle of a viscoelastic material.

Question 90

A 32-year-old woman presents with knee pain. Radiographs show an eccentric, lytic epiphyseal lesion in the proximal tibia extending right to the subchondral bone. Biopsy confirms a giant cell tumor of bone. She is treated with denosumab prior to surgical curettage. What is the mechanism of action of this medication?

Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand). By binding to RANKL, denosumab prevents it from activating the RANK receptor on the surface of osteoclasts and their precursors. Giant cell tumor (GCT) of bone consists of neoplastic stromal cells that express abnormally high levels of RANKL, which recruit numerous reactive osteoclast-like giant cells responsible for the aggressive bone resorption. Denosumab effectively breaks this cycle.

Question 91

Fracture healing can occur via different biologic pathways depending on the mechanical environment. Which of the following clinical scenarios primarily utilizes intramembranous ossification rather than endochondral ossification?

Explanation

Distraction osteogenesis (e.g., using an Ilizarov frame) primarily occurs via intramembranous ossification, a process where bone forms directly from mesenchymal stem cells without a cartilaginous intermediate. Endochondral ossification, which involves a cartilage model that is subsequently replaced by bone, is the primary mechanism of secondary fracture healing (callus formation) seen in nonoperative management or relative stability (intramedullary nailing). Longitudinal physeal growth also occurs via endochondral ossification.

Question 92

Regarding the structural zonal anatomy of normal articular cartilage, which of the following statements most accurately describes the superficial (tangential) zone?

Explanation

The superficial (tangential) zone makes up the top 10-20% of articular cartilage. It is characterized by having the highest concentration of water and collagen, and the lowest concentration of proteoglycans. The collagen fibers are oriented parallel to the joint surface to resist shear stress, and the chondrocytes are flattened. The deep zone, in contrast, contains the highest proteoglycan content, spherical chondrocytes arranged in columns, and collagen fibers oriented perpendicular to the joint to resist compressive forces.

Question 93

A 65-year-old male presents with generalized bone pain, fatigue, and a recent pathologic fracture of the proximal humerus. Laboratory testing reveals hypercalcemia, anemia, and renal insufficiency. A skeletal survey demonstrates multiple punched-out lytic lesions in the skull and pelvis. The neoplastic cells primarily responsible for this condition are derived from which of the following cell lineages?

Explanation

The patient's clinical presentation is classic for multiple myeloma, fulfilling the CRAB criteria (hyperCalcemia, Renal insufficiency, Anemia, Bone lesions). Multiple myeloma is a plasma cell dyscrasia characterized by the neoplastic proliferation of monoclonal plasma cells, which are terminally differentiated B-lymphocytes. These malignant cells secrete monoclonal immunoglobulins and stimulate aggressive osteoclastic bone resorption via upregulation of RANKL and downregulation of osteoprotegerin (OPG).

Question 94

A 19-year-old male collegiate basketball player presents for evaluation of a knee injury. On physical examination, he is noted to have a tall stature, arachnodactyly, pectus excavatum, and generalized joint hypermobility. He also reports a history of lens dislocation. The defective protein in this underlying genetic condition is most critical for the structural integrity of which of the following?

Explanation

The clinical features (tall stature, arachnodactyly, pectus excavatum, joint hypermobility, ectopia lentis) are hallmark signs of Marfan syndrome. Marfan syndrome is an autosomal dominant connective tissue disorder caused by mutations in the FBN1 gene, which encodes fibrillin-1. Fibrillin-1 is a glycoprotein that serves as the essential structural scaffold for elastin microfibrils in the extracellular matrix, most notably affecting the aorta, ligaments, and ciliary zonules. Defective Type I collagen causes Osteogenesis Imperfecta.

Question 95

Tranexamic acid (TXA) is routinely administered to reduce perioperative blood loss and transfusion requirements in total joint arthroplasty. The specific pharmacological mechanism of action of TXA is best described by which of the following?

Explanation

Tranexamic acid (TXA) is a synthetic derivative of the amino acid lysine and functions as an antifibrinolytic agent. It works by competitively binding to the lysine-binding sites on plasminogen, which prevents its activation into plasmin. By inhibiting the formation of plasmin, TXA blocks the degradation of fibrin clots (fibrinolysis), thereby stabilizing clots and reducing surgical bleeding. Direct inhibition of Factor Xa is the mechanism of drugs like rivaroxaban.

Question 96

A 14-year-old boy presents with a 2-month history of worsening thigh pain that frequently awakens him at night. Radiographs reveal a permeative, diaphyseal lesion in the femur with a lamellated 'onion-skin' periosteal reaction. A core needle biopsy is performed, demonstrating a uniform population of small blue cells. Which of the following chromosomal translocations is most strongly associated with this patient's diagnosis?

Explanation

The clinical and radiographic presentation, along with the biopsy finding of uniform small round blue cells, strongly supports the diagnosis of Ewing sarcoma. Ewing sarcoma is classically associated with the t(11;22)(q24;q12) translocation, which occurs in approximately 85% to 90% of cases and results in the EWS-FLI1 fusion protein. Synovial sarcoma is associated with t(X;18). The Philadelphia chromosome, t(9;22), is associated with chronic myelogenous leukemia (CML). Myxoid liposarcoma is characterized by t(12;16), and alveolar rhabdomyosarcoma is associated with t(2;13).

Question 97

A 32-year-old woman presents with persistent knee pain. Radiographs reveal an eccentric, purely lytic, epiphyseal-metaphyseal lesion in the distal femur extending to the subchondral bone without a sclerotic margin. Biopsy confirms a giant cell tumor of bone. She is scheduled for curettage but is prescribed a targeted biologic therapy preoperatively to downstage the tumor. What is the precise mechanism of action of this medication?

Explanation

The patient is describing Denosumab, which is used in the treatment of Giant Cell Tumor (GCT) of bone. The neoplastic cells in GCT are the mononuclear spindle-shaped stromal cells, which express high levels of RANK Ligand (RANKL). This RANKL recruits and activates reactive, non-neoplastic multinucleated giant cells (osteoclast-like cells) that cause the massive bone destruction seen in this disease. Denosumab is a fully humanized monoclonal antibody that binds directly to RANKL, preventing it from interacting with the RANK receptor on the osteoclast-like giant cells. It does not bind to the RANK receptor itself.

Question 98

In the staging and prognosis of adult soft-tissue sarcomas of the extremities, which of the following parameters is widely considered the single most important predictor of overall survival and the development of distant metastasis?

Explanation

While tumor size and depth are important staging variables (T-stage) that influence local recurrence and surgical planning, histologic grade is the single most important prognostic factor for overall survival and the risk of distant metastasis in adult soft-tissue sarcomas. Both the Musculoskeletal Tumor Society (MSTS/Enneking) and the American Joint Committee on Cancer (AJCC) staging systems heavily incorporate histologic grade (based on differentiation, mitotic count, and tumor necrosis) to dictate the stage grouping and systemic treatment algorithms.

Question 99

A 45-year-old man undergoes a biopsy of a painful, medullary lesion in his proximal humerus. Radiographs had shown endosteal scalloping and 'rings and arcs' calcifications. The biopsy confirms a Grade 2 conventional central chondrosarcoma. Somatic mutations in which of the following genes are most commonly implicated in the pathogenesis of this tumor and its benign precursor (enchondroma)?

Explanation

Mutations in the isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) genes are found in a large majority (>50-70%) of central enchondromas and conventional central chondrosarcomas. They are also the genetic hallmark of Ollier disease and Maffucci syndrome. EXT1 and EXT2 mutations are associated with multiple hereditary exostoses (osteochondromas). GNAS mutations are found in fibrous dysplasia. TP53 and RB1 are tumor suppressor genes whose mutations are classically associated with osteosarcoma.

Question 100

A 62-year-old woman with a history of metastatic breast cancer presents with severe right hip pain. The pain is functionally limiting and severely aggravated by weight-bearing. Radiographs show a purely lytic metastatic lesion in the peritrochanteric region of the right femur. The maximum diameter of the lesion occupies approximately 25% of the bone cortex. According to Mirels' criteria, what is her score, and what is the standard recommendation?

Explanation

Mirels' criteria are used to predict the risk of pathologic fracture and guide the decision for prophylactic fixation. The score is calculated based on four categories, each scoring 1 to 3 points. Site: upper extremity (1), lower extremity (2), peritrochanteric (3). Pain: mild (1), moderate (2), functional/mechanical (3). Lesion type: blastic (1), mixed (2), lytic (3). Size: <1/3 cortex (1), 1/3-2/3 cortex (2), >2/3 cortex (3). For this patient: Site = peritrochanteric (3); Pain = functional (3); Lesion = lytic (3); Size = 25% cortex (<1/3) (1). Total score = 3+3+3+1 = 10. A Mirels' score of 9 or greater carries a high risk of fracture, and prophylactic internal fixation is recommended.

Orthopedic Board Review Mock Exam #603: 100 High-Yield MCQs

Orthopedic Basic 2026 MCQs: Board Review Questions & Answers (Part 3)

Key Takeaway

Orthopedic Basic 2026 MCQs: Board Review Questions & Answers (Part 3)

Chapter Index