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Self Assessment Examination Adult S Review | Dr Hutaif - ...

Orthopedics Hyperguide Review | Dr Hutaif General Ortho -...

23 Apr 2026 116 min read 120 Views
Illustration of q a yearold woman - Dr. Mohammed Hutaif

Key Takeaway

Here are the crucial details you must know about ORTHOPEDICS HYPERGUIDE 2022 MCQ1001-1051. Articular cartilage comprises collagen (60%), proteoglycans (25-35%), and noncollagenous proteins (15-20%) as its primary macromolecules. Collagen types II, IX, and XI form cross-banded fibrils, with decorin and fibromodulin stabilizing the type II network. In a q a yearold woman, understanding this composition is crucial, particularly the superficial zone's high collagen and low proteoglycan concentrations, essential for joint function.

Orthopedics Hyperguide Review | Dr Hutaif General Ortho -...

Comprehensive 100-Question Exam


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Question 1

What is the primary mechanism of bone healing when absolute stability is achieved with a lag screw and neutralization plate?





Explanation

Absolute stability (strain < 2%) leads to primary bone healing, which occurs via Haversian remodeling (cutting cones) crossing the fracture site. There is no visible callus (which is characteristic of endochondral ossification/secondary healing).

Question 2

Which of the following bone graft options possesses osteoconductive, osteoinductive, and osteogenic properties?





Explanation

Iliac crest bone autograft (ICBG) is the gold standard because it is the only option listed that contains living cells (osteogenic), growth factors like BMPs (osteoinductive), and a collagen/mineral scaffold (osteoconductive). Allografts and DBM lack live osteoprogenitor cells.

Question 3

In articular cartilage, which zone contains the highest concentration of water and collagen fibers oriented parallel to the joint surface?





Explanation

The superficial zone (Zone 1) of articular cartilage is characterized by collagen fibers aligned parallel to the articular surface to resist shear stress, and it has the highest water content.

Question 4

In the context of locked plating for a comminuted diaphyseal fracture, increasing the working length of the plate has which of the following biomechanical effects?





Explanation

Increasing the working length of a locking plate (the distance between the innermost screws on either side of the fracture) decreases the stiffness of the construct. This distributes strain over a larger area, thereby decreasing the strain at the fracture site and promoting secondary bone healing through callus formation.

Question 5

What is the mechanism of action of denosumab in the treatment of osteoporosis?





Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL, preventing it from interacting with RANK on osteoclasts and their precursors. This inhibits osteoclast formation, function, and survival. Bisphosphonates inhibit farnesyl pyrophosphate synthase.

Question 6

During tensile testing of a normal tendon, the initial 'toe region' of the stress-strain curve represents which of the following physiological events?





Explanation

The toe region of the stress-strain curve (up to about 2% strain) represents the straightening or 'uncrimping' of the normally crimped collagen fibers. Continued stress leads to the linear (elastic) region, followed by microscopic failure (yield point) and then macroscopic failure.

Question 7

Which collagen type is the most abundant in normal bone matrix?





Explanation

Type I collagen accounts for about 90% of the organic matrix of bone. Type II is found in articular cartilage. Type III is found in skin, blood vessels, and early fracture callus. Type X is found in the hypertrophic zone of the physis.

Question 8

A 45-year-old malnourished patient presents with bleeding gums, petechiae, and joint pain. A deficiency in which of the following vitamins is responsible for this condition, and what is its role in collagen synthesis?





Explanation

Scurvy is caused by Vitamin C deficiency. Vitamin C (ascorbic acid) is a required cofactor for prolyl hydroxylase and lysyl hydroxylase, which hydroxylate proline and lysine residues in procollagen. This step is essential for the stable triple-helix formation of collagen.

Question 9

During the normal gait cycle, at what phase does the maximum force on the hip joint occur?





Explanation

Maximum force across the hip joint occurs during mid-stance when the abductors (primarily gluteus medius) must contract forcefully to stabilize the pelvis while the entire body weight is supported on one leg. The joint reaction force can reach 2.5 to 3 times body weight.

Question 10

When a stainless steel screw is placed through a titanium plate in a physiological environment, which of the following phenomena is most likely to occur?





Explanation

Galvanic corrosion occurs when two dissimilar metals (like stainless steel and titanium) are placed in contact within an electrolytic solution (like bodily fluids). The less noble metal (typically stainless steel) undergoes accelerated anodic dissolution.

Question 11

A 25-year-old male sustains a closed tibia fracture. Which of the following pressure measurements is most indicative of the need for an emergent fasciotomy?





Explanation

The delta pressure (Diastolic BP - Compartment Pressure) is the most reliable indicator of compartment syndrome. A delta pressure of less than 30 mmHg signifies inadequate tissue perfusion and is an absolute indication for emergent fasciotomy.

Question 12

According to the Sunderland classification, a fourth-degree nerve injury involves disruption of which of the following structures?





Explanation

Sunderland IV injury involves disruption of the axon, endoneurium, and perineurium, leaving only the epineurium intact. Sunderland I is neuropraxia (myelin). II is axonotmesis (axon disrupted, endoneurium intact). III includes endoneurium disruption.

Question 13

A patient presents with an acutely swollen and painful knee. Joint aspiration yields synovial fluid with a WBC count of 85,000 cells/µL, 90% polymorphonuclear leukocytes, and a glucose level significantly lower than serum glucose. What is the most likely diagnosis?





Explanation

A synovial fluid WBC count greater than 50,000 cells/µL (often >80,000 in reality) with >75-90% PMNs and low glucose is highly indicative of septic arthritis. Inflammatory arthritis (RA, gout) typically has WBC counts between 2,000 and 50,000 cells/µL.

Question 14

Which of the following cytokines is most strongly implicated as an osteoclast activating factor in the pathogenesis of lytic bone lesions in multiple myeloma?





Explanation

Interleukin-6 (IL-6) is a critical cytokine in multiple myeloma. It acts as a potent osteoclast-activating factor and also promotes the growth and survival of myeloma cells, leading to characteristic lytic bone lesions.

Question 15

In which zone of the physis do Salter-Harris type I fractures most commonly occur?





Explanation

Salter-Harris fractures typically propagate through the mechanically weakest zone of the physis, which is the zone of cell hypertrophy (specifically the zone of cell maturation and degeneration) because it lacks abundant collagen and calcified matrix.

Question 16

A 65-year-old male presents with increasing hat size, hearing loss, and anterior bowing of the tibiae. Laboratory evaluation is most likely to show which of the following?





Explanation

The patient has Paget's disease of bone (osteitis deformans). In Paget's disease, there is disorganized bone remodeling, leading to markedly elevated alkaline phosphatase, but serum calcium and phosphorus levels typically remain normal.

Question 17

Cefazolin is widely used as a prophylactic antibiotic in orthopedic surgery. What is its primary mechanism of action?





Explanation

Cefazolin is a first-generation cephalosporin (a beta-lactam). Beta-lactams exert their bactericidal effect by binding to penicillin-binding proteins (PBPs), thereby inhibiting the cross-linking of peptidoglycan during bacterial cell wall synthesis.

Question 18

Bone Morphogenetic Proteins (BMPs) initiate intracellular signaling primarily through which of the following pathways?





Explanation

BMPs are members of the TGF-beta superfamily. They bind to serine/threonine kinase cell surface receptors, which phosphorylate intracellular Smad proteins (typically Smad 1, 5, and 8). The phosphorylated Smads complex with Smad 4 and translocate to the nucleus to regulate gene transcription.

Question 19

A 24-year-old male with bilateral femur fractures develops respiratory distress, confusion, and a petechial rash on his chest 36 hours post-injury. What is the major pathophysiologic mechanism leading to the pulmonary manifestations of this syndrome?





Explanation

The patient has Fat Embolism Syndrome (FES). The pathophysiology involves both mechanical obstruction by fat globules and a biochemical response where fat breaks down into toxic free fatty acids (FFAs). FFAs cause direct endothelial damage in the pulmonary circulation, leading to capillary leak and ARDS.

Question 20

When evaluating the pullout strength of a cortical screw, which of the following design alterations will yield the greatest increase in pullout strength?





Explanation

Pullout strength is directly proportional to the outer diameter of the thread, the length of engagement, and inversely proportional to the thread pitch (distance between threads). Decreasing the pitch means more threads engage the bone per unit length, thus increasing pullout strength.

Question 21

A 4-year-old boy presents with multiple fractures and a 'rugger-jersey' spine appearance on radiographs. Labs show normal calcium, normal phosphorus, and elevated acid phosphatase. Which of the following is the underlying molecular defect most commonly associated with this condition?





Explanation

Osteopetrosis is caused by defective osteoclast function, leading to dense, brittle bones and a 'rugger-jersey' spine. A common mutation is a deficiency in carbonic anhydrase II, which normally produces the acidic environment required for osteoclasts to resorb bone.

Question 22

A 15-year-old boy presents with a painful mass in the diaphysis of his femur. Radiographs show a permeative, moth-eaten lesion with an 'onion-skin' periosteal reaction. Core biopsy is performed. Which of the following chromosomal translocations is most strongly associated with this lesion?





Explanation

Ewing sarcoma typically presents in the diaphysis of long bones with an onion-skin periosteal reaction. It is classically characterized by the t(11;22) translocation, resulting in the EWS-FLI1 fusion protein. t(X;18) is associated with Synovial Sarcoma; t(2;13) with Alveolar Rhabdomyosarcoma; and t(12;16) with Myxoid Liposarcoma.

Question 23

During the early stages of primary osteoarthritis, which of the following biochemical changes occurs first in the articular cartilage matrix?





Explanation

In early osteoarthritis, the initial change in articular cartilage is an increase in water content and a decrease in proteoglycan concentration. This leads to a less stiff matrix and increased permeability. In later stages, the water content eventually decreases.

Question 24

A new point-of-care test for periprosthetic joint infection is implemented in an orthopedic clinic where the prevalence of the disease is very high, compared to a primary care setting where the prevalence is low. Assuming the sensitivity and specificity of the test remain constant, how will the predictive values change in the orthopedic clinic compared to the primary care setting?





Explanation

Predictive values are dependent on the prevalence of the disease in the tested population. A higher prevalence increases the positive predictive value (PPV) and decreases the negative predictive value (NPV), while sensitivity and specificity are intrinsic to the test and remain constant.

Question 25

A 35-year-old man sustains an Achilles tendon rupture and undergoes surgical repair. During the remodeling phase of tendon healing, which of the following collagen transitions primarily occurs to increase the tensile strength of the repair site?





Explanation

Tendon healing progresses through inflammatory, proliferative, and remodeling phases. In the early proliferative phase, fibroblasts primarily synthesize Type III collagen. During the remodeling phase, Type I collagen replaces Type III collagen, aligning along the lines of tension to increase the tissue's mechanical strength.

Question 26

A 60-year-old woman with long-standing rheumatoid arthritis complains of occipital headaches and upper extremity paresthesias. Flexion-extension cervical spine radiographs reveal an anterior atlantodens interval (ADI) of 8 mm. What is the most appropriate next step in management?





Explanation

An ADI > 3 mm in adults indicates C1-C2 instability. Because this patient is symptomatic (myelopathy/headaches) and her ADI is > 5 mm, she is at high risk for catastrophic neurologic compromise. A posterior C1-C2 fusion is indicated. Occipitocervical fusion is typically reserved for cranial settling (vertical migration).

Question 27

A 5-year-old boy presents with bone pain, petechiae, and swollen gums. Radiographs of the knee show a dense zone of provisional calcification (white line of Frankel) and a radiolucent zone adjacent to the physis (Trummerfeld zone). The pathogenesis of this condition is due to a failure in which of the following processes?





Explanation

The clinical and radiographic findings describe Scurvy (Vitamin C deficiency). Vitamin C is a required cofactor for prolyl hydroxylase and lysyl hydroxylase, which are responsible for the hydroxylation of proline and lysine residues during collagen synthesis. This defect weakens the structural integrity of collagen.

Question 28

A 68-year-old man presents with severe back pain. Radiographs demonstrate multiple punched-out lytic lesions in the vertebral bodies and skull. Laboratory tests reveal hypercalcemia and anemia. Which of the following is the most appropriate next step to confirm the diagnosis of this condition?





Explanation

The patient has classic signs of Multiple Myeloma (CRAB: hyperCalcemia, Renal involvement, Anemia, Bone lytic lesions). Diagnosis is confirmed with serum/urine protein electrophoresis (demonstrating an M-spike) and bone marrow biopsy showing >10% clonal plasma cells. A Tc-99m bone scan is typically 'cold' and unhelpful due to absent osteoblastic activity.

Question 29

A 12-year-old boy with severe Hemophilia A presents with chronic knee swelling and restricted range of motion. Radiographs show epiphyseal overgrowth and joint space narrowing. Which of the following is the primary mechanism of cartilage destruction in hemophilic arthropathy?





Explanation

Hemophilic arthropathy is driven by recurrent hemarthroses. Blood in the joint leads to the accumulation of iron/hemosiderin in the synovium and cartilage, which directly stimulates synovial hypertrophy, inflammatory cytokine release, and iron-mediated chondrocyte apoptosis.

Question 30

A 4-year-old girl is evaluated for bowing of the legs. Laboratory evaluation reveals normal serum calcium, low serum phosphate, and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. Which of the following is the most appropriate medical treatment for this patient?





Explanation

The patient has X-linked hypophosphatemic rickets (XLHR), caused by a PHEX mutation leading to elevated FGF-23, which wastes phosphate in the kidneys. The classic treatment requires oral phosphate repletion combined with active Vitamin D (calcitriol) to prevent secondary hyperparathyroidism. Burosumab is a newer specific therapy, but among standard options, phosphate plus calcitriol is correct.

Question 31

Demineralized bone matrix (DBM) is commonly used as a bone graft substitute in spine fusion surgery. Which of the following best describes the biological properties of DBM?





Explanation

DBM provides a structural scaffold (osteoconductive) and contains retained bone morphogenetic proteins (BMPs) that stimulate bone formation (osteoinductive). Because it is sterilized and processed, it lacks live cells, so it is not osteogenic. Autograft is the only graft type that possesses all three properties.

Question 32

An orthopedic surgeon prescribes rivaroxaban for venous thromboembolism prophylaxis following a total hip arthroplasty. What is the mechanism of action of this medication?





Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that work by specifically inhibiting Factor Xa. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin inhibits vitamin K epoxide reductase. Heparin/LMWH acts via antithrombin III.

Question 33

A patient sustains a closed midshaft humerus fracture resulting in a radial nerve palsy. Electromyography (EMG) at 4 weeks shows fibrillation potentials. According to the Sunderland classification, a third-degree peripheral nerve injury is characterized by the disruption of which structures?





Explanation

In the Sunderland classification: 1st degree is Neuropraxia (focal block). 2nd degree is Axonotmesis (axon disrupted, endoneurium intact). 3rd degree involves disruption of the axon and endoneurium, but the perineurium remains intact. 4th degree adds perineurium disruption. 5th degree is complete transection (Neurotmesis).

Question 34

A 6-year-old child presents to the emergency department with hip pain and a limp. He refuses to bear weight. His temperature is 38.6°C (101.5°F). Labs show WBC 13,000/mm3, ESR 45 mm/hr, and CRP 2.5 mg/dL. According to the Kocher criteria, what is the probability that this child has septic arthritis rather than transient synovitis?





Explanation

The Kocher criteria for pediatric septic arthritis of the hip are: 1) Non-weight bearing, 2) Temperature > 38.5°C, 3) ESR > 40 mm/hr, 4) WBC > 12,000/mm3. This patient meets all 4 criteria. The probability of septic arthritis is approximately 99% when all 4 criteria are present. (3 criteria = 93%, 2 criteria = 40%, 1 criterion = 3%).

Question 35

A 2-year-old girl with blue sclerae, dentinogenesis imperfecta, and multiple prior fractures is diagnosed with Osteogenesis Imperfecta (OI). According to the Sillence classification, which type of OI is typically lethal in the perinatal period?





Explanation

Sillence Classification of OI: Type I is the most common and mildest form. Type II is lethal in the perinatal period due to severe pulmonary complications and in utero fractures. Type III is the most severe non-lethal form, leading to progressive deformity. Type IV is intermediate in severity.

Question 36

A 65-year-old man presents with an acutely swollen, red, and painful knee. Aspiration yields cloudy yellow fluid with a WBC count of 45,000 cells/mm3. Polarized light microscopy reveals rhomboid-shaped crystals that are weakly positively birefringent. What is the composition of these crystals?





Explanation

Rhomboid-shaped, weakly positively birefringent crystals are characteristic of pseudogout, which is caused by the deposition of calcium pyrophosphate dihydrate (CPPD) crystals in the joint. Monosodium urate crystals (gout) are needle-shaped and strongly negatively birefringent.

Question 37

A 20-year-old man presents with a 6-month history of a dull, aching thigh pain that is significantly worse at night and dramatically relieved by taking ibuprofen. Radiographs demonstrate a diaphyseal cortical thickening with a small radiolucent nidus. The relief of pain by NSAIDs in this condition is due to the inhibition of which molecule highly produced by the nidus?





Explanation

The clinical and radiographic presentation is classic for an osteoid osteoma. The nidus of an osteoid osteoma uniquely produces high levels of prostaglandins, particularly PGE2, which cause the intense nighttime pain. NSAIDs inhibit cyclooxygenase (COX), rapidly reducing PGE2 synthesis and providing dramatic pain relief.

Question 38

During a physical therapy session, a patient is performing a quadriceps strengthening exercise on a specialized machine that provides accommodating resistance to maintain a constant angular velocity of the knee joint throughout the entire range of motion. What type of muscle contraction is this?





Explanation

Isokinetic contractions occur when a muscle contracts at a constant velocity throughout the range of motion. Because mechanical advantage changes with joint angle, maintaining a constant velocity requires specialized equipment (like a Cybex machine) that provides accommodating resistance. Isometric equals constant length; Isotonic equals constant tension.

Question 39

A 45-year-old immigrant presents with a severe, unilateral lower extremity deformity characterized by a flail limb, fixed equinus, and profound muscle atrophy. Sensation in the limb is completely normal. He reports having a severe febrile illness followed by paralysis as a child. What is the primary pathologic target of the infectious agent responsible for this patient's condition?





Explanation

The clinical picture of asymmetric flaccid paralysis with intact sensation from a childhood illness suggests Poliomyelitis. The poliovirus specifically targets and destroys the anterior horn cells (motor neurons) in the spinal cord, leading to pure lower motor neuron signs without any sensory deficits.

Question 40

In total joint arthroplasty, the phenomenon of 'stress shielding' occurs when an implant carries a disproportionate share of the mechanical load, leading to resorption of the surrounding bone. Comparing common orthopedic implant materials, which of the following characteristics makes Titanium alloys less prone to causing stress shielding than Cobalt-Chromium alloys?





Explanation

Titanium alloys have a lower modulus of elasticity compared to Cobalt-Chromium (CoCr) or stainless steel, making Titanium more flexible and mechanically closer to the stiffness of cortical bone. This similarity allows for better load sharing with the host bone, thereby reducing stress shielding and subsequent osteopenia.

Question 41

To improve the longevity of ultra-high-molecular-weight polyethylene (UHMWPE) bearings in total joint arthroplasty, cross-linking is performed followed by either remelting, annealing, or Vitamin E doping. What is the primary biomechanical advantage of incorporating Vitamin E (alpha-tocopherol) compared to the remelting process?





Explanation

Remelting eliminates free radicals created during irradiation but alters the crystalline structure, which reduces fatigue strength and crack propagation resistance. Vitamin E scavenges free radicals without the need for thermal remelting, thereby preventing oxidation while preserving the polymer's fatigue strength and mechanical properties.

Question 42

A 25-year-old male presents with multiple cartilaginous tumors of the hands and long bones, as well as several bluish, soft-tissue masses on his extremities. Radiographs demonstrate multiple enchondromas. What genetic mutation is most strongly associated with this patient's underlying syndrome?





Explanation

The patient has Maffucci syndrome, characterized by multiple enchondromas and soft tissue hemangiomas. Both Ollier disease and Maffucci syndrome are strongly associated with somatic mosaic mutations in the IDH1 and IDH2 (isocitrate dehydrogenase) genes. EXT1/EXT2 mutations are seen in Multiple Hereditary Exostoses. GNAS1 is associated with fibrous dysplasia.

Question 43

During an anterior intrapelvic (modified Stoppa) approach for an acetabular fracture, the surgeon must identify and protect the 'corona mortis' to prevent catastrophic hemorrhage. This vascular structure typically represents an anastomosis between which two vessel systems?





Explanation

The corona mortis (crown of death) is a vascular anastomosis between the external iliac or inferior epigastric vessels and the obturator vessels. It is located over the superior pubic ramus, typically 4 to 9 cm from the pubic symphysis, and is at high risk of iatrogenic injury during anterior pelvic surgical approaches.

Question 44

In a patient with advanced cervical spondylotic myelopathy, which of the following magnetic resonance imaging (MRI) findings of the spinal cord is most highly predictive of a poor potential for clinical recovery following surgical decompression?





Explanation

Focal T2 hyperintensity in the spinal cord can indicate transient edema, inflammation, or early myelomalacia. However, when combined with T1 hypointensity, it indicates established cystic myelomalacia, necrosis, and permanent spinal cord damage. This combination serves as a strong negative predictor for neurologic recovery post-decompression.

Question 45

A 3-month-old female with developmental dysplasia of the hip (DDH) has been treated in a Pavlik harness for two weeks. During a follow-up exam, she demonstrates an absent patellar reflex and lacks active knee extension on the treated side. What is the most appropriate next step in management?





Explanation

The patient has developed a femoral nerve palsy, a known complication of excessive hip flexion in a Pavlik harness. The appropriate management is to loosen the anterior straps (decreasing flexion) or temporarily remove the harness if the palsy does not quickly resolve. Excessive abduction (tight posterior straps) can lead to avascular necrosis of the femoral head.

Question 46

A patient sustains a severe laceration to the ulnar nerve at the level of the medial epicondyle. Upon examination, the patient has completely preserved function of the first dorsal interosseous muscle and adductor pollicis, despite profound ulnar sensory loss in the hand. What anatomical variant best explains this clinical picture?





Explanation

The Martin-Gruber anastomosis is a communicating neural branch carrying motor fibers from the median nerve (or anterior interosseous nerve) to the ulnar nerve in the proximal forearm. In patients with this variant, a high ulnar nerve lesion (e.g., at the elbow) may spare ulnar-innervated intrinsic hand muscles because their motor supply bypassed the elbow via the median nerve.

Question 47

When comparing structural properties of various grafts used for anterior cruciate ligament (ACL) reconstruction, which of the following grafts exhibits the highest initial ultimate tensile load prior to physiological incorporation?





Explanation

Biomechanical studies demonstrate that the quadrupled hamstring graft has the highest ultimate tensile load (~4140 N). A 10-mm BPTB has a tensile strength of ~2977 N. A 10-mm quadriceps tendon is ~2185 N. The native, intact ACL has an ultimate tensile load of approximately 2160 N. Despite these differences in initial strength, long-term clinical stability is generally comparable.

Question 48

A 55-year-old female presents with stage IIB adult acquired flatfoot deformity (posterior tibial tendon dysfunction). Her deformity is flexible but demonstrates >30% uncoverage of the talar head on AP foot radiographs. Along with a flexor digitorum longus (FDL) transfer, which of the following bony procedures is most biomechanically appropriate to address her specific multiplanar deformity?





Explanation

Stage IIB adult acquired flatfoot deformity is characterized by a flexible deformity with significant forefoot abduction (>30% talonavicular uncoverage). A medial displacement calcaneal osteotomy (MDCO) primarily addresses hindfoot valgus, but a lateral column lengthening is necessary to correct the severe forefoot abduction. Combined MDCO and lateral column lengthening (often with FDL transfer) is standard for Stage IIB.

Question 49

In total hip arthroplasty (THA), accurately restoring the hip center of rotation is critical for optimizing biomechanics. Compared to a superiorly and laterally placed center of rotation, moving the center of rotation medially and inferiorly achieves which of the following effects?





Explanation

Placing the acetabular component medially and inferiorly (anatomic position) increases the abductor moment arm and decreases the body weight moment arm. According to biomechanical equilibrium, a larger abductor moment arm decreases the required abductor force, which in turn significantly decreases the overall joint reaction force across the hip.

Question 50

Bone morphogenetic proteins (BMPs) play a crucial role in osteoinduction and fracture healing. They act by binding to specific cell-surface receptors. Which of the following describes the primary intracellular signaling pathway activated directly by BMPs?





Explanation

BMPs belong to the TGF-beta superfamily. They bind to Type I and Type II serine/threonine kinase receptors. Upon binding, these receptors phosphorylate receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These subsequently complex with Co-Smad (Smad 4) and translocate to the nucleus to initiate osteogenic gene transcription.

Question 51

During a posteromedial approach to the proximal tibia for fixation of a complex Schatzker IV plateau fracture, the surgeon develops the primary deep interval to expose the posterior aspect of the medial tibial condyle. Which two structures define this standard internervous/anatomical interval?





Explanation

The classic posteromedial approach to the tibial plateau utilizes the interval between the medial head of the gastrocnemius (which is retracted posterolaterally to protect the neurovascular bundle) and the pes anserinus / semimembranosus (which is retracted anteromedially). This safely exposes the posteromedial cortex of the proximal tibia.

Question 52

A 12-year-old gymnast presents with back pain and a grade II isthmic spondylolisthesis at L5-S1. Radiographic spinopelvic parameters are measured. Which of the following parameters is a fixed, morphological characteristic of the pelvis that strongly correlates with the magnitude of shear stress at the lumbosacral junction and the risk of slip progression?





Explanation

Pelvic incidence (PI) is a fixed anatomical parameter unique to each individual, defined as the angle between a line perpendicular to the sacral endplate at its midpoint and a line connecting this point to the axis of the femoral heads. PI = PT + SS. A high pelvic incidence correlates with a steeper sacral slope, leading to higher shear forces at L5-S1 and predisposing to isthmic spondylolisthesis progression.

Question 53

In Scaphoid Nonunion Advanced Collapse (SNAC), the pattern of degenerative arthritis predictably progresses through specific stages. Unlike Scapholunate Advanced Collapse (SLAC), the radiolunate joint is typically spared in SNAC wrists. What biomechanical mechanism accounts for the preservation of the radiolunate joint in a SNAC wrist?





Explanation

In a SNAC wrist, the proximal pole of the scaphoid remains firmly attached to the lunate via the intact scapholunate (SL) ligament. This maintains the lunate in a relatively extended but congruous position within the spherical lunate fossa of the radius, thereby preserving the radiolunate joint. The distal scaphoid fragment flexes, leading to progressive arthritis at the radioscaphoid and midcarpal joints.

Question 54

A 28-year-old female undergoes a biopsy for a slowly enlarging, calcified soft-tissue mass situated adjacent to the knee joint. Histology demonstrates a biphasic pattern of spindle cells and epithelial cells. Cytogenetic analysis confirms a t(X;18)(p11;q11) translocation. What is the most likely diagnosis?





Explanation

Synovial sarcoma classically presents in young adults as a juxta-articular mass. It frequently shows calcifications on radiographs (~30%). Histologically it can be biphasic (spindle and epithelial components) or monophasic. The pathognomonic cytogenetic abnormality is the t(X;18) translocation, resulting in the SYT-SSX fusion gene.

Question 55

A 24-year-old elite volleyball player presents with vague posterior shoulder pain and weakness in external rotation. MRI reveals isolated atrophy of the teres minor. Compression of the neurovascular bundle in the quadrilateral space is suspected. Which of the following forms the inferior boundary of this anatomical space?





Explanation

The quadrilateral space is bordered superiorly by the teres minor, inferiorly by the teres major, medially by the long head of the triceps, and laterally by the surgical neck of the humerus. It contains the axillary nerve and the posterior circumflex humeral artery. Compression here leads to quadrilateral space syndrome.

Question 56

A 40-year-old male sustains a 'terrible triad' injury of the elbow (elbow dislocation, radial head fracture, and coronoid fracture). During surgical reconstruction, after fixation of the coronoid and replacement of the comminuted radial head, the elbow remains unstable in extension and supination. What is the next most appropriate step in the surgical sequence?





Explanation

The standard algorithm for a terrible triad injury works from deep to superficial and typically involves restoring the anterior buttress (coronoid), restoring the radiocapitellar contact (radial head fix/replace), and then restoring the lateral stabilizing structures by repairing the LUCL to the lateral epicondyle. If the elbow remains unstable after LUCL repair, MCL repair or a hinged external fixator is considered.

Question 57

Osteogenesis imperfecta (OI) is a genetic connective tissue disorder characterized by bone fragility. The majority of OI cases are caused by mutations in the COL1A1 or COL1A2 genes. At the molecular level, what is the primary consequence of these mutations?





Explanation

OI is primarily caused by mutations in COL1A1 or COL1A2. The most common pathogenic mechanism is a single-base substitution that replaces a crucial glycine residue (the smallest amino acid, which fits inside the helix core) with a bulkier amino acid. This disrupts the tight coiling of the Type I collagen triple helix, leading to structurally abnormal or severely depleted Type I collagen.

Question 58

The Lisfranc ligament is essential for the stability of the tarsometatarsal joint complex. Which of the following accurately describes the precise anatomical origin and insertion of this critical ligament?





Explanation

The Lisfranc ligament is an intra-articular ligament that extends from the plantar-lateral aspect of the medial cuneiform to the plantar-medial base of the second metatarsal. There is no direct ligamentous connection between the bases of the first and second metatarsals, making the Lisfranc ligament the critical primary stabilizer linking the medial and middle columns.

Question 59

During a posterior-stabilized total knee arthroplasty (TKA), trial components are placed. The surgeon notes that the knee is well-balanced in extension, but the flexion gap is unacceptably tight (lift-off and difficult to flex past 90 degrees). Which of the following intraoperative maneuvers is most appropriate to resolve this specific imbalance?





Explanation

A tight flexion gap with a balanced extension gap means the posterior aspect of the femoral component is too prominent or the tibia is lacking slope. Options to increase the flexion gap without affecting the extension gap include downsizing the femoral component (with anterior referencing, this moves the posterior condylar cut anteriorly) or increasing the posterior slope of the tibial cut. Resecting more distal femur or releasing the posterior capsule affects the extension gap.

Question 60

A 22-year-old male hockey player presents with groin pain exacerbated by hip flexion and internal rotation. Radiographs reveal an aspherical femoral head-neck junction with an alpha angle of 70 degrees. During hip arthroscopy for this 'cam-type' femoroacetabular impingement (FAI), where is the characteristic articular cartilage damage (chondral delamination) most likely to be found?





Explanation

Cam FAI is caused by an abnormally shaped femoral head-neck junction (alpha angle >55 degrees) engaging the acetabulum during flexion and internal rotation. This creates massive shear forces at the anterosuperior acetabular rim, classically leading to chondral delamination from the subchondral bone, peeling inward from the labro-chondral junction in the anterosuperior quadrant.

Question 61

A 45-year-old male presents with a painful, lytic lesion in the proximal humeral epiphysis. Biopsy reveals cells with abundant clear cytoplasm, distinct borders, and centrally located nuclei scattered among regions of hyaline cartilage. What is the most appropriate definitive management for this lesion?





Explanation

The clinical and histological presentation is highly characteristic of clear cell chondrosarcoma. Unlike chondroblastoma (which also occurs in the epiphysis but typically affects patients under 20 and is treated with curettage), clear cell chondrosarcoma is a low-grade malignant tumor. It is characteristically resistant to chemotherapy and radiation. The standard of care to prevent local recurrence and metastasis is wide surgical resection.

Question 62

A 22-year-old female presents with persistent back pain that frequently awakens her at night and is only partially relieved by NSAIDs. Imaging shows a 2.5 cm expansile, lytic lesion in the posterior elements of L3 with scattered calcifications and surrounding sclerosis. Which of the following is the most likely diagnosis?





Explanation

Osteoblastomas are benign, bone-forming tumors that are histologically similar to osteoid osteomas but are distinguished by being larger than 2 cm in diameter. They frequently occur in the posterior elements of the spine in young adults. Unlike osteoid osteomas, the pain associated with osteoblastoma is often less reliably relieved by NSAIDs and salicylates.

Question 63

A 4-year-old boy presents with severe bowing of the lower extremities. Laboratory tests reveal normal serum calcium, abnormally low serum phosphate, and elevated alkaline phosphatase. Genetic testing identifies a mutation in the PHEX gene. Which of the following best describes the pathophysiology of this condition?





Explanation

The patient has X-linked hypophosphatemic rickets (XLHR), the most common form of heritable rickets. A mutation in the PHEX gene leads to decreased degradation and subsequent overproduction of Fibroblast Growth Factor 23 (FGF23). Elevated FGF23 decreases renal tubular reabsorption of phosphate (causing phosphate wasting) and inhibits 1-alpha-hydroxylase, decreasing active Vitamin D levels.

Question 64

A surgeon is revising a femur fracture nonunion originally fixed with a 316L stainless steel plate. The surgeon plans to add a lag screw across the nonunion site using a titanium screw. Which of the following types of corrosion is most likely to occur if these two distinct metals are in direct contact in vivo?





Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (such as stainless steel and titanium) are placed in direct physical contact within a conductive fluid medium (like body fluids). The less noble metal becomes the anode and corrodes at an accelerated rate. Mixing metals in orthopedic implants should generally be avoided to prevent this complication.

Question 65

A 14-year-old boy from Connecticut presents with an atraumatic, swollen, and mildly painful right knee. Aspiration yields a WBC count of 45,000 cells/mm3 with 85% neutrophils. Serology and PCR confirm Lyme arthritis. Assuming no neurological or cardiac involvement, what is the recommended initial management?





Explanation

Lyme arthritis (late-stage disease caused by Borrelia burgdorferi) presenting without neurologic involvement is treated medically, not surgically. The first-line treatment is a 28-day course of oral doxycycline or amoxicillin. IV ceftriaxone is reserved for cases that are refractory to oral antibiotics or those presenting with neurological or severe cardiac manifestations.

Question 66

A 65-year-old female with long-standing rheumatoid arthritis is undergoing preoperative evaluation for a total hip arthroplasty. Flexion-extension cervical spine radiographs reveal an anterior atlantodens interval (ADI) of 11 mm and a posterior atlantodens interval (PADI) of 13 mm. She is neurologically intact. What is the most appropriate management regarding her cervical spine?





Explanation

In patients with rheumatoid arthritis, an anterior atlantodens interval (ADI) > 9-10 mm or a posterior atlantodens interval (PADI, or space available for the cord) < 14 mm are strong indications for surgical stabilization (C1-C2 fusion) due to the high risk of catastrophic neurological injury, even if currently asymptomatic. The hip arthroplasty should be postponed until the cervical spine is stabilized.

Question 67

Increasing the radiation dose during the cross-linking process of ultra-high molecular weight polyethylene (UHMWPE) for total joint arthroplasty has which of the following mechanical effects?





Explanation

Highly cross-linked polyethylene is manufactured by exposing UHMWPE to radiation. While this significantly increases wear resistance (decreasing volumetric wear and lowering the risk of osteolysis), it concomitantly alters the mechanical properties, leading to a reduction in ultimate tensile strength, yield strength, and fatigue strength. Remelting is subsequently used to reduce free radicals and prevent oxidative degradation.

Question 68

A 12-year-old boy presents with a painful, swollen thigh. Radiographs show a permeative diaphyseal lesion with a periosteal 'onion-skin' reaction. Biopsy reveals small, round blue cells that are heavily positive for CD99. Which of the following chromosomal translocations is most characteristic of this tumor?





Explanation

Ewing sarcoma is a small round blue cell tumor characteristically positive for CD99 (MIC2). The most common genetic mutation driving the tumor is the t(11;22)(q24;q12) translocation, which results in the EWS-FLI1 fusion protein. t(X;18) is associated with synovial sarcoma; t(12;16) with myxoid liposarcoma; t(2;13) with alveolar rhabdomyosarcoma; and t(9;22) with CML.

Question 69

According to Perren's strain theory of fracture healing, what is the maximum interfragmentary strain that will still permit primary (osteonal/contact) bone healing?





Explanation

Perren's strain theory dictates that primary (direct or osteonal) bone healing occurs when absolute stability is achieved. This requires an interfragmentary strain of less than 2%. Strains between 2% and 10% tolerate the formation of cartilage (secondary bone healing via callus formation), while strains above 10% result in granulation tissue and eventual nonunion.

Question 70

A patient undergoes a primary total hip arthroplasty via a posterior approach. Postoperatively, the patient experiences recurrent posterior dislocations. Radiographic evaluation shows that the acetabular component is placed in 5 degrees of anteversion. Which of the following best explains the etiology of this instability?





Explanation

The normal 'safe zone' for acetabular cup anteversion is approximately 15 to 20 degrees. A cup placed in 5 degrees of anteversion is functionally retroverted, which significantly predisposes the patient to posterior dislocation, particularly with hip flexion and internal rotation.

Question 71

Which of the following lower extremity amputations is associated with the highest percentage increase in metabolic energy expenditure during walking compared to a normal physiological baseline?





Explanation

Energy expenditure during gait increases significantly as the level of amputation moves proximally, and is higher in dysvascular patients than traumatic patients. A dysvascular transfemoral amputation increases energy expenditure by up to 100-120%, whereas traumatic transfemoral is ~60%, dysvascular transtibial is ~40-50%, and traumatic transtibial is ~25%.

Question 72

During the proximal extension of the volar (Henry) approach to the forearm, the surgeon develops an internervous plane. Which of the following correctly identifies the muscles and their respective nerve supplies forming this proximal interval?





Explanation

The proximal portion of the volar (Henry) approach to the radius utilizes the true internervous plane between the brachioradialis (innervated by the radial nerve) and the pronator teres (innervated by the median nerve). Distally, the plane shifts between the brachioradialis and the flexor carpi radialis.

Question 73

A 5-year-old child with a history of multiple low-energy fractures, blue sclerae, and dentinogenesis imperfecta is diagnosed with osteogenesis imperfecta. This condition is most commonly caused by a genetic mutation leading to the substitution of which specific amino acid in the collagen triple helix?





Explanation

Osteogenesis imperfecta is typically caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes. The most common defect is a missense mutation that substitutes a bulky amino acid for glycine in the repeating Gly-X-Y sequence of the type I collagen triple helix, destabilizing the entire structure.

Question 74

A 32-year-old female presents with knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy shows multinucleated giant cells in a stroma of mononuclear cells. The mononuclear cells express which of the following crucial factors driving the pathogenesis of this tumor?





Explanation

In a giant cell tumor (GCT) of bone, the true neoplastic cells are the mononuclear stromal cells. These cells overexpress Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL), which binds to RANK on osteoclast precursors, recruiting and activating the multinucleated, reactive osteoclast-like giant cells that cause bone resorption. This is the mechanism targeted by Denosumab.

Question 75

According to the Musculoskeletal Infection Society (MSIS) criteria for periprosthetic joint infection (PJI), which of the following is considered a 'major' criterion, sufficient to definitively establish a diagnosis of PJI on its own?





Explanation

According to the MSIS criteria, there are two major criteria for definitive diagnosis of PJI: 1) A sinus tract communicating with the prosthesis, or 2) A pathogen isolated by culture from at least two separate tissue or fluid samples. The other options are minor criteria.

Question 76

A 28-year-old male is brought to the ED after a severe crush injury. Radiographs show a widened pubic symphysis of 3.5 cm and widening of the right sacroiliac joint anteriorly and posteriorly. This corresponds to a Young-Burgess Anteroposterior Compression (APC) Type III injury. Which of the following ligamentous structures is disrupted in an APC III injury but typically remains intact in an APC II injury?





Explanation

In the Young-Burgess classification, an APC I injury involves only symphyseal widening (< 2.5 cm). APC II involves rupture of the symphysis, anterior sacroiliac ligaments, sacrotuberous, and sacrospinous ligaments, but the posterior sacroiliac ligaments remain intact (opening book). APC III implies a complete disruption, including the robust posterior sacroiliac ligaments, leading to full hemipelvis instability.

Question 77

In the biomechanics of articular cartilage, which lubrication mechanism is characterized by the elastic deformation of the articular surfaces under load, thereby increasing the surface area and trapping a pressurized fluid film between them?





Explanation

Elastohydrodynamic lubrication occurs when the load is high enough to cause elastic deformation of the compliant articular cartilage. This deformation increases the contact surface area and alters the geometry, trapping a fluid film (synovial fluid) that separates the surfaces, thereby protecting them during rapid motion under high loads.

Question 78

A 68-year-old male presents with deep, aching thigh pain and reports that his hat size has increased over the past few years. Radiographs of the femur reveal cortical thickening, coarsened trabeculae, and a prominent bowing deformity. Histological examination of the affected bone would most likely demonstrate which of the following findings?





Explanation

The clinical picture is classic for Paget's disease of bone (osteitis deformans). The hallmark histological finding is a 'mosaic' or 'jigsaw puzzle' pattern of lamellar bone created by prominent, haphazard cement lines due to erratic and uncoupled osteoclast resorption and subsequent frantic osteoblast bone formation.

Question 79

Following a closed humerus shaft fracture, a patient develops a complete radial nerve palsy. Electromyography (EMG) at 4 weeks shows fibrillation potentials in the brachioradialis. According to Seddon's classification, the nerve injury is determined to be an axonotmesis. Which of the following characterizes this specific type of nerve injury?





Explanation

In Seddon's classification, Neuropraxia is a focal conduction block (myelin injury) without axonal disruption (recovers quickly). Axonotmesis involves disruption of the axon and myelin sheath, leading to Wallerian degeneration distally, but the connective tissue tubes (endoneurium, perineurium, epineurium) remain partially or fully intact, allowing for targeted axonal regeneration. Neurotmesis is complete nerve transection.

Question 80

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





Explanation

Tranexamic acid is a synthetic analog of the amino acid lysine. It functions as an antifibrinolytic by reversibly and competitively binding to the lysine receptor sites on plasminogen. This prevents plasminogen from binding to fibrin, thereby inhibiting its activation to plasmin and preventing the degradation of fibrin clots.

Question 81

A 45-year-old patient undergoes an Achilles tendon repair. During rehabilitation, the physical therapist applies a constant load to the tendon over time, resulting in continued progressive deformation of the tendon. Which biomechanical property is described?





Explanation

Creep is defined as the progressive deformation of a material under a constant load over time. Viscoelastic materials, such as tendons and ligaments, exhibit this property. Stress relaxation, conversely, is the decrease in stress over time when a material is held at a constant strain (deformation).

Question 82

A 65-year-old male presents with severe back pain. Radiographs show multiple punched-out lytic lesions in the skull and a compression fracture at L2. Laboratory workup reveals hypercalcemia and anemia. What is the most likely diagnosis?





Explanation

Multiple myeloma is the most common primary malignancy of bone in adults. It classically presents with punched-out lytic lesions (without a sclerotic rim), hypercalcemia, renal failure, anemia, and bone pain. Prostate cancer typically presents with blastic metastatic lesions.

Question 83

Following a closed humerus fracture, a patient develops a radial nerve palsy. An EMG performed at 4 weeks shows fibrillation potentials but no motor unit action potentials. The nerve injury involves disruption of the axons and myelin sheath, but the endoneurium, perineurium, and epineurium remain intact. Which Sunderland grade corresponds to this injury?





Explanation

A Sunderland Grade II injury corresponds to axonotmesis where the axon and myelin sheath are disrupted, but the endoneurial tubes, perineurium, and epineurium are intact. Recovery is generally complete as the intact endoneurium guides regenerating axons at a rate of 1 mm/day.

Question 84

A 72-year-old man presents with increasing head size (reports needing a larger hat) and deep aching bone pain in his right femur. Laboratory testing shows a significantly elevated alkaline phosphatase but normal calcium and phosphorus. Histological examination of the bone is most likely to show:





Explanation

Paget disease (osteitis deformans) is characterized by increased, disorganized bone turnover. The hallmark histologic finding is a 'mosaic pattern' of lamellar bone with irregular, prominent cement lines indicating haphazard bone resorption and formation.

Question 85

A 70-year-old female with severe osteoporosis and multiple vertebral compression fractures is started on teriparatide. Which of the following best describes the mechanism of action of this medication?





Explanation

Teriparatide is a recombinant parathyroid hormone (PTH 1-34) analog. When administered intermittently (daily subcutaneous injection), it has an anabolic effect, directly stimulating osteoblastic bone formation. Continuous endogenous PTH, however, leads to bone resorption. Bisphosphonates inhibit farnesyl pyrophosphate synthase, and denosumab binds RANKL.

Question 86

A 19-year-old male complains of severe, unrelenting right thigh pain that is distinctly worse at night and dramatically relieved by NSAIDs. Radiographs show a small radiolucent nidus surrounded by dense sclerotic bone in the proximal femur. Which of the following substances is produced in high amounts by the cells within the nidus?





Explanation

The clinical presentation is classic for an osteoid osteoma. The nidus contains osteoblasts that produce high levels of Prostaglandin E2 (PGE2), which mediates the severe nocturnal pain and explains the dramatic response to NSAIDs (cyclooxygenase inhibitors).

Question 87

In the context of orthopedic implants, which of the following material properties describes the ability to absorb energy and plastically deform prior to failure?





Explanation

Toughness is represented by the total area under the stress-strain curve and indicates the amount of energy a material can absorb before failure. It incorporates both the strength and ductility of the material.

Question 88

The formation of a biofilm on an orthopedic implant makes eradication of infection extremely difficult. Which of the following is the primary constituent of the extracellular polymeric substance (EPS) matrix that protects the bacteria in a biofilm?





Explanation

The extracellular polymeric substance (EPS) or 'slime' layer of a biofilm is composed primarily of polysaccharides (along with proteins, extracellular DNA, and lipids). This matrix protects the bacteria from host immune responses and restricts the penetration of antimicrobial agents.

Question 89

A 42-year-old man presents with dactylitis of his index finger, nail pitting, and asymmetric oligoarthritis. Radiographs of his hands reveal marginal erosions with adjacent bone proliferation, described as a 'pencil-in-cup' deformity. He is negative for rheumatoid factor. Which HLA association is most commonly linked to this patient's condition?





Explanation

The patient has psoriatic arthritis, characterized by dactylitis, nail pitting, and 'pencil-in-cup' deformities. It is a seronegative spondyloarthropathy associated with the HLA-B27 allele. HLA-DR4 is associated with Rheumatoid Arthritis.

Question 90

A 60-year-old male with a history of a metal-on-metal total hip arthroplasty presents with groin pain and a palpable mass. Workup reveals an adverse local tissue reaction (ALTR) or pseudotumor. Which type of hypersensitivity reaction is primarily responsible for this pathology?





Explanation

Adverse local tissue reactions (ALTR) or pseudotumors associated with metal-on-metal implants are characterized pathologically by an ALVAL (aseptic lymphocytic vasculitis-associated lesion) response, which is a Type IV (delayed, T-cell mediated) hypersensitivity reaction to metal ions (cobalt and chromium).

Question 91

A 28-year-old female presents with a slowly enlarging, deep soft tissue mass near her knee joint. Biopsy reveals a biphasic tumor with both epithelial and spindle cell components. Cytogenetic analysis shows a t(X;18)(p11;q11) translocation. What is the diagnosis?





Explanation

Synovial sarcoma classically occurs in young adults, near (but rarely within) large joints like the knee. It often exhibits a biphasic histology (epithelial and spindle cells) and is uniquely associated with the t(X;18)(p11;q11) chromosomal translocation, resulting in the SYT-SSX fusion gene.

Question 92

A 45-year-old female presents with diffuse bone pain and muscle weakness. Laboratory studies reveal low serum calcium, low serum phosphate, elevated alkaline phosphatase, and elevated parathyroid hormone (PTH). Radiographs show pseudofractures (Looser zones) in the femoral neck. What is the most likely diagnosis?





Explanation

The clinical and laboratory profile (low calcium, low phosphate, high ALP, high PTH) along with Looser zones (pseudofractures) is classic for osteomalacia, typically due to severe vitamin D deficiency. Secondary hyperparathyroidism occurs as a physiological response to the hypocalcemia.

Question 93

Following a total knee arthroplasty, a patient is prescribed rivaroxaban for deep vein thrombosis prophylaxis. What is the specific mechanism of action of this medication?





Explanation

Rivaroxaban and apixaban are direct Factor Xa inhibitors. Dabigatran is a direct thrombin (IIa) inhibitor. Warfarin inhibits vitamin K epoxide reductase. Heparin and LMWH enhance antithrombin III activity.

Question 94

A surgeon revises a femur fracture initially treated with a stainless steel plate and screws. Due to stripped screw holes, the surgeon replaces several screws with titanium screws, leaving the stainless steel plate in place. What biomechanical complication is most likely to occur?





Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in contact within a conductive medium (body fluid). The electrochemical potential difference causes the less noble metal to corrode rapidly. Mixing distinct metal types in implants should be avoided.

Question 95

A patient sustained a closed crush injury to the forearm 6 weeks ago. Electromyography (EMG) is performed to assess nerve recovery. Which of the following EMG findings indicates active denervation and a poor prognosis for spontaneous recovery if persistent?





Explanation

Fibrillation potentials and positive sharp waves at rest on an EMG indicate active denervation of the muscle (individual muscle fibers firing spontaneously without central nerve input). Polyphasic potentials and large amplitude MUAPs indicate reinnervation and collateral sprouting.

Question 96

Compared to normal walking, what is the approximate increase in energy expenditure required for a patient ambulating with a unilateral traumatic transfemoral (above-knee) amputation?





Explanation

Energy expenditure during gait increases as the level of amputation moves proximally. The approximate increases in energy expenditure are: unilateral Syme's (15%), unilateral transtibial (25-30%), unilateral transfemoral (60-65%), and bilateral transfemoral (>200%).

Question 97

A midshaft tibia fracture is treated with a cast. Callus formation is noted on radiographs at 4 weeks. Which type of collagen is the predominant structural component of the soft callus prior to its replacement by woven bone?





Explanation

Secondary bone healing occurs via endochondral ossification. During the soft callus phase, chondrocytes produce a cartilaginous matrix predominantly composed of Type II collagen. This is subsequently replaced by woven bone (Type I collagen) during the hard callus phase.

Question 98

A 30-year-old female presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femur extending to the subchondral bone without a sclerotic margin. Biopsy reveals multinucleated giant cells interspersed among mononuclear stromal cells. Denosumab is considered for treatment. What is the molecular target of this medication in this specific tumor?





Explanation

In Giant Cell Tumor of bone, the true neoplastic cells are the mononuclear stromal cells, which produce copious amounts of RANK Ligand (RANKL). This RANKL recruits normal macrophages and induces their fusion into osteoclast-like multinucleated giant cells. Denosumab binds and inhibits RANKL produced by the stromal cells.

Question 99

A 65-year-old male presents with acute swelling and severe pain in his right knee. Aspiration yields cloudy synovial fluid with a WBC count of 45,000 cells/mm3. Polarized light microscopy reveals rhomboid-shaped crystals with weak positive birefringence. What is the composition of these crystals?





Explanation

The clinical and microscopic findings are diagnostic of pseudogout (CPPD). The crystals are made of Calcium Pyrophosphate Dihydrate, which are classically rhomboid-shaped and exhibit weak positive birefringence under polarized light. Monosodium urate (gout) crystals are needle-shaped and strongly negatively birefringent.

Question 100

In normal articular cartilage, the organization of collagen fibers and chondrocytes varies by depth. Which zone is characterized by collagen fibers oriented perpendicular to the joint surface and chondrocytes arranged in columnar profiles?





Explanation

In the deep (radial) zone of articular cartilage, collagen fibers are oriented perpendicular to the articular surface, and chondrocytes are arranged in vertical columns. This structural arrangement provides maximal resistance to compressive forces. The superficial zone has collagen parallel to the surface to resist shear forces.

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