Which of the following prophylactic regimens for the prevention of deep venous thrombosis after knee arthroplasty has received a grade 1A recommendation in favor of its use from the American College of Chest Physicians (ACCP) in the 2004 guidelines?

Warfarin with a targeted international normalized ratio (INR) of 2.0 to 3.0 for 10 to 14 days

Figures 11a and 11b show the radiographs of a 50-year-old man who was struck by a car. Treatment should consist of

open reduction and internal fixation.

Fixed hyperextension of the metatarsophalangeal joint is associated with

dorsal subluxation of the interossei.

Free Orthopedics MCQ Review | Dr Hutaif General Orthopedics

Question 1

A 45-year-old male undergoes revision ORIF for a nonunion of the femur. The surgeon uses a stainless steel plate and titanium screws. Which of the following best describes the likely consequence of mixing these metals in vivo?

Explanation

Mixing dissimilar metals with different anodic/cathodic potentials in an electrolytic environment (body fluid) leads to galvanic corrosion. Stainless steel and titanium have different potentials, making the stainless steel more susceptible to anodic dissolution when coupled with titanium.

Question 2

What is the primary function of the superficial (tangential) zone of articular cartilage?

Explanation

The superficial (tangential) zone of articular cartilage consists of flattened chondrocytes and collagen fibers oriented parallel to the joint surface. Its primary biomechanical role is to resist shear forces. Compressive forces are primarily resisted by the middle and deep zones, where proteoglycan concentration is higher and collagen fibers are arranged vertically.

Question 3

A 65-year-old female presents with generalized bone pain and proximal muscle weakness. Laboratory studies reveal low serum calcium, low phosphorus, elevated alkaline phosphatase, and elevated parathyroid hormone (PTH). A bone biopsy would most likely show which of the following?

Explanation

The clinical and laboratory picture (low Ca, low PO4, high Alk Phos, high PTH) is classic for osteomalacia, often due to Vitamin D deficiency. Bone biopsy in osteomalacia typically shows increased unmineralized osteoid (wide osteoid seams) due to a defect in bone mineralization. 'Normal bone mineralization with decreased bone mass' describes osteoporosis, and a 'mosaic pattern' is indicative of Paget's disease.

Question 4

A pediatric patient is diagnosed with achondroplasia. Which of the following best describes the genetic mutation and its effect on the epiphyseal plate?

Explanation

Achondroplasia is caused by an autosomal dominant gain-of-function mutation in the FGFR3 gene. This leads to constitutive activation of the receptor, which abnormally inhibits chondrocyte proliferation in the proliferative zone of the physis, resulting in rhizomelic dwarfism.

Question 5

Which modification to a cortical screw design will most significantly increase its pullout strength?

Explanation

Pullout strength of a screw is directly related to the outer diameter, thread depth, and thread density (number of threads per inch), and the shear strength of the host material. Decreasing the thread pitch increases the number of threads per unit length (higher thread density), thereby increasing the pullout strength.

Question 6

A 15-year-old boy presents with knee pain. Radiographs reveal a mixed lytic and sclerotic lesion in the distal femoral metaphysis with a Codman triangle. A biopsy confirms conventional high-grade osteosarcoma. Which of the following is the most important prognostic factor for this patient?

Explanation

In conventional high-grade osteosarcoma, the degree of tumor necrosis after neoadjuvant chemotherapy is the most critical prognostic indicator. Greater than 90% necrosis indicates a good response and correlates strongly with improved long-term disease-free survival.

Question 7

A 32-year-old male sustains an open tibia fracture in a motorcycle crash. The wound measures 8 cm, involves extensive muscle stripping, and requires a free rotational flap for coverage. There is no major arterial injury. According to the Gustilo-Anderson classification, what is the grade of this injury?

Explanation

Gustilo-Anderson Type IIIB fractures are open fractures with extensive soft-tissue injury, periosteal stripping, and bone exposure, requiring a local or free flap for soft tissue coverage. Type IIIA can be covered primarily without a flap. Type IIIC involves an arterial injury requiring repair for limb salvage.

Question 8

Which of the following constructs promotes primary (direct) bone healing without callus formation?

Explanation

Primary (direct) bone healing occurs under conditions of absolute stability, which is achieved by interfragmentary compression. A lag screw and neutralization plate provide absolute stability, preventing micro-motion and allowing for Haversian remodeling without visible callus. Options like nails, bridge plates, external fixators, and casts provide relative stability, leading to secondary bone healing via callus formation.

Question 9

A 14-year-old girl presents with bilateral cavovarus foot deformities. Family history is positive for similar foot shapes. Genetic testing reveals a duplication of the PMP22 gene. Which physical exam finding is most likely to be present?

Explanation

The duplication of PMP22 causes Charcot-Marie-Tooth (CMT) disease type 1A, a hereditary motor and sensory neuropathy. The characteristic cavovarus deformity is driven by an imbalance where the anterior tibialis and peroneus brevis become weak early, while the posterior tibialis and peroneus longus remain relatively strong. Deep tendon reflexes are typically decreased or absent, not hyperactive.

Question 10

A 68-year-old male is scheduled for a total hip arthroplasty. He has a history of atrial fibrillation and is currently taking Rivaroxaban. According to current perioperative guidelines, when should the Rivaroxaban be discontinued prior to surgery?

Explanation

Rivaroxaban is a direct Factor Xa inhibitor. For major orthopedic surgery with a high bleeding risk, current guidelines generally recommend discontinuing Rivaroxaban 3 days (approx. 72 hours) prior to surgery in patients with normal renal function to allow for adequate drug clearance.

Question 11

Which Bone Morphogenetic Protein (BMP) is FDA-approved for use in acute open tibial shaft fractures?

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA approved for use in acute, open tibial shaft fractures when applied via an absorbable collagen sponge. BMP-7 (Osteogenic Protein-1) historically had a Humanitarian Device Exemption for recalcitrant tibial nonunions.

Question 12

A patient sustains a closed midshaft humerus fracture and presents with a complete radial nerve palsy. An EMG performed at 4 weeks shows fibrillation potentials in the brachioradialis and no voluntary motor unit action potentials. However, clinical recovery is noted at 12 weeks. According to the Seddon classification, what was the most likely initial nerve injury?

Explanation

Axonotmesis involves disruption of the axon and myelin sheath, but the supporting connective tissue (endoneurium, perineurium, epineurium) remains intact. Wallerian degeneration occurs distally, causing fibrillation potentials on EMG at 3-4 weeks. The intact tubes allow for directed axonal regeneration (1 mm/day), leading to spontaneous clinical recovery. Neuropraxia does not display fibrillation potentials since the axon remains intact.

Question 13

In evaluating synovial fluid from a painful knee, which of the following profiles is most characteristic of an inflammatory arthropathy such as Rheumatoid Arthritis?

Explanation

Inflammatory synovial fluid (Group II) is typically translucent to opaque, yellow/green, with decreased viscosity and a WBC count ranging from 2,000 to 50,000 cells/mm3, containing >50% PMNs. Non-inflammatory (OA) fluid has <2,000 WBCs. Septic arthritis fluid typically has >50,000 WBCs with >90% PMNs.

Question 14

What is the primary purpose of highly cross-linking ultra-high molecular weight polyethylene (UHMWPE) used in total joint arthroplasty?

Explanation

Cross-linking UHMWPE using gamma irradiation significantly improves its resistance to adhesive and abrasive wear, thereby minimizing wear debris and reducing the incidence of periprosthetic osteolysis. However, this process sacrifices mechanical properties, decreasing ultimate tensile strength, fatigue strength, and ductility.

Question 15

A patient undergoes a four-compartment fasciotomy for acute compartment syndrome of the leg following a tibia fracture. Which of the following muscles is located in the deep posterior compartment?

Explanation

The deep posterior compartment of the leg contains the tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles. The gastrocnemius and soleus reside in the superficial posterior compartment. The peroneus brevis is lateral, and the extensor digitorum longus is anterior.

Question 16

A 14-year-old male presents for evaluation of scoliosis. He is tall and thin with a wingspan greater than his height, and hypermobile joints. A mutation in the FBN1 gene is suspected. Which associated ocular abnormality is most characteristic of this condition?

Explanation

The patient has Marfan syndrome, caused by an autosomal dominant mutation in the FBN1 gene (fibrillin-1). A classic ocular finding in Marfan syndrome is superior (upward) lens dislocation (ectopia lentis). By contrast, homocystinuria, which can present with a similar habitus, is typically associated with inferior (downward) lens dislocation.

Question 17

During the remodeling phase of tendon healing, which type of collagen is primarily synthesized to replace the initial provisional matrix?

Explanation

During the initial proliferative phase of tendon healing, fibroblasts rapidly deposit Type III collagen, forming a disorganized provisional matrix. In the remodeling phase, this is gradually replaced by thicker, highly organized Type I collagen, which restores the tensile strength of the mature tendon.

Question 18

A 24-year-old male sustains bilateral closed femur fractures in an MVC. On hospital day 2, he develops confusion, tachypnea, and a petechial rash over his axillae. Which of the following is considered the primary biochemical mechanism for the pulmonary manifestations of his condition?

Explanation

The patient is presenting with Fat Embolism Syndrome (FES). According to the biochemical theory of FES, marrow fat enters the bloodstream and is degraded by tissue lipases into free fatty acids (FFAs). These FFAs are highly toxic to the pulmonary capillary endothelium, precipitating Acute Respiratory Distress Syndrome (ARDS).

Question 19

A biopsy of a destructive diaphyseal bone lesion in an 8-year-old boy shows sheets of small round blue cells. Cytogenetic analysis reveals a t(11;22)(q24;q12) translocation. Which fusion gene product is most likely responsible for the pathogenesis of this tumor?

Explanation

The scenario describes Ewing Sarcoma, which presents histologically as small round blue cells. The hallmark cytogenetic abnormality is the t(11;22)(q24;q12) translocation, creating the EWS-FLI1 fusion protein. SYT-SSX is found in synovial sarcoma, and TLS-CHOP is associated with myxoid liposarcoma.

Question 20

What is the primary cellular mechanism by which nitrogen-containing bisphosphonates (e.g., Alendronate) inhibit osteoclast function?

Explanation

Nitrogen-containing bisphosphonates (like alendronate and zoledronate) inhibit bone resorption by inhibiting farnesyl pyrophosphate (FPP) synthase in the mevalonate pathway. This prevents prenylation of small GTPase proteins (such as Ras, Rho, and Rac) that are essential for osteoclast survival and ruffled border formation. Non-nitrogenous bisphosphonates form toxic ATP analogues.

Question 21

According to Perren's strain theory, what is the maximum interfragmentary strain tolerated by lamellar bone during fracture healing?

Explanation

According to Perren's strain theory, lamellar bone can only tolerate up to 2% strain before failing. In contrast, cartilage tolerates up to 10% strain, and granulation tissue can withstand up to 100% strain.

Question 22

A surgeon decides to increase the diameter of a solid titanium intramedullary nail from 10 mm to 12 mm. By what approximate factor will the torsional rigidity of the nail increase?

Explanation

The torsional rigidity of a solid cylinder is proportional to the fourth power of its radius (r^4). Increasing the diameter by a factor of 1.2 increases the rigidity by 1.2^4, which is approximately 2.07.

Question 23

Which of the following is the earliest biochemical change seen in the articular cartilage of a patient developing osteoarthritis?

Explanation

The earliest biochemical change in osteoarthritis is an increase in the water content of the articular cartilage. This is accompanied by a decrease in proteoglycan concentration and an alteration in the structural collagen network.

Question 24

A 70-year-old female is treated with denosumab for severe osteoporosis. Which of the following best describes the molecular mechanism of this medication?

Explanation

Denosumab is a human monoclonal antibody that binds directly to RANKL, preventing it from interacting with the RANK receptor on osteoclasts. This effectively inhibits osteoclast formation, function, and survival, thereby decreasing bone resorption.

Question 25

A new diagnostic test for periprosthetic joint infection is evaluated at a tertiary referral center. If the prevalence of the infection in the study population increases, which of the following statistical parameters will also increase?

Explanation

Positive predictive value (PPV) is highly dependent on the prevalence of the disease in the test population. As disease prevalence increases, the PPV of a given test increases, while the negative predictive value (NPV) decreases.

Question 26

Demineralized bone matrix (DBM) is commonly used in orthopedic surgery to augment fracture healing. Which of the following best describes its primary biologic properties?

Explanation

DBM provides a structural scaffold for new bone growth (osteoconductive) and contains bone morphogenetic proteins (BMPs) that stimulate local cells to form bone (osteoinductive). It lacks viable bone-forming cells, so it is not osteogenic.

Question 27

Which of the following modifications to a cortical screw design would most significantly increase its pullout strength from bone?

Explanation

Pullout strength is directly proportional to the outer diameter, thread depth, and length of engagement, and inversely proportional to thread pitch. Decreasing the thread pitch increases the number of threads engaged in the bone, increasing pullout strength.

Question 28

According to the Sunderland classification of nerve injuries, a third-degree injury is characterized by the disruption of which of the following microanatomical structures?

Explanation

A Sunderland third-degree nerve injury involves disruption of the axon and the endoneurium, while the perineurium remains intact. Recovery is often unpredictable and variable due to the development of endoneurial scarring.

Question 29

A 25-year-old male presents with a slow-growing, painful mass in the plantar aspect of his foot. Biopsy reveals a biphasic spindle cell neoplasm. Which of the following chromosomal translocations is most characteristic of this tumor?

Explanation

The clinical presentation and histologic description are classic for synovial sarcoma. This aggressive tumor is genetically characterized by the t(X;18) translocation, resulting in the fusion of the SYT and SSX genes.

Question 30

Tranexamic acid (TXA) is frequently used in major orthopedic surgeries to reduce intraoperative and postoperative blood loss. What is the primary mechanism of action of TXA?

Explanation

Tranexamic acid is a synthetic analog of the amino acid lysine. It competitively inhibits the activation of plasminogen to plasmin, thereby acting as an antifibrinolytic agent that prevents the degradation of fibrin clots.

Question 31

A 55-year-old male presents with a transverse fracture of the tibial shaft after a minor fall. Laboratory results show normal serum calcium, normal phosphorus, and markedly elevated alkaline phosphatase. Which of the following is the most likely underlying condition?

Explanation

Paget disease of bone is characterized by accelerated, disorganized bone turnover resulting in an isolated, markedly elevated alkaline phosphatase. Serum calcium and phosphorus levels typically remain normal.

Question 32

During the pathogenesis of prosthetic joint infections, Staphylococcus epidermidis utilizes which of the following components to facilitate initial adherence and biofilm formation on the metal implant surface?

Explanation

Staphylococcus epidermidis establishes infections on implants by forming a biofilm, which begins with adherence followed by the production of a protective slime layer. This process is heavily mediated by the production of polysaccharide intercellular adhesin (PIA).

Question 33

A patient walks with a compensatory Trendelenburg gait, shifting their trunk laterally over the stance leg. This gait adaptation is primarily designed to reduce the mechanical demand on which of the following muscles?

Explanation

Shifting the trunk laterally over the stance leg (abductor lurch) decreases the lever arm of the body's center of gravity relative to the hip joint. This reduces the joint reactive force and the force required by the ipsilateral gluteus medius to maintain a level pelvis.

Question 34

During the descent phase of a deep squat, the quadriceps muscle actively lengthens to control the downward movement. Which type of muscle contraction is taking place?

Explanation

An eccentric contraction occurs when a muscle generates tension while it actively lengthens. This type of contraction is highly energy-absorbent and is commonly used for deceleration, such as controlling the descent of a squat.

Question 35

Which of the following factors is the primary driver of galvanic corrosion when two different orthopaedic metals are placed in direct contact within the human body?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in electrical contact within an electrolytic solution like bodily fluids. The less noble metal acts as an anode and undergoes accelerated corrosion.

Question 36

A 12-year-old child with a history of recurrent fractures, delayed dentition, and anemia is diagnosed with osteopetrosis. The pathogenesis of this condition is primarily related to a defect in which of the following?

Explanation

Osteopetrosis is caused by defective osteoclast function, frequently due to a mutation in carbonic anhydrase II. This enzyme is crucial for generating the acidic environment required by osteoclasts to dissolve the bone mineral matrix.

Question 37

Following a flexor tendon repair in zone II, the surgeon initiates an early passive mobilization protocol. What is the primary biological rationale for this early motion?

Explanation

Early mobilization of repaired flexor tendons promotes intrinsic healing, which is driven by tenocytes residing within the tendon itself. This limits extrinsic healing, thereby preventing the formation of restrictive peritendinous adhesions.

Question 38

Achondroplasia is the most common form of short-limbed dwarfism. Which of the following best describes the underlying genetic mutation and its primary effect on bone growth?

Explanation

Achondroplasia is an autosomal dominant disorder caused by a gain-of-function (activating) mutation in the FGFR3 gene. This overactivity abnormally inhibits chondrocyte proliferation in the proliferative zone of the physis, stunting endochondral ossification.

Question 39

Which of the following ultrastructural components allows for direct communication and the exchange of nutrients between adjacent osteocytes within dense cortical bone?

Explanation

Osteocytes reside within lacunae and extend long cytoplasmic processes through microscopic channels called canaliculi. These networks connect via gap junctions, facilitating essential intercellular communication and nutrient transport.

Question 40

A patient develops a methicillin-resistant Staphylococcus aureus (MRSA) infection following a total knee arthroplasty and is treated with intravenous vancomycin. What is the specific mechanism of action of this antibiotic?

Explanation

Vancomycin is a glycopeptide antibiotic that inhibits bacterial cell wall synthesis. It acts by binding tightly to the D-alanyl-D-alanine terminus of cell wall precursor units, preventing their incorporation into the growing peptidoglycan matrix.

Question 41

A new titanium alloy is undergoing biomechanical testing for intramedullary nail fabrication. The material deforms under an applied load and returns precisely to its original shape once the load is removed. The maximum stress this material can withstand before permanently deforming is known as which of the following?

Explanation

The yield point marks the end of the elastic region on a stress-strain curve. Stress applied beyond this point will result in plastic, irreversible deformation of the material.

Question 42

According to Perren's strain theory of fracture healing, primary bone healing with direct lamellar bone formation can only occur if the interfragmentary strain is kept below what maximum threshold?

Explanation

Lamellar bone formation requires absolute stability, tolerating a maximum of 2% strain. Woven bone can tolerate up to 10% strain, while granulation tissue can withstand up to 100% strain.

Question 43

In the manufacturing of highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) for total joint arthroplasty, gamma irradiation is used to promote cross-linking. Which subsequent processing step is critical to prevent long-term oxidative degradation of the implant?

Explanation

Gamma irradiation generates free radicals that leave the polyethylene susceptible to oxidation in vivo. Remelting or annealing the plastic eliminates these free radicals, significantly improving long-term wear resistance.

Question 44

Articular cartilage relies on a specific biochemical composition to withstand compressive and shear forces. Which of the following collagens comprises 90-95% of the collagen in normal hyaline cartilage, and which cell type is responsible for its production?

Explanation

Type II collagen is the primary structural framework of articular cartilage, providing tensile strength. It is synthesized exclusively by resident chondrocytes within the extracellular matrix.

Question 45

Tranexamic acid (TXA) is extensively utilized in orthopedic procedures, such as total knee arthroplasty, to minimize perioperative blood loss. What is the specific mechanism of action of this agent?

Explanation

TXA is a synthetic analog of the amino acid lysine. It reversibly binds to plasminogen, blocking its interaction with fibrin and preventing its activation to plasmin, thereby inhibiting fibrinolysis.

Question 46

A 72-year-old male complains of deep, aching bone pain in his right leg and an increasing hat size over the past year. Radiographs demonstrate marked cortical thickening and bowing of the tibia. A bone biopsy of the affected tibia is most likely to reveal which histologic characteristic?

Explanation

The clinical presentation is classic for Paget's disease of bone. The pathognomonic histologic finding is a chaotic, mosaic pattern of lamellar bone with prominent cement lines due to poorly coordinated osteoclast and osteoblast activity.

Question 47

A pediatric patient is diagnosed with achondroplasia, the most common form of short-limb dwarfism. This condition is primarily driven by a gain-of-function mutation in the FGFR3 gene. Which specific anatomic zone of the physis is most severely inhibited by this mutation?

Explanation

The FGFR3 mutation in achondroplasia leads to constitutive activation of the receptor, which negatively regulates bone growth. This defect primarily impairs chondrocyte multiplication in the zone of proliferation.

Question 48

A patient develops a late chronic prosthetic joint infection caused by Staphylococcus epidermidis. The profound resistance of this organism to systemic antimicrobial therapy is primarily mediated by which of the following mechanisms?

Explanation

S. epidermidis produces an extracellular polysaccharide matrix (glycocalyx) that allows it to adhere to implants and form a robust biofilm. This biofilm physically shields the bacteria from both host immunity and systemic antibiotics.

Question 49

During the correction of a severe knee flexion contracture, a physical therapist applies a static, constant force splint. Over several weeks, the soft tissues progressively lengthen despite the force remaining unchanged. This viscoelastic property is termed:

Explanation

Creep describes the time-dependent, progressive deformation of a viscoelastic material when subjected to a constant applied load. Stress relaxation, conversely, is the decrease in stress over time under a constant strain.

Question 50

A spine surgeon elects to use demineralized bone matrix (DBM) to augment a posterolateral fusion. Based on its biologic processing, DBM provides which of the following combinations of bone-grafting properties?

Explanation

DBM consists of a collagen scaffold that provides osteoconductivity, and it contains native bone morphogenetic proteins (BMPs) that provide osteoinductivity. It is acellular, so it lacks osteogenic properties.

Question 51

Denosumab is highly effective in treating postmenopausal osteoporosis by aggressively reducing osteoclast-mediated bone resorption. What is the specific molecular target of this monoclonal antibody?

Explanation

Denosumab specifically binds to and inhibits RANKL, preventing it from interacting with the RANK receptor on osteoclasts. This effectively halts osteoclast differentiation, function, and survival.

Question 52

Following a complete traumatic transection of the radial nerve, primary microscopic repair is performed. The distal nerve segment predictably undergoes Wallerian degeneration. Which cell type is primarily responsible for clearing myelin debris and forming the bands of Büngner to guide regenerating axons?

Explanation

In the peripheral nervous system, Schwann cells divide and work with macrophages to clear debris during Wallerian degeneration. They then align to form the bands of Büngner, providing a regenerative pathway for new axons.

Question 53

A 14-year-old male presents with knee pain. Imaging reveals a permeative, destructive metaphyseal lesion in the distal femur with a classic 'sunburst' periosteal reaction. Genetic analysis of this patient's tumor cells is most likely to reveal an inactivating mutation in which of the following tumor suppressor genes?

Explanation

The clinical and radiographic presentation is pathognomonic for osteosarcoma. Pathogenesis is strongly associated with mutations in the TP53 (associated with Li-Fraumeni syndrome) and Rb tumor suppressor genes.

Question 54

A surgeon utilizes a bridge plating technique for a comminuted midshaft femur fracture. To optimize secondary bone healing via callus formation, the surgeon deliberately skips screw holes near the fracture site. Increasing the 'working length' of this construct has which specific biomechanical effect?

Explanation

The working length is the distance between the innermost screws on either side of a fracture. Increasing the working length increases the construct's flexibility, thereby decreasing strain across the fracture gap and promoting secondary healing.

Question 55

During the active phase of bone remodeling, osteoclasts attach to the bone surface and form a sealed zone. The dissolution of the inorganic hydroxyapatite mineral at the ruffled border is primarily achieved by which mechanism?

Explanation

Osteoclasts utilize carbonic anhydrase II to generate protons, which are actively pumped into the sealed zone to lower the pH and dissolve the inorganic hydroxyapatite. Proteases like cathepsin K subsequently degrade the organic collagen matrix.

Question 56

A 45-year-old male presents with an acutely hot, swollen first metatarsophalangeal joint. Aspiration reveals yellow, cloudy fluid with a leukocyte count of 55,000 cells/mm3 (70% PMNs). Polarized microscopy demonstrates negatively birefringent, needle-shaped crystals. What is the most appropriate initial management?

Explanation

The microscopic finding of negatively birefringent, needle-shaped monosodium urate crystals confirms acute gout. The first-line treatment for an acute attack is NSAIDs (e.g., indomethacin), colchicine, or corticosteroids; allopurinol is contraindicated during an acute flare.

Question 57

Which of the following recombinant human bone morphogenetic proteins (rhBMPs) is currently FDA-approved for use as an alternative to autogenous bone graft in the treatment of acute, open tibial shaft fractures treated with an intramedullary nail?

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved for use in acute open tibial shaft fractures stabilized with an IM nail. It is also approved for anterior lumbar interbody fusions (ALIF).

Question 58

Patients with end-stage renal disease frequently develop renal osteodystrophy and secondary hyperparathyroidism. Which primary defect in vitamin D metabolism is the primary catalyst for this metabolic bone cascade?

Explanation

In chronic kidney disease, the failing kidneys lose the ability to produce 1-alpha-hydroxylase. This prevents the final conversion of 25-hydroxyvitamin D into its active form, 1,25-dihydroxyvitamin D, leading to hypocalcemia and secondary hyperparathyroidism.

Question 59

When analyzing the stress-strain curve of a normal articular cartilage specimen loaded in tension, an initial non-linear 'toe region' is observed before the curve becomes linear. What microscopic structural change corresponds to this toe region?

Explanation

The 'toe region' represents the low-strain phase where the naturally wavy or crimped collagen fibers begin to straighten out (uncrimp) without structurally failing. Once straight, the tissue enters the linear elastic region.

Question 60

A 35-year-old male smoker sustains a closed midshaft tibia fracture treated with a cast. At 6 months, he presents with pain, and radiographs demonstrate a prominent hypertrophic nonunion (elephant foot appearance). What is the primary underlying cause of this specific type of nonunion?

Explanation

Hypertrophic nonunions are characterized by abundant callus formation, indicating that the biological environment and blood supply are excellent. The nonunion persists purely due to excessive mechanical instability, and the treatment is rigid stabilization.

Question 61

In the stress-strain curve of orthopedic biomaterials, the slope of the linear elastic region represents which of the following properties?

Explanation

The slope of the linear elastic portion of the stress-strain curve defines Young's modulus, indicating the material's stiffness. Toughness is the area under the curve, and yield strength marks the end of elastic deformation.

Question 62

Which type of collagen is most predominant in the hypertrophic zone of the physis and is essential for normal endochondral ossification?

Explanation

Type X collagen is specifically synthesized by hypertrophic chondrocytes in the growth plate. It facilitates matrix mineralization and is a critical structural component during endochondral ossification.

Question 63

A 10-year-old child presents with recurrent fractures, anemia, and cranial nerve palsies. Radiographs show a "bone-within-a-bone" appearance. A defect in which of the following cellular mechanisms is most likely responsible?

Explanation

Osteopetrosis is caused by defective osteoclast function, frequently due to a mutation in carbonic anhydrase II, leading to an inability to acidify Howship's lacuna. This prevents normal bone resorption, resulting in dense, brittle bones.

Question 64

Following a complete peripheral nerve transection (neurotmesis), Wallerian degeneration occurs distal to the injury. Which cell type is primarily responsible for clearing the myelin debris in the peripheral nervous system?

Explanation

In the peripheral nervous system, Wallerian degeneration involves clearing axonal and myelin debris distal to the injury. This process is primarily executed by local Schwann cells and recruited macrophages.

Question 65

During the early stance phase of the normal gait cycle, the quadriceps muscle contracts to control knee flexion. This type of muscle contraction is best described as:

Explanation

An eccentric contraction occurs when a muscle lengthens while under tension, serving to decelerate a joint. The quadriceps act eccentrically during early stance to absorb shock and prevent the knee from buckling.

Question 66

A newborn is evaluated for short-limbed dwarfism, frontal bossing, and midface hypoplasia. Radiographs reveal rhizomelic shortening. The underlying genetic mutation results in which of the following cellular effects?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3, which leads to constitutive inhibition of chondrocyte proliferation in the proliferative zone of the physis.

Question 67

Osteolysis surrounding a total hip arthroplasty with a highly cross-linked polyethylene liner is primarily driven by an inflammatory response to particulate debris. Which of the following cells is the primary mediator of this osteolytic process?

Explanation

Macrophages phagocytose polyethylene wear particles and release inflammatory cytokines like TNF-alpha and IL-6. This cascade stimulates osteoclast differentiation and activity via the RANK/RANKL pathway, leading to periprosthetic osteolysis.

Question 68

During the remodeling phase of tendon healing, the biomechanical strength of the tendon improves significantly. This is primarily due to which of the following processes?

Explanation

The remodeling phase is characterized by a decrease in cellularity and the progressive replacement of weaker type III collagen with stronger, longitudinally aligned type I collagen. This transition is crucial for restoring the tendon's tensile strength.

Question 69

A surgeon utilizes a demineralized bone matrix (DBM) allograft for a spinal fusion. Which of the following biologic properties does DBM possess?

Explanation

DBM is osteoconductive due to its collagenous matrix and osteoinductive due to preserved bone morphogenetic proteins (BMPs). It lacks live osteoprogenitor cells, so it is not osteogenic.

Question 70

A patient is prescribed rivaroxaban for venous thromboembolism prophylaxis following a total knee arthroplasty. What is the specific mechanism of action of this medication?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants that work by selectively inhibiting Factor Xa. Unlike low-molecular-weight heparins, they directly inhibit the target without requiring antithrombin III.

Question 71

A 4-year-old child fed exclusively boiled cow's milk presents with irritability, bleeding gums, and subperiosteal hematomas. The primary defect in bone formation is related to the impaired function of which of the following enzymes?

Explanation

The patient exhibits classic signs of scurvy (Vitamin C deficiency). Vitamin C is an essential cofactor for prolyl and lysyl hydroxylase, which are required for the intracellular hydroxylation of proline and lysine during collagen synthesis.

Question 72

A new diagnostic test for periprosthetic joint infection is evaluated against the gold standard. Out of 100 infected cases, the test correctly identifies 90. Out of 100 uninfected cases, the test falsely identifies 20 as infected. What is the negative predictive value of this test?

Explanation

Negative Predictive Value (NPV) is True Negatives / (True Negatives + False Negatives). TN = 80 (100-20), FN = 10 (100-90). NPV = 80 / (80 + 10) = 80/90 = 88.8%.

Question 73

A 70-year-old man undergoes total hip arthroplasty for secondary osteoarthritis. Radiographs demonstrate cortical thickening and coarse trabeculae in the proximal femur. Preoperative labs show an isolated elevation of alkaline phosphatase. Histologic evaluation of the femoral head would most likely show:

Explanation

The patient's presentation is indicative of Paget's disease of bone, which features hyperactive osteoclasts and chaotic osteoblastic bone formation. The classic histologic hallmark is a 'mosaic' or 'jigsaw puzzle' pattern of lamellar bone with irregular cement lines.

Question 74

Polymethylmethacrylate (PMMA) bone cement is frequently used in joint arthroplasty. The addition of barium sulfate to PMMA serves which of the following purposes?

Explanation

Barium sulfate or zirconium dioxide is added to PMMA to make the cement radiopaque, allowing for postoperative radiographic evaluation of the cement mantle. These additives act as stress risers and actually decrease the ultimate tensile strength of the cement.

Question 75

During secondary fracture healing, the biomechanical stability of the fracture determines the type of tissue that forms in the gap according to Perren's strain theory. If the strain at the fracture gap is between 2% and 10%, which tissue will primarily form?

Explanation

According to Perren's strain theory, low strain (<2%) allows primary bone healing. Intermediate strain (2-10%) promotes the formation of a fibrocartilaginous callus, while high strain (>10%) results in granulation tissue and potential nonunion.

Question 76

According to the inverse square law of radiation exposure, if an orthopedic surgeon increases their distance from the fluoroscopy source from 1 meter to 3 meters, the radiation exposure is reduced to what fraction of the original dose?

Explanation

The inverse square law dictates that radiation intensity is inversely proportional to the square of the distance from the source. Tripling the distance (from 1 to 3 meters) reduces the exposure to 1/(3^2) or 1/9 of the original dose.

Question 77

A 15-year-old boy presents with a destructive diaphyseal lesion of the femur and a large soft tissue mass. Biopsy reveals small round blue cells. Cytogenetic analysis is most likely to reveal which of the following translocations?

Explanation

The clinical presentation and biopsy findings are characteristic of Ewing sarcoma. Over 90% of Ewing sarcomas exhibit the t(11;22)(q24;q12) translocation, resulting in the EWS-FLI1 fusion protein.

Question 78

In the pathogenesis of osteoarthritis, matrix metalloproteinases (MMPs) play a critical role in articular cartilage degradation. Which specific MMP is primarily responsible for the cleavage of type II collagen?

Explanation

MMP-13 (collagenase-3) is the principal enzyme responsible for the degradation of the type II collagen network in the pathogenesis of osteoarthritis. Its expression is highly upregulated in osteoarthritic cartilage.

Question 79

When inserting a cortical screw into a diaphysis, the "pull-out strength" of the screw is directly proportional to which of the following variables?

Explanation

The pull-out strength of a bone screw is directly proportional to the outer thread diameter, the length of thread engagement in the bone, and the shear strength of the surrounding bone. The inner root diameter dictates the screw's resistance to fatigue bending failure.

Question 80

A 68-year-old female with severe osteoporosis is treated with alendronate. Bisphosphonates inhibit osteoclast-mediated bone resorption by interfering with which of the following intracellular pathways?

Explanation

Nitrogen-containing bisphosphonates, such as alendronate, inhibit farnesyl pyrophosphate synthase within the mevalonate pathway. This prevents the prenylation of small GTPase proteins (e.g., Ras, Rho), disrupting osteoclast function and leading to apoptosis.

Question 81

A viscoelastic material subjected to a constant load will continuously deform over time. This specific biomechanical phenomenon is known as which of the following?

Explanation

Creep is the progressive deformation of a viscoelastic material under a constant load over time. Stress relaxation refers to a decrease in stress over time when the material is held at a constant deformation.

Question 82

A 70-year-old female is prescribed alendronate for osteoporosis. Which of the following best describes the cellular mechanism of action of this medication?

Explanation

Nitrogen-containing bisphosphonates like alendronate inhibit farnesyl pyrophosphate synthase within the mevalonate pathway. This prevents osteoclast ruffled border formation and induces osteoclast apoptosis.

Question 83

When comparing articular cartilage changes in early osteoarthritis to normal aging, which of the following changes is characteristic of osteoarthritis but NOT normal aging?

Explanation

In osteoarthritis, the earliest change is an increase in water content and a decrease in total proteoglycan content. In contrast, normal aging cartilage typically demonstrates a decrease in water content.

Question 84

During skeletal muscle contraction, the release of calcium from the sarcoplasmic reticulum directly initiates the power stroke by binding to which of the following structures?

Explanation

Calcium released from the sarcoplasmic reticulum binds to Troponin C. This causes a conformational change that moves tropomyosin, exposing the myosin-binding sites on the actin filament.

Question 85

A patient sustains a peripheral nerve injury characterized by loss of axonal continuity and endoneurial tube disruption, but with an intact perineurium. What is the corresponding Sunderland classification?

Explanation

A third-degree Sunderland nerve injury involves disruption of the axon and endoneurium, while the perineurium and epineurium remain intact. This corresponds to a severe axonotmesis in Seddon's classification.

Question 86

A 25-year-old male presents with a deep, slow-growing soft tissue mass in his foot. Biopsy reveals a biphasic pattern of spindle and epithelial cells. Which of the following chromosomal translocations is most strongly associated with this diagnosis?

Explanation

The presentation is classic for a synovial sarcoma, which is characterized by the t(X;18)(p11;q11) translocation resulting in the SYT-SSX fusion gene. The t(11;22) translocation is seen in Ewing sarcoma.

Question 87

In total joint arthroplasty, highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is used primarily to reduce volumetric wear. Which of the following mechanical properties is typically DECREASED as a result of the cross-linking process?

Explanation

Highly cross-linking UHMWPE significantly reduces abrasive and adhesive wear but reduces its fatigue resistance and ductility. This makes the material more susceptible to fatigue crack propagation and fracture.

Question 88

A 60-year-old male presents with acute knee pain and effusion. Joint aspiration yields synovial fluid with 45,000 WBCs/mcL. Polarized light microscopy reveals weakly positively birefringent rhomboid-shaped crystals. What is the most likely composition of these crystals?

Explanation

Weakly positively birefringent, rhomboid-shaped crystals are diagnostic of calcium pyrophosphate dihydrate (CPPD) deposition disease (pseudogout). Monosodium urate crystals are needle-shaped and strongly negatively birefringent.

Question 89

During secondary fracture healing, the transition from the soft callus to the hard callus phase is primarily characterized by which of the following biological processes?

Explanation

The conversion of the cartilaginous soft callus into a hard bony callus occurs via endochondral ossification. Intramembranous ossification occurs directly from mesenchymal stem cells without a cartilage intermediate.

Question 90

A 4-year-old boy presents with refusal to walk, bleeding gums, and petechiae. Radiographs reveal a densely sclerotic metaphyseal line and a lucent zone immediately diaphyseal to it. A deficiency in a vitamin required for which of the following cellular processes is the likely cause?

Explanation

The child has scurvy due to Vitamin C deficiency. Vitamin C is an essential cofactor for prolyl and lysyl hydroxylase, required for the hydroxylation of proline and lysine during collagen synthesis.

Question 91

A 72-year-old male presents with increasing hat size, hearing loss, and bowing of his tibiae. Radiographs show mixed lytic and sclerotic lesions. The primary cellular defect in this disease process is localized to which of the following?

Explanation

Paget disease of bone is initiated by overactive, large, and multinucleated osteoclasts. The subsequent intense osteoblastic response results in the chaotic deposition of mechanically weak woven and lamellar bone.

Question 92

Bone morphogenetic proteins (BMPs) are used clinically to augment bone healing. Which specific intracellular signaling molecules are directly phosphorylated by activated BMP receptors?

Explanation

BMPs bind to serine/threonine kinase receptors, which directly phosphorylate intracellular Smad 1, 5, and 8 proteins. These active Smads then complex with Smad 4 to translocate to the nucleus and regulate target gene transcription.

Question 93

A neonate is diagnosed with malignant infantile osteopetrosis. The defective gene most commonly associated with this autosomal recessive condition encodes for which of the following?

Explanation

Malignant infantile osteopetrosis is most commonly caused by a mutation in the TCIRG1 gene (vacuolar proton pump) or Carbonic Anhydrase II. This leads to the inability of osteoclasts to create the acidic environment needed to resorb bone.

Question 94

The slope of the linear elastic region of a material's stress-strain curve represents which of the following biomechanical properties?

Explanation

Young's modulus (modulus of elasticity) is defined as the slope of the stress-strain curve in the linear elastic region, representing the stiffness of the material. Toughness is the area under the entire stress-strain curve.

Question 95

Following total hip arthroplasty, a patient is started on rivaroxaban for deep vein thrombosis prophylaxis. This medication exerts its anticoagulant effect by directly inhibiting which of the following?

Explanation

Rivaroxaban is an oral direct Factor Xa inhibitor that disrupts the coagulation cascade by preventing the conversion of prothrombin to thrombin. Dabigatran, in contrast, is a direct thrombin inhibitor.

Question 96

In a growing child with nutritional rickets, which zone of the physis is most significantly widened and disorganized?

Explanation

In rickets, failure of mineralization in the zone of provisional calcification leads to a massive accumulation of unmineralized cartilage. This specifically widens the hypertrophic zone of the physis.

Question 97

What is the most abundant non-collagenous protein found in the extracellular matrix of mature bone?

Explanation

Osteocalcin is the most abundant non-collagenous protein in bone matrix and is secreted by osteoblasts. It is dependent on Vitamin K for carboxylation and plays a critical role in regulating bone mineralization.

Question 98

A 40-year-old patient presents with a swollen, painful knee. Synovial fluid analysis reveals a WBC count of 85,000 cells/mcL with 92% polymorphonuclear leukocytes. Glucose level in the fluid is markedly lower than serum levels. What is the most likely diagnosis?

Explanation

A synovial fluid WBC count greater than 50,000 cells/mcL with a high PMN percentage (>90%) and low glucose is highly suspicious for bacterial septic arthritis until proven otherwise.

Question 99

A modular total hip arthroplasty fails due to micro-motion at the head-neck taper junction, leading to wear debris and local tissue reaction. Which type of corrosion primarily occurs as a result of this mechanical disruption of the passivation layer?

Explanation

Fretting corrosion occurs at the contact areas between materials under load, such as taper junctions. Micro-motion repeatedly strips the protective oxide passivation layer, accelerating oxidative wear.

Question 100

Continuous endogenous secretion of parathyroid hormone (PTH) primarily stimulates osteoclastogenesis by binding directly to receptors on which of the following cells?

Explanation

PTH receptors are located on osteoblasts, not osteoclasts. PTH binding stimulates osteoblasts to upregulate RANKL expression, which subsequently binds to RANK on osteoclast precursors to drive their differentiation.

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