Which of the following is considered a specific advantage of using COX-2 inhibitors over COX-1 inhibitors?

Does not affect platelet function and can be used during the perioperative period

An obese 4-year-old boy has infantile Blount’s disease. Radiographs reveal a metaphyseal-diaphyseal angle of 18 degrees and a depression of the medial proximal tibial physis. Management should consist of

proximal tibial osteotomy that produces 10 degrees of valgus.

Figure 23 shows the postoperative radiograph of a patient who underwent an anterior cruciate ligament (ACL) reconstruction (with bone-patella tendon-bone autograft) that failed. He initially had loss of flexion postoperatively. What is the most likel

Anterior placement of the femoral tunnel

Which of the following best describes heat stroke? Review Topic

Hyperthermia, central nervous system dysfunction, and loss of thermoregulatory function

Free Orthopedic Review Quiz | Dr Hutaif Orthopedics

Question 1

A 55-year-old postmenopausal woman is evaluated for osteoporosis. She is started on denosumab. Which of the following best describes the primary mechanism of action of this medication?

Explanation

Denosumab is a monoclonal antibody that targets RANKL, preventing it from binding to RANK on the surface of osteoclasts and their precursors. This inhibits osteoclastogenesis and decreases bone resorption. Bisphosphonates inhibit farnesyl pyrophosphate synthase. Romosozumab activates Wnt signaling.

Question 2

During the clinical examination of a knee ligament, a constant force is applied, and the ligament is observed to slowly elongate over time. Which viscoelastic property does this describe?

Explanation

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

Question 3

A 14-year-old boy presents with a painful mass in his left distal femur. Radiographs show a destructive, permeative lesion with a 'sunburst' periosteal reaction. Biopsy confirms high-grade osteosarcoma. Which of the following genetic alterations is most frequently associated with the pathogenesis of this condition?

Explanation

High-grade intramedullary osteosarcoma is highly associated with mutations in tumor suppressor genes, specifically Rb and p53 (Li-Fraumeni syndrome). t(11;22) is characteristic of Ewing sarcoma. t(X;18) is seen in synovial sarcoma. t(2;13) is seen in alveolar rhabdomyosarcoma. GNAS1 mutations are associated with fibrous dysplasia.

Question 4

A 12-year-old obese boy presents with left thigh pain and a limp for 3 weeks. Examination reveals an obligate external rotation of the left hip during flexion. Radiographs confirm a mild slipped capital femoral epiphysis (SCFE). Contralateral prophylactic in situ pinning is most strongly indicated in which of the following scenarios?

Explanation

Prophylactic pinning of the contralateral hip in SCFE is controversial but is strongly recommended in patients with endocrine disorders (e.g., hypothyroidism, renal osteodystrophy, growth hormone deficiency) or those receiving radiation therapy, as they have a significantly higher risk of bilateral involvement.

Question 5

A 32-year-old man sustains a closed tibial shaft fracture. Twelve hours later, he complains of severe pain out of proportion to the injury, unrelieved by intravenous opioids. Which of the following intracompartmental pressure measurements is the most accepted threshold for performing a four-compartment fasciotomy?

Explanation

The delta p (diastolic blood pressure minus compartment pressure) is the most reliable indicator for acute compartment syndrome. A delta p of less than 30 mm Hg is an absolute indication for emergency fasciotomy, as perfusion to the muscle and nerve tissues is critically compromised.

Question 6

In aseptic loosening of a total hip arthroplasty, which of the following cell types is primarily responsible for internalizing particulate wear debris and initiating the osteolytic cascade?

Explanation

Macrophages are the primary effector cells in the pathogenesis of aseptic loosening (particle disease). They phagocytize wear debris (such as polyethylene particles) and subsequently release pro-inflammatory cytokines (e.g., TNF-alpha, IL-1, IL-6), which stimulate osteoclastic bone resorption.

Question 7

A 60-year-old man with poorly controlled diabetes presents with a rapidly spreading erythematous, exquisitely tender swelling of his right leg. Crepitus is palpated. Gram stain of fluid from a blister reveals a polymicrobial infection including Gram-positive cocci and Gram-negative rods. What is the most appropriate initial management?

Explanation

The patient is presenting with signs of necrotizing fasciitis (Type I, polymicrobial, common in diabetics). The presence of crepitus, rapid spread, and severe pain demands immediate surgical debridement. Delaying surgery for imaging or relying solely on antibiotics significantly increases mortality.

Question 8

According to Lewinnek's 'safe zone' for acetabular cup placement in total hip arthroplasty, which of the following parameters is associated with the lowest risk of dislocation?

Explanation

Lewinnek described a safe zone for acetabular component positioning to minimize the risk of dislocation. This zone is classically defined as 40 degrees +/- 10 degrees of abduction (inclination) and 15 degrees +/- 10 degrees of anteversion. Option B (40 degrees inclination, 15 degrees anteversion) falls squarely in the middle of this safe zone.

Question 9

Following an anterior cruciate ligament (ACL) reconstruction using a bone-patellar tendon-bone autograft, the graft undergoes a multi-phase biologic incorporation process. During which phase is the graft mechanically at its weakest?

Explanation

An ACL autograft undergoes histologic phases: necrosis, revascularization/proliferation, remodeling, and maturation. The graft is structurally at its weakest during the revascularization and cellular proliferation phase (typically between 6 and 12 weeks postoperatively) because the initial necrotic collagen framework is being broken down by enzymes before new organized collagen can mature.

Question 10

A 65-year-old man complains of bilateral calf pain that occurs after walking two blocks. The pain is relieved when he leans forward on a shopping cart or sits down. He states he can ride a stationary bicycle for 30 minutes without symptoms. These clinical findings are most characteristic of:

Explanation

This is a classic presentation of neurogenic claudication caused by lumbar spinal stenosis. Symptoms are typically exacerbated by extension (walking upright) which narrows the spinal canal, and relieved by flexion (leaning on a cart, sitting, cycling) which widens the canal and neural foramina. Vascular claudication would typically cause pain during cycling as well due to muscular oxygen demand.

Question 11

A 72-year-old man presents with dull aching pain in his right thigh and an enlarging hat size. Radiographs of the femur show cortical thickening, coarse trabeculae, and a radiolucent V-shaped leading edge. Histologic evaluation of the affected bone would most likely demonstrate:

Explanation

The clinical presentation is classic for Paget's disease of bone. The V-shaped leading edge is the 'blade of grass' sign of the lytic phase. The hallmark histologic finding in the mixed phase of Paget's disease is a 'mosaic pattern' of lamellar bone with prominent, haphazard cement lines due to chaotic bone resorption and formation.

Question 12

A 60-year-old man presents with back pain, anemia, hypercalcemia, and renal insufficiency. Radiographs show multiple punched-out lytic lesions in the skull and pelvis. Which of the following imaging modalities is generally considered LEAST useful in identifying the full extent of his skeletal lesions?

Explanation

Multiple myeloma lesions are purely osteolytic and lack the reactive osteoblastic bone formation required for uptake on a standard Technetium-99m bone scan. Therefore, bone scans often yield false-negative results in myeloma and are considered the least useful modality compared to WBLDCT, skeletal surveys, MRI, or PET-CT.

Question 13

A surgeon decides to use a stainless steel screw to secure a titanium plate during open reduction and internal fixation of a femur fracture. The patient subsequently develops a localized reaction. Which of the following types of corrosion is most likely occurring at the implant interface?

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are in physical contact within a conductive fluid medium (such as body fluids). This creates an electrochemical cell, leading to the accelerated corrosion of the less noble metal.

Question 14

Articular cartilage has a highly specialized extracellular matrix. Which of the following components is primarily responsible for the tissue's ability to resist compressive forces?

Explanation

The ability of articular cartilage to withstand compressive forces is primarily provided by proteoglycans (predominantly aggrecan), which are highly negatively charged and attract water, creating a swelling pressure (Donnan osmotic effect). The Type II collagen network provides tensile strength.

Question 15

A 4-week-old female infant is diagnosed with developmental dysplasia of the hip (DDH) and placed in a Pavlik harness. Two weeks later, the parents notice she is not kicking her affected leg as much as the other. Examination reveals an inability to actively extend the knee on the affected side. This complication is most likely due to excessive:

Explanation

The complication described is a femoral nerve palsy, which manifests as weakness of the quadriceps (inability to extend the knee). In a Pavlik harness, this is caused by excessive hip flexion, which stretches or compresses the femoral nerve. Excessive abduction can lead to avascular necrosis (AVN) of the femoral head.

Question 16

According to the Advanced Trauma Life Support (ATLS) classification of hemorrhagic shock, which of the following clinical signs is first observed when a patient transitions into Class III shock (31-40% blood volume loss)?

Explanation

Class III hemorrhagic shock (31-40% blood loss) is the point at which compensatory mechanisms fail, and the patient first develops measurable hypotension. Class II shock manifests with tachycardia and a narrow pulse pressure, but systolic blood pressure remains normal.

Question 17

A 68-year-old man is prescribed alendronate for the prevention of osteoporotic fragility fractures. At the cellular level, this medication ultimately inhibits bone resorption by which of the following mechanisms?

Explanation

Nitrogen-containing bisphosphonates (like alendronate) inhibit farnesyl pyrophosphate synthase, a key enzyme in the mevalonate pathway. This prevents the prenylation of small GTP-binding proteins (like Ras, Rho, and Rab) essential for osteoclast function and survival, ultimately leading to osteoclast apoptosis and decreased bone resorption.

Question 18

A 25-year-old man sustains a stab wound to the lateral aspect of his chest wall. He subsequently demonstrates winging of the scapula, especially when pushing against a wall. Which of the following muscles is paralyzed?

Explanation

Winging of the scapula, particularly prominent with forward pushing, is the classic sign of paralysis of the serratus anterior muscle. This muscle is innervated by the long thoracic nerve (C5, C6, C7), which courses superficially along the lateral chest wall, making it vulnerable to penetrating trauma or surgical injury.

Question 19

A 45-year-old typist is undergoing an open carpal tunnel release. To avoid injury to the recurrent motor branch of the median nerve, the surgeon must be aware of its most common anatomical variation. The recurrent branch typically innervates which of the following muscle groups?

Explanation

The recurrent motor branch of the median nerve supplies the thenar muscles: the abductor pollicis brevis, opponens pollicis, and the superficial head of the flexor pollicis brevis. (The deep head of FPB and adductor pollicis are supplied by the ulnar nerve). During carpal tunnel release, recognizing its course is critical to prevent iatrogenic thenar denervation.

Question 20

A 55-year-old patient with long-standing uncontrolled diabetes presents with a profoundly swollen, warm, and erythematous left foot. Radiographs demonstrate severe periarticular debris, fragmentation of the tarsal bones, and joint subluxation. There is no open wound. According to the Eichenholtz classification of Charcot arthropathy, what stage does this represent?

Explanation

The Eichenholtz classification describes the natural history of Charcot arthropathy. Stage 0 is the acute inflammatory phase (normal radiographs or mild osteopenia). Stage I (Fragmentation) is characterized by acute inflammation, joint laxity, subluxation, and radiographic evidence of bone fragmentation and periarticular debris. Stage II (Coalescence) shows decreased inflammation, absorption of fine debris, and early fusion.

Question 21

A 35-year-old male sustains a severe open tibial shaft fracture treated with intramedullary nailing. The surgeon considers an osteoinductive adjunct to promote bone healing. Which of the following recombinant human bone morphogenetic proteins (rhBMPs) is FDA-approved specifically for acute, open tibial shaft fractures treated with an intramedullary nail?

Explanation

rhBMP-2 is an osteoinductive protein that acts by stimulating mesenchymal stem cells to differentiate into osteoblasts. It is FDA approved for use in acute open tibial shaft fractures treated with an IM nail, as well as for anterior lumbar interbody fusion (ALIF). rhBMP-7 (OP-1) was previously approved under a Humanitarian Device Exemption for recalcitrant long bone nonunions but is no longer widely commercially available. rhBMP-3 is actually osteoinhibitory.

Question 22

Which of the following screw modifications will most significantly increase its theoretical pullout strength when placed into cancellous bone?

Explanation

Pullout strength of a cortical or cancellous screw is determined primarily by the formula: Pullout strength = S × L × π × D, where S is the shear strength of the surrounding bone, L is the length of engagement of the threads in the bone, and D is the outer (major or thread) diameter of the screw. Increasing the outer diameter, increasing the length of engagement, and increasing the shear strength of the bone all directly increase pullout strength.

Question 23

In the structural organization of articular cartilage, which zone is characterized by the highest concentration of water, the lowest concentration of proteoglycans, and type II collagen fibers oriented parallel to the articular surface?

Explanation

The superficial (tangential) zone makes up 10-20% of articular cartilage thickness. It has the highest water content (approx. 80%), the lowest proteoglycan content, and densely packed type II collagen fibers arranged parallel to the joint surface to resist shear forces.

Question 24

A 14-year-old boy presents with multiple fractures after minor trauma, recurrent dental infections, and mild hearing loss. Radiographs show diffusely dense, 'marble-like' bones with absent medullary canals. Which of the following cellular mechanisms is primarily defective in this condition?

Explanation

The clinical and radiographic presentation is classic for osteopetrosis (marble bone disease). Osteopetrosis is caused by defective osteoclast function, most commonly a failure to form the ruffled border, leading to impaired bone resorption. Common genetic defects include mutations in carbonic anhydrase II (CAII) and TCIRG1.

Question 25

According to Perren's strain theory of fracture healing, lamellar bone can only form when the interfragmentary strain at the fracture site is below what percentage?

Explanation

Perren's strain theory dictates that tissues cannot form in an environment where the strain exceeds their tolerance. Granulation tissue tolerates up to 100% strain, fibrocartilage tolerates up to 10%, and true lamellar bone formation requires a stable environment with an interfragmentary strain of less than 2%.

Question 26

A 62-year-old female with long-standing rheumatoid arthritis presents with neck pain and mild upper extremity clumsiness. Lateral flexion-extension radiographs of the cervical spine demonstrate atlantoaxial subluxation (AAS). Which of the following radiographic measurements is widely considered an absolute indication for surgical stabilization?

Explanation

In rheumatoid arthritis, the posterior atlantodental interval (PADI), also known as the space available for the cord (SAC), is the most reliable predictor of neurologic compromise and recovery. A PADI of less than 14 mm indicates critical spinal canal narrowing and is a strong indication for surgical stabilization, even if neurological symptoms are mild.

Question 27

A 9-year-old boy from Massachusetts presents with acute right knee swelling and a large effusion. He has no fever and can bear weight. Knee aspiration yields a synovial white blood cell count of 55,000 cells/mm³ with 85% polymorphonuclear leukocytes. Enzyme-linked immunosorbent assay (ELISA) and Western blot are positive for Borrelia burgdorferi. What is the most appropriate initial management?

Explanation

Lyme arthritis frequently presents with massive joint effusions, most commonly in the knee, with synovial WBC counts that can mimic septic arthritis (often 20,000 to >60,000). The first-line treatment for Lyme arthritis without systemic neurologic involvement is a 28-day course of oral antibiotics (doxycycline in children >8, amoxicillin, or cefuroxime).

Question 28

A 4-year-old boy is evaluated for short stature and significant bowing of the lower extremities. Laboratory studies reveal normal serum calcium, profoundly decreased serum phosphate, normal PTH, and normal 25-hydroxyvitamin D. However, 1,25-dihydroxyvitamin D is inappropriately normal. Which of the following is the most likely genetic defect?

Explanation

The clinical and laboratory profile (hypophosphatemia, normal calcium, normal PTH, and inappropriately normal 1,25-dihydroxyvitamin D despite low phosphate) is classic for X-linked hypophosphatemic rickets. This is caused by an inactivating mutation in the PHEX gene, leading to excessive levels of FGF23, which suppresses renal phosphate reabsorption and 1-alpha-hydroxylase activity.

Question 29

Which of the following proprioceptive structures is located in series with extrafusal muscle fibers and is primarily responsible for monitoring muscle tension, thereby preventing excessive force during a maximal contraction?

Explanation

The Golgi tendon organ is located at the myotendinous junction in series with extrafusal muscle fibers. It is an inhibitory receptor (via Ib afferent fibers) that monitors muscle tension and acts to reflexively relax the muscle when tension becomes dangerously high, preventing structural damage.

Question 30

Low-molecular-weight heparin (LMWH) is frequently utilized for venous thromboembolism prophylaxis following total joint arthroplasty. What is the primary mechanism of action of LMWH?

Explanation

Low-molecular-weight heparins (e.g., enoxaparin) act by binding to and potentiating antithrombin III. Unlike unfractionated heparin, which has roughly equal anti-Factor Xa and anti-Factor IIa activity, LMWH has a higher ratio of anti-Factor Xa to anti-Factor IIa activity (typically 2:1 to 4:1) because its shorter polysaccharide chains are less able to form the ternary complex required to inhibit thrombin.

Question 31

A 45-year-old patient undergoing an open reduction internal fixation of a distal radius fracture under an axillary block suddenly develops perioral numbness, tinnitus, tonic-clonic seizures, and subsequent cardiovascular collapse. Intravenous 20% lipid emulsion therapy is rapidly initiated. What was the most likely regional anesthetic agent used?

Explanation

The presentation is classic for local anesthetic systemic toxicity (LAST). Bupivacaine is a highly lipophilic, long-acting amide local anesthetic with a strong affinity for cardiac voltage-gated sodium channels. This strong binding makes it profoundly cardiotoxic compared to other agents. Intravenous lipid emulsion acts as a 'lipid sink' to draw the lipophilic bupivacaine away from the myocardium and central nervous system.

Question 32

In orthopedic implant constructs, combining a stainless steel plate with titanium screws is generally discouraged primarily due to the increased risk of which of the following processes?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different anodic indices (electropotentials) are in direct physical contact within an electrolytic medium (such as bodily fluids). The less noble metal (stainless steel) acts as an anode and corrodes at an accelerated rate, while the more noble metal (titanium) acts as a cathode.

Question 33

Compared to baseline ambulation in an able-bodied individual, which of the following lower extremity amputations theoretically requires the greatest increase in energy expenditure during ambulation?

Explanation

Energy expenditure increases significantly as the level of lower extremity amputation moves proximally, largely due to the loss of the knee joint. A unilateral transtibial amputation increases energy cost by roughly 25%, and bilateral transtibial by roughly 40%. A unilateral transfemoral amputation increases energy cost by 60-70%, making it more energetically demanding than a bilateral transtibial amputation.

Question 34

A 7-year-old boy with spastic diplegic cerebral palsy develops an iatrogenic 'crouch gait', characterized by excessive hip flexion, knee flexion, and ankle dorsiflexion during the stance phase. Which of the following prior surgical interventions is the most common iatrogenic cause of this specific gait pattern?

Explanation

Over-lengthening of the Achilles tendon (tendo-Achilles lengthening) in a patient with spastic diplegia weakens the critical plantarflexion-knee extension couple. Without competent plantarflexors to control the forward progression of the tibia over the foot during stance, the tibia falls forward, causing the knee to buckle into flexion. This leads to an iatrogenic, highly disabling crouch gait.

Question 35

In Sunderland's classification of peripheral nerve injuries, a third-degree injury is defined by disruption of the axon and which of the following connective tissue layers?

Explanation

Sunderland classification: 1st degree (Neuropraxia) = local myelin damage; 2nd degree (Axonotmesis) = axon severed, all connective tissue intact; 3rd degree = axon and endoneurium disrupted (perineurium intact); 4th degree = axon, endoneurium, and perineurium disrupted (epineurium intact); 5th degree (Neurotmesis) = complete transection of the nerve.

Question 36

In mature cortical bone, the Haversian systems (osteons) run parallel to the long axis of the bone. Which of the following structures is responsible for connecting adjacent Haversian canals to facilitate vascular communication between osteons?

Explanation

Volkmann canals are transverse or oblique channels in cortical bone that contain blood vessels and connect adjacent longitudinally running Haversian canals to each other, as well as to the periosteal and endosteal blood supplies.

Question 37

A 65-year-old female with a history of breast cancer presents with progressive right thigh pain. Radiographs reveal a 3.5 cm lytic lesion in the peritrochanteric region of the proximal femur, occupying 60% of the cortical diameter. Which of the following parameters are utilized in the Mirels' scoring system to determine the indication for prophylactic internal fixation?

Explanation

Mirels' criteria are used to quantify the risk of pathologic fracture in long bones affected by metastatic lesions. The score (ranging from 4 to 12) is based on four variables: Site (upper limb, lower limb, peritrochanteric), Pain (mild, moderate, functional/severe), Lesion nature (blastic, mixed, lytic), and Size (<1/3, 1/3-2/3, >2/3 cortical diameter). A score of 9 or greater indicates high risk and warrants prophylactic fixation.

Question 38

Which of the following pharmacological agents has been shown in prospective randomized trials to significantly decrease the incidence of Complex Regional Pain Syndrome (CRPS) following closed treatment of distal radius fractures?

Explanation

Vitamin C (ascorbic acid), typically given at a dose of 500 mg daily for 50 days following injury, has been shown in several randomized controlled trials to significantly reduce the risk of developing Complex Regional Pain Syndrome (CRPS) after distal radius fractures.

Question 39

A 68-year-old male presents with deep, aching bone pain in his right femur and an enlarging hat size. Laboratory tests show a significantly elevated serum alkaline phosphatase but normal calcium and phosphorus. A biopsy of the femur is most likely to demonstrate which of the following pathognomonic histological features?

Explanation

The clinical scenario is classic for Paget's disease of bone (osteitis deformans). The pathognomonic histological finding in the mixed and late/sclerotic phases is a mosaic, 'jigsaw puzzle' pattern of lamellar bone with prominent, haphazard cement lines, reflecting the chaotic, rapid cycles of bone resorption and formation.

Question 40

A patient develops a suspected metal hypersensitivity reaction following a total knee arthroplasty, presenting with a painful, swollen joint and overlying eczematous dermatitis. This reaction is primarily mediated by which of the following components of the immune system?

Explanation

Metal hypersensitivity (e.g., to nickel, cobalt, or chromium) is a Type IV (delayed-type) hypersensitivity reaction. Unlike Types I, II, and III, which are antibody-mediated, Type IV is a cell-mediated response driven primarily by sensitized T lymphocytes that release pro-inflammatory cytokines upon antigen re-exposure.

Question 41

A 35-year-old man undergoes closed reduction and cast application for a closed tibial shaft fracture. Secondary bone healing is anticipated. During the cartilaginous callus phase, which collagen type is predominantly synthesized before being replaced by woven bone?

Explanation

Secondary bone healing involves a soft cartilaginous callus phase characterized by the production of Type II collagen by chondrocytes. As the cartilage undergoes endochondral ossification, hypertrophic chondrocytes produce Type X collagen, followed by osteoblasts producing Type I collagen in the hard callus.

Question 42

When evaluating the mechanical properties of orthopedic implant materials, the stiffness of a material is represented on the stress-strain curve. Which of the following best defines the Young's modulus of a material?

Explanation

Young's modulus (modulus of elasticity) is determined by the slope of the stress-strain curve in the linear, elastic region. It represents the inherent stiffness of the material, with a steeper slope indicating a stiffer material.

Question 43

A 68-year-old man presents with increasing right thigh pain and bowing of his femur. Radiographs show cortical thickening and trabecular coarsening. Serum alkaline phosphatase is markedly elevated, but calcium and phosphorus are normal. If a bone biopsy were performed, what characteristic histological finding would be expected?

Explanation

Paget's disease of bone is characterized by increased, disorganized bone remodeling. Histologically, it presents as a mosaic pattern of lamellar bone with prominent, haphazardly arranged cement lines due to irregular periods of osteoclastic resorption and osteoblastic formation.

Question 44

A 72-year-old woman with a history of a vertebral fragility fracture is prescribed alendronate. At the cellular level, this medication inhibits bone resorption primarily by inhibiting which of the following enzymes?

Explanation

Nitrogen-containing bisphosphonates, such as alendronate, inhibit farnesyl pyrophosphate synthase within the mevalonate pathway. This disruption prevents the prenylation of small GTPase proteins (like Ras and Rho), ultimately leading to osteoclast apoptosis and decreased bone resorption.

Question 45

A 10-year-old boy presents with severe, unrelenting diaphyseal thigh pain. Radiographs reveal a permeative lytic lesion with an "onion-skin" periosteal reaction in the femur diaphysis. Which of the following chromosomal translocations is pathognomonic for this tumor?

Explanation

The clinical and radiographic presentation is classic for Ewing sarcoma. Over 90% of Ewing sarcomas are driven by the t(11;22)(q24;q12) translocation, which fuses the EWS gene on chromosome 22 with the FLI1 gene on chromosome 11.

Question 46

A 45-year-old mechanic complains of chronic vague pain in the dorsal proximal forearm and weakness when extending his fingers and thumb, though wrist extension is largely preserved. He has no sensory deficits. The affected nerve is most likely compressed by which of the following structures?

Explanation

The patient has Posterior Interosseous Nerve (PIN) syndrome, characterized by weakness in finger/thumb extension and preserved radial wrist extension. The most common site of PIN compression is the Arcade of Frohse, the proximal fascial edge of the superficial head of the supinator muscle.

Question 47

A 40-year-old woman with celiac disease presents with diffuse bone pain and proximal muscle weakness. Radiographs reveal bilateral pseudofractures (Looser zones) in the femoral neck. Which of the following serum laboratory profiles is most consistent with her diagnosis?

Explanation

The patient has osteomalacia secondary to malabsorption of Vitamin D. This typically presents with low serum calcium, low serum phosphorus (due to secondary hyperparathyroidism), and elevated alkaline phosphatase.

Question 48

A newborn sustains multiple fractures during an uncomplicated vaginal delivery. Examination reveals blue sclerae and joint hyperlaxity. This condition is primarily caused by a mutation affecting the synthesis of which of the following?

Explanation

Osteogenesis imperfecta (OI) is an autosomal dominant disorder characterized by bone fragility and blue sclerae. It is caused by mutations in the COL1A1 or COL1A2 genes, which lead to quantitative or qualitative defects in Type I collagen.

Question 49

An orthopedic surgeon plans to use a stainless steel screw to secure a titanium plate during fracture fixation. This combination of different metals in an electrolytic environment places the construct at highest risk for which type of corrosion?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within a conductive fluid. This creates an electrochemical cell, leading to accelerated corrosion of the less noble metal.

Question 50

A 30-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic, epiphyseal-metaphyseal lesion in the proximal tibia extending to the subchondral bone. Biopsy shows multinucleated giant cells. The targeted therapy (denosumab) for this specific tumor inhibits which of the following?

Explanation

Giant cell tumors of bone consist of neoplastic mononuclear stromal cells that express high levels of RANKL, which recruits and activates the reactive multinucleated osteoclast-like giant cells. Denosumab is a monoclonal antibody that binds and inhibits RANKL, effectively reducing osteoclastic bone destruction.

Question 51

Articular cartilage relies on a unique structural organization to withstand compressive and shear forces. In which zone of articular cartilage are the type II collagen fibers oriented perpendicular to the joint surface?

Explanation

In the deep (radial) zone of articular cartilage, chondrocytes are arranged in columns, and the Type II collagen fibers are oriented perpendicularly to the joint surface to resist compressive forces. The superficial zone has parallel fibers to resist shear, while the middle zone has randomly oriented fibers.

Question 52

A 55-year-old woman with chronic kidney disease presents with a painful, radiolucent lesion in her right humerus. Laboratory tests reveal markedly elevated parathyroid hormone (PTH) and alkaline phosphatase. Biopsy of the lesion shows fibrovascular tissue, hemosiderin deposition, and numerous multinucleated giant cells. What is the most likely diagnosis?

Explanation

A Brown tumor (osteitis fibrosa cystica) is a localized accumulation of fibrous tissue, giant cells, and hemosiderin (giving a brown color) that occurs in the setting of severe primary or secondary hyperparathyroidism. It resolves with treatment of the underlying endocrine condition.

Question 53

A 65-year-old man presents with a delayed, low-grade periprosthetic joint infection of his total hip arthroplasty, caused by Staphylococcus epidermidis. The persistence and antibiotic resistance of this organism on the implant surface are primarily mediated by the production of which extracellular substance?

Explanation

Staphylococcus epidermidis is a common cause of indolent periprosthetic joint infections due to its ability to form a robust biofilm. The biofilm is composed of a self-produced extracellular polymeric substance known as glycocalyx, which shields the bacteria from host immune responses and antibiotics.

Question 54

When inserting a cortical screw for fracture fixation, which of the following alterations in screw design will maximize its pullout strength from the bone?

Explanation

Pullout strength of a screw is directly proportional to the volume of bone engaged by the threads. It is maximized by increasing the outer diameter, decreasing the inner (core) diameter (which increases thread depth), and decreasing the thread pitch.

Question 55

Tranexamic acid (TXA) is routinely used in total joint arthroplasty to reduce perioperative blood loss. Which of the following describes the specific mechanism of action of TXA?

Explanation

Tranexamic acid is an antifibrinolytic agent that works as a synthetic analog of the amino acid lysine. It reversibly and competitively binds to the lysine receptor sites on plasminogen, preventing its activation into plasmin and thereby stabilizing the fibrin clot.

Question 56

A massive structural cortical allograft is utilized for reconstruction following tumor resection. Compared to an autograft, what is the primary disadvantage regarding the biological incorporation of this massive allograft via creeping substitution?

Explanation

Massive structural cortical allografts incorporate through creeping substitution, but this process is very slow and superficial. The central portion of the allograft typically remains necrotic and unremodeled for years, making it susceptible to late fatigue fractures and nonunions.

Question 57

In the pathogenesis of Rheumatoid Arthritis joint destruction, what is the primary cell type within the hyperplastic synovium (pannus) responsible for the direct production of matrix metalloproteinases (MMPs) that degrade articular cartilage?

Explanation

In rheumatoid arthritis, the inflammatory pannus is driven by an interplay of immune cells. Fibroblast-like synoviocytes (Type B synoviocytes) are the primary effector cells that, once activated by cytokines, produce high levels of matrix metalloproteinases (MMPs) leading to cartilage destruction.

Question 58

A surgeon plans to use a stainless steel plate with titanium screws for a fracture fixation. Which of the following best describes the primary risk associated with mixing these two metals in vivo?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in an electrolytic solution like body fluid, creating an electrochemical gradient. The less noble metal acts as an anode and undergoes accelerated corrosion.

Question 59

In normal adult articular cartilage, what is the predominant orientation and type of collagen found in the deep zone (radiata)?

Explanation

The deep zone of articular cartilage is composed primarily of Type II collagen fibers oriented perpendicular to the joint surface. This arrangement provides high compressive strength and securely anchors the cartilage to the calcified zone across the tidemark.

Question 60

A 4-year-old child presents with multiple fractures and hepatosplenomegaly. Radiographs reveal diffuse osteosclerosis and a "bone-within-a-bone" appearance. A genetic defect in carbonic anhydrase II is identified. This mutation primarily impairs which of the following cellular functions?

Explanation

Carbonic anhydrase II deficiency causes osteopetrosis by preventing osteoclasts from generating the protons needed to acidify the resorptive pit (Howship lacuna). This leads to defective bone resorption, resulting in overly dense but brittle bones.

Question 61

During the incorporation of a cortical bone allograft, the process of "creeping substitution" is observed. Which of the following sequences best describes the initial cellular events in this process?

Explanation

Cortical bone grafts incorporate via creeping substitution, an extremely slow process where osteoclasts first resorb the existing Haversian canals. This is immediately followed by osteoblasts laying down new bone within these widened canals.

Question 62

A patient is prescribed subcutaneous enoxaparin for deep vein thrombosis prophylaxis following a total hip arthroplasty. What is the primary mechanism of action of this pharmacological agent?

Explanation

Enoxaparin is a low-molecular-weight heparin (LMWH) that binds to antithrombin III. This binding induces a conformational change that preferentially accelerates the inhibition of Factor Xa compared to thrombin (Factor IIa).

Question 63

A 60-year-old man presents with a slowly enlarging, painful pelvic mass. Imaging shows a lytic lesion with "ring and arc" calcifications. Biopsy demonstrates atypical chondrocytes in a hyaline cartilage matrix. What is the most appropriate definitive management?

Explanation

The diagnosis is conventional chondrosarcoma, which is characteristically highly resistant to both chemotherapy and radiation therapy. Wide surgical excision is the mainstay of treatment for localized disease to ensure negative margins.

Question 64

When inserting a cortical screw for fracture fixation, which of the following geometric properties of the screw most significantly determines its pullout strength?

Explanation

The pullout strength of a bone screw is most highly correlated with its outer (thread) diameter. Other major contributing factors include the length of thread engagement in the bone and the shear strength of the surrounding bone.

Question 65

A patient develops a periprosthetic joint infection caused by Staphylococcus epidermidis. The pathogenesis heavily relies on the formation of a biofilm. Which of the following primarily mediates the initial adherence of the bacteria to the orthopedic implant?

Explanation

Initial adherence to an implant is mediated by bacterial adhesins like FnBPs binding to host proteins (such as fibronectin and fibrinogen) that immediately coat the implant surface. Following this attachment phase, the bacteria produce a polysaccharide glycocalyx to establish the mature biofilm.

Question 66

According to Perren's strain theory of fracture healing, which of the following statements regarding secondary bone healing is true?

Explanation

Perren's strain theory dictates that tissue will only form if the local strain is less than the tissue's tolerance. Granulation tissue can tolerate up to 100% strain, allowing it to initially bridge the fracture gap and stabilize the fragments so stiffer tissues can subsequently form.

Question 67

A 5-year-old boy is evaluated for bowing of the legs and short stature. Laboratory tests show low serum phosphate, normal calcium, normal PTH, and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. Which of the following is the primary pathophysiologic defect?

Explanation

The patient has X-linked hypophosphatemic rickets caused by a PHEX gene mutation, leading to unchecked elevation of FGF-23 levels. Excess FGF-23 inhibits renal phosphate reabsorption in the proximal tubule, resulting in severe renal phosphate wasting.

Question 68

During the repair of a zone II flexor tendon laceration, a surgeon utilizes techniques to minimize adhesion formation. Adhesions in tendon healing are primarily associated with which of the following processes?

Explanation

Tendon healing occurs via concurrent intrinsic (tenocytes within the tendon) and extrinsic pathways. The extrinsic healing pathway relies on inflammatory cells and fibroblasts invading from the surrounding peritendinous tissues, which is primarily responsible for scar adhesion formation.

Question 69

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic epiphyseal lesion in the proximal tibia. Biopsy shows mononuclear cells interspersed with multinucleated giant cells. If pharmacological treatment is indicated, which of the following medications directly targets the pathophysiology of this lesion?

Explanation

Giant Cell Tumor of Bone is characterized by neoplastic mononuclear stromal cells that express high levels of RANKL, driving the recruitment of reactive osteoclast-like giant cells. Denosumab, a monoclonal antibody against RANKL, directly inhibits this essential pathway.

Question 70

A patient sustains a closed midshaft humerus fracture and presents with a secondary radial nerve palsy. According to the Seddon classification, if this is an axonotmesis, which of the following histologic structures remains intact?

Explanation

In an axonotmesis, the axon and its myelin sheath are disrupted, leading to Wallerian degeneration distally. However, the surrounding connective tissue framework (endoneurium, perineurium, and epineurium) remains completely intact, allowing for potential spontaneous axonal regeneration.

Question 71

Which glycosaminoglycan is the most abundant in the extracellular matrix of adult articular cartilage?

Explanation

Chondroitin sulfate is the most abundant glycosaminoglycan in articular cartilage, followed by keratan sulfate. Its concentration decreases with age, whereas keratan sulfate concentration increases over time.

Question 72

A 65-year-old man presents with hearing loss and increasing hat size. Radiographs show a "cotton wool" appearance of the skull. Which of the following is the primary cellular abnormality initiating this disease process?

Explanation

Paget's disease is primarily a disorder of osteoclast function, characterized by large, multinucleated osteoclasts leading to massive bone resorption. This is followed by a secondary, disorganized osteoblastic response.

Question 73

When two dissimilar metals, such as a titanium plate and stainless steel screws, are placed in contact within the body's electrolyte-rich environment, which type of corrosion is most likely to occur?

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals are in direct contact within an electrolyte solution, leading to an electrochemical gradient and material breakdown.

Question 74

A 10-year-old girl presents with a diaphyseal mass in her right fibula. Biopsy shows a uniform population of small, round, blue cells. Which specific chromosomal translocation is most diagnostic for this condition?

Explanation

Ewing sarcoma is classically associated with the t(11;22) chromosomal translocation. This specific genetic alteration results in the formation of the EWS-FLI1 fusion protein.

Question 75

During secondary fracture healing, at which specific stage does the peak expression of Type II collagen occur?

Explanation

Type II collagen is the primary collagen type found in cartilage and reaches peak expression during the soft callus (chondrogenic) phase of secondary fracture healing. Type I collagen peaks later during hard callus formation.

Question 76

Which of the following bone graft substitutes possesses osteoinductive properties but definitively lacks osteoconductive and osteogenic properties?

Explanation

Demineralized bone matrix (DBM) provides osteoinductive proteins, such as BMPs, but lacks a structural scaffolding for osteoconduction and contains no live cells for osteogenesis.

Question 77

Which specific modification to a standard cortical screw design will most effectively increase its pullout strength in dense bone?

Explanation

Pullout strength is directly proportional to the volume of bone caught between the screw threads. Decreasing the thread pitch (creating more threads per unit length) effectively increases pullout strength.

Question 78

A patient is scheduled for a total hip arthroplasty and will receive rivaroxaban for postoperative venous thromboembolism prophylaxis. What is the specific mechanism of action of this drug?

Explanation

Rivaroxaban is an oral anticoagulant that works by directly inhibiting Factor Xa, interrupting the common pathway of the coagulation cascade. Dabigatran, in contrast, is a direct thrombin (Factor IIa) inhibitor.

Question 79

A 55-year-old man is diagnosed with a primary conventional grade II chondrosarcoma of the proximal humerus based on a biopsy. Radiographs show intralesional stippled calcifications. What is the most appropriate definitive management?

Explanation

Conventional chondrosarcomas are historically highly resistant to both chemotherapy and radiation therapy. The standard of care for intermediate and high-grade chondrosarcoma is wide surgical resection alone.

Question 80

During an eccentric muscle contraction, how does the force-velocity relationship primarily differ from that of a concentric muscle contraction?

Explanation

In an eccentric contraction, the muscle lengthens while contracting against a load. The force generated increases as the velocity of lengthening increases until a plateau is reached, generating forces greater than maximum isometric force.

Question 81

A 4-year-old boy presents with progressive bowing of his lower extremities. Laboratory evaluation reveals profound hypophosphatemia, normal serum calcium, normal PTH, and elevated alkaline phosphatase. Which of the following is the most likely diagnosis?

Explanation

X-linked hypophosphatemic rickets is driven by an excess of FGF-23, leading to severe renal phosphate wasting. This results in isolated hypophosphatemia with classically normal calcium and parathyroid hormone levels.

Question 82

A 24-year-old motorcyclist sustains a traction injury to his brachial plexus. He exhibits complete loss of shoulder abduction and external rotation, and significant weakness in elbow flexion. Which nerve roots are most likely involved?

Explanation

This classic presentation represents Erb's palsy, involving the upper trunk (C5, C6 nerve roots). It compromises the suprascapular, axillary, and musculocutaneous nerves, resulting in a "waiter's tip" clinical posture.

Question 83

In total joint arthroplasty, which distinct type of wear is characterized by cyclic load transferring between two bearing surfaces resulting in subsurface microcracks and subsequent macroscopic delamination?

Explanation

Fatigue wear occurs from repetitive cyclical loading that creates subsurface shear stresses, leading to microcracks that coalesce and cause macroscopic material delamination. This is classically seen in non-crosslinked polyethylene components.

Question 84

A 32-year-old man sustains a closed high-energy tibial shaft fracture. Two hours post-injury, he complains of severe leg pain disproportionate to the injury. Which early clinical finding is the most sensitive for impending acute compartment syndrome?

Explanation

Pain with passive stretch of the muscles in the affected compartment is typically the most sensitive and earliest clinical sign of acute compartment syndrome. Pulselessness, pallor, and paralysis are late and notoriously unreliable signs.

Question 85

Stress shielding around an orthopedic implant occurs because the metallic implant is significantly stiffer than the surrounding bone. Among the following orthopedic biomaterials, which has a modulus of elasticity closest to that of human cortical bone?

Explanation

Titanium alloy has a modulus of elasticity of approximately 110 GPa, which is lower than stainless steel (~200 GPa) and cobalt-chromium (~240 GPa), making it the closest metal to human cortical bone (~15-20 GPa).

Question 86

A patient sustains a high-energy fracture-dislocation of the midfoot involving the tarsometatarsal joints. Which ligament is primarily responsible for maintaining the structural stability of the second metatarsal base to the medial cuneiform?

Explanation

The Lisfranc ligament connects the medial cuneiform directly to the base of the second metatarsal. It lacks an intermetatarsal ligament between the first and second rays, making this specific ligament critical for midfoot stability.

Question 87

A 20-year-old man complains of severe, aching thigh pain that is notoriously worse at night and dramatically improves with NSAIDs. Radiographs reveal focal cortical thickening in the proximal femur with a central radiolucent nidus. What is the principal molecular mediator responsible for the intense pain in this lesion?

Explanation

Osteoid osteomas produce extremely high local levels of Prostaglandin E2 (PGE2) due to marked upregulation of COX-2 expression within the nidus. This accounts for the intense nocturnal pain and the dramatic relief provided by NSAIDs.

Question 88

A 3-year-old girl is brought to the emergency department with an acute limp and refuses to bear weight on her right leg. Joint aspiration of the hip reveals a synovial fluid white blood cell count of 85,000 cells/mm3 with 90% polymorphonuclear leukocytes. What is the most common causative organism of septic arthritis in this age group?

Explanation

Staphylococcus aureus remains the single most common organism causing septic arthritis in children overall. While Kingella kingae is increasingly recognized in children under 4 years old, S. aureus represents the classic and most frequent isolate.

Question 89

In a healthy native diarthrodial joint during high-load and low-speed activities (e.g., initiating standing from a chair), which type of cartilage lubrication mechanism is predominantly active to minimize friction?

Explanation

Boundary lubrication depends on a distinct molecular layer of lubricin adhering directly to the articular surfaces. It is the primary mechanism preventing surface wear and reducing friction during conditions of extreme load and low speed.

Question 90

A 70-year-old woman is treated for severe osteoporosis with a targeted medication that directly stimulates osteoblastic bone formation by acting as a recombinant fragment of human parathyroid hormone. Which medication was prescribed?

Explanation

Teriparatide is a recombinant fragment of human parathyroid hormone (PTH 1-34) that acts as a powerful anabolic agent to directly stimulate new bone formation. Unlike bisphosphonates or denosumab, it builds bone rather than primarily inhibiting resorption.

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