Orthopedic Ob Basic Review | Dr Hutaif Basic Science Re -...

Question 1

A 65-year-old female is treated with a monoclonal antibody that targets a key cytokine involved in bone resorption. Which of the following is the specific mechanism of action of this drug at the cellular level?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, preventing it from binding to the RANK receptor on osteoclast precursors, thereby inhibiting osteoclast differentiation, activation, and survival. It acts as a pharmacologic mimic of osteoprotegerin (OPG).

Question 2

When evaluating the mechanical properties of a novel orthopedic implant alloy, a load-deformation curve is generated. What biomechanical property is represented by the total area under the stress-strain curve up to the point of failure?

Explanation

Toughness is defined as the total amount of energy a material can absorb before it fractures, which corresponds to the entire area under the stress-strain curve. Young's modulus is the slope of the elastic region, measuring stiffness. Yield strength is the point where plastic (irreversible) deformation begins.

Question 3

In normal human articular cartilage, which zone is characterized by the largest diameter collagen fibrils oriented strictly perpendicular to the joint surface and the lowest water content?

Explanation

The deep (radial) zone of articular cartilage contains the largest diameter collagen fibrils, which are oriented perpendicular to the joint surface to resist compressive loads. It also has the highest concentration of proteoglycans and the lowest water content of the uncalcified zones.

Question 4

A 70-year-old man presents with aseptic loosening 15 years after a total hip arthroplasty. Tissue retrieved during revision surgery shows numerous macrophages containing particulate debris. Which of the following is the primary cytokine released by these macrophages that drives the subsequent osteolysis?

Explanation

Polyethylene wear debris is phagocytosed by macrophages, which cannot digest the particles. This leads to the release of pro-inflammatory cytokines, most notably TNF-alpha, IL-1, and IL-6. These cytokines stimulate osteoclastogenesis and lead to periprosthetic osteolysis.

Question 5

An orthopedic researcher conducts a randomized controlled trial comparing two different types of intramedullary nails for tibial shaft fractures. The study finds no statistically significant difference in union rates (p = 0.12). However, a true difference does exist in the general population. Which of the following best describes this statistical scenario?

Explanation

A Type II error (beta error) occurs when a study fails to reject a false null hypothesis—meaning it finds no significant difference when one truly exists in nature. Statistical Power (1 - beta) is the probability of avoiding a Type II error, often due to an inadequately small sample size.

Question 6

During the proliferative phase of fracture healing, mesenchymal stem cells differentiate into osteoblasts. Which of the following intracellular signaling pathways is most essential for committing these mesenchymal precursors exclusively to the osteoblast lineage?

Explanation

The Wnt/beta-catenin signaling pathway is critical for promoting osteoblast differentiation from mesenchymal stem cells while simultaneously inhibiting chondrogenic and adipogenic lineages. LRP5/6 are key co-receptors in this pathway.

Question 7

A neonate is diagnosed with the most common form of short-limb dwarfism. The underlying genetic defect involves a point mutation in the gene encoding for Fibroblast Growth Factor Receptor 3 (FGFR3). What is the cellular consequence of this specific mutation in the epiphyseal plate?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3 (most commonly G380R). In normal growth plates, FGFR3 acts as a negative regulator of bone growth. The mutation leads to constitutive activation of the receptor, causing profound inhibition of chondrocyte proliferation in the proliferative zone of the physis.

Question 8

When a constant sustained load is applied to an anterior cruciate ligament allograft over a prolonged period, the ligament slowly elongates over time without any increase in the applied force. Which of the following viscoelastic properties does this describe?

Explanation

Creep is defined as the progressive deformation (elongation) of a viscoelastic material when subjected to a constant load over time. Stress relaxation, conversely, is the decrease in stress (force) within a material over time when it is held at a constant length or strain.

Question 9

An elite marathon runner undergoes a muscle biopsy of the gastrocnemius. The predominant muscle fibers identified are rich in mitochondria, myoglobin, and rely on oxidative metabolism. Which of the following best characterizes the mechanical properties of these specific fibers compared to the other major fiber type?

Explanation

The biopsy describes Type I muscle fibers (slow-twitch, red fibers). They are highly oxidative, rich in mitochondria and myoglobin, and are characterized by a slow contraction speed but a high resistance to fatigue, making them ideal for endurance activities.

Question 10

A surgeon uses a stainless steel screw to secure a titanium plate during the open reduction and internal fixation of a distal radius fracture. Which type of corrosion is most likely to occur at the interface of these two different metals?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different anodic indices are placed in electrical contact within a conductive fluid. The less noble metal (anode) undergoes accelerated corrosion. Stainless steel and titanium should generally not be mixed to prevent this electrochemical process.

Question 11

Fibroblast Growth Factor 23 (FGF-23) plays a critical role in phosphate homeostasis. Which of the following actions is a direct physiological effect of elevated circulating FGF-23 levels?

Explanation

FGF-23 is a hormone primarily secreted by osteocytes in response to hyperphosphatemia. It acts on the kidneys to decrease phosphate reabsorption (by downregulating NaPi-IIa and NaPi-IIc cotransporters) and decreases the synthesis of active 1,25-dihydroxyvitamin D by inhibiting 1-alpha-hydroxylase.

Question 12

An orthopedic surgeon is selecting an intramedullary nail for a femur fracture. If the surgeon chooses a solid titanium nail with a radius of 12 mm instead of a 10 mm nail of the identical material, by approximately what factor does the bending rigidity increase?

Explanation

The bending rigidity of a solid cylinder is proportional to the area moment of inertia, which is proportional to the radius to the fourth power (r^4). (12/10)^4 = (1.2)^4 = 2.0736. Therefore, the bending rigidity increases by a factor of approximately 2.07.

Question 13

During the stance phase of normal human gait, the peak compressive loads across the articular cartilage of the knee can exceed several times body weight. Under these high-load, high-motion conditions, what is the primary mode of joint lubrication?

Explanation

Under high load and motion conditions, articular cartilage relies on elastohydrodynamic lubrication. The pressure of the fluid film causes elastic deformation of the articular cartilage, increasing the surface area of contact and distributing the load over a wider area.

Question 14

A 55-year-old male develops a late periprosthetic joint infection after a total knee arthroplasty. Cultures yield Staphylococcus epidermidis. The chronicity of this infection is largely attributed to the organism's ability to produce a protective biofilm. Which of the following is the key component responsible for the initial adhesion phase to the implant in this process?

Explanation

The initial attachment of Staphylococcus epidermidis to a polymer or metallic implant surface is mediated primarily by surface proteins such as autolysin (AtlE) and fibronectin-binding proteins. After this primary attachment, the bacteria proliferate and secrete polysaccharide intercellular adhesin (PIA), which forms the bulk of the mature extracellular polymeric biofilm matrix.

Question 15

A 28-year-old male presents with a deep, slow-growing soft tissue mass in his thigh. Core needle biopsy demonstrates a biphasic pattern of epithelial-like cells and spindle cells. Molecular analysis reveals a t(X;18)(p11;q11) translocation. What fusion gene is created by this specific chromosomal abnormality?

Explanation

Synovial sarcoma is characterized by the t(X;18)(p11;q11) translocation, which fuses the SYT gene on chromosome 18 with one of the SSX genes (usually SSX1 or SSX2) on the X chromosome. EWS-FLI1 is characteristic of Ewing sarcoma t(11;22); FUS-CHOP is seen in myxoid liposarcoma t(12;16).

Question 16

A patient suffers a closed crush injury to the forearm. Electromyography and nerve conduction studies performed 4 weeks later suggest a nerve injury characterized by axonal disruption and loss of endoneurial tube continuity, but an intact perineurium and epineurium. According to the Sunderland classification, what grade is this nerve injury?

Explanation

A Sunderland Grade III injury involves loss of continuity of the axon and the endoneurium, but the perineurium and epineurium remain intact. Because the endoneurial tubes are disrupted, regenerating axons may become misdirected, leading to incomplete or aberrant recovery compared to Grade II injuries.

Question 17

An orthopaedic surgeon utilizes a massive structural cortical allograft for a segmental defect reconstruction following tumor resection. Which of the following best describes the classical sequence of events during the creeping substitution of this cortical bone graft?

Explanation

Creeping substitution in cortical bone grafts involves osteoclastic resorption cutting cones that bore through the dead cortical bone, closely followed by osteoblasts that lay down new lamellar bone. This process temporarily weakens the graft mechanically before it incorporates and regains strength.

Question 18

While reviewing the histological properties of dense regular connective tissue, it is noted that ligaments and tendons have slightly different ultrastructural compositions. Compared to a typical tendon, a typical ligament contains a higher proportion of which of the following?

Explanation

Tendons and ligaments are both primarily composed of Type I collagen. However, ligaments generally have slightly less Type I collagen and a higher proportion of proteoglycans, elastin, and Type III collagen compared to tendons, giving them slightly more extensibility.

Question 19

Following an acute diaphyseal fracture, a hematoma forms and an inflammatory cascade is initiated. Experimental depletion of which of the following immune cell populations has been shown to completely halt the transition from the inflammatory phase to the cartilaginous reparative phase of fracture healing?

Explanation

Macrophages (particularly the transition from M1 pro-inflammatory to M2 anti-inflammatory phenotypes) are absolutely critical for fracture healing. Studies have shown that experimental depletion of macrophages disrupts the formation of the cartilaginous callus and arrests fracture healing in the inflammatory phase.

Question 20

A 68-year-old male is prescribed rivaroxaban for deep vein thrombosis prophylaxis following a total hip arthroplasty. At which specific point in the coagulation cascade does this agent exert its primary therapeutic effect?

Explanation

Rivaroxaban is a direct oral anticoagulant (DOAC) that functions by directly and reversibly inhibiting Factor Xa, preventing the conversion of prothrombin to thrombin. Dabigatran directly inhibits thrombin (Factor IIa). Heparins work by activating antithrombin III.

Question 21

A 2-year-old child presents with multiple unexplained fractures and a radiograph showing a 'bone-within-bone' appearance.

Genetic testing reveals a mutation in the gene encoding Carbonic Anhydrase II. Which of the following best describes the resulting cellular dysfunction?

Explanation

Carbonic Anhydrase II mutation causes an autosomal recessive form of osteopetrosis. This enzyme is essential for osteoclasts to produce protons, which are pumped into Howship's lacuna to dissolve bone mineral.

Question 22

A surgeon plans to use a stainless steel screw through a titanium locking plate to treat a periarticular fracture. What specific biomechanical or biomaterial phenomenon is most likely to occur at the screw-plate interface?

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within an electrolytic environment. Stainless steel is less noble (more anodic) than titanium, causing the stainless steel to corrode while the titanium is protected.

Question 23

During a biomechanical study, a human Achilles tendon is subjected to tensile testing at varying strain rates. According to the viscoelastic properties of ligaments and tendons, what is the expected result when the tendon is loaded at a very high strain rate compared to a low strain rate?

Explanation

Ligaments and tendons exhibit viscoelasticity, meaning their biomechanical behavior is strain-rate dependent. At high strain rates, they become stiffer and can withstand a higher ultimate failure load before rupturing.

Question 24

A 5-year-old boy presents with short stature, frontal bossing, and rhizomelic shortening of the limbs.

This condition is caused by a gain-of-function mutation in FGFR3. Which zone of the physis is primarily affected by this mutation?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3, which constitutively inhibits chondrocyte proliferation. This primary defect manifests in the proliferative zone of the growth plate, leading to dwarfism.

Question 25

A patient undergoes spinal fusion using a massive structural cortical bone allograft. Compared to a cancellous autograft, which of the following accurately describes the incorporation process of the cortical allograft?

Explanation

Structural cortical grafts incorporate via creeping substitution, where osteoclasts first resorb the dead bone (cutting cones) before osteoblasts lay down new bone. This causes the graft to become temporarily weaker before full incorporation.

Question 26

In total hip arthroplasty, highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is frequently utilized to improve longevity. What is the primary biomechanical tradeoff of increasing the radiation dose to increase cross-linking in UHMWPE?

Explanation

Highly cross-linked polyethylene significantly reduces volumetric and adhesive wear in joint arthroplasty. However, the cross-linking process decreases the material's ductility, fracture toughness, and resistance to fatigue crack propagation.

Question 27

A patient sustains a closed humerus fracture and presents with a radial nerve palsy. Electromyography at 4 weeks shows fibrillation potentials but no motor unit action potentials. If this represents a Sunderland third-degree nerve injury, which of the following structures remains intact?

Explanation

In a Sunderland third-degree injury, the axon, myelin sheath, and endoneurium are disrupted, but the perineurium and epineurium remain intact. This causes Wallerian degeneration but maintains the fascicular architecture for potential regeneration.

Question 28

A surgeon plans to use a stainless steel screw through a titanium plate. What is the primary mechanism of implant failure associated with mixing these two metals?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in an electrolytic solution like body fluid. This leads to the transfer of electrons and accelerated corrosion of the less noble metal, which is why mixing titanium and stainless steel is generally avoided.

Question 29

Romosozumab is an anabolic agent used for the treatment of severe osteoporosis. Which of the following best describes its mechanism of action at the cellular level?

Explanation

Romosozumab is a monoclonal antibody that binds and inhibits sclerostin. Because sclerostin is a natural inhibitor of the Wnt/beta-catenin pathway, its inhibition leads to increased osteoblastogenesis and enhanced bone formation.

Question 30

Aggrecan is the most abundant proteoglycan in articular cartilage. Its ability to draw water into the extracellular matrix and resist compressive loads is primarily due to which of the following glycosaminoglycan side chains?

Explanation

Aggrecan contains numerous chondroitin sulfate and keratan sulfate glycosaminoglycan (GAG) chains. The high density of negative charges on these sulfated GAGs creates a large osmotic swelling pressure that draws water into the tissue.

Question 31

During material testing of a new orthopedic alloy, the specimen is loaded in tension. The point on the stress-strain curve where the material transitions from elastic to plastic deformation is known as the:

Explanation

The yield point marks the transition between elastic (reversible) and plastic (irreversible) deformation on a stress-strain curve. Loading beyond this point results in permanent deformation of the implant.

Question 32

When applying tensile force to a normal ligament, the initial non-linear, low-stiffness portion of the load-elongation curve is primarily due to:

Explanation

The "toe region" of the ligament load-elongation curve represents the straightening or uncrimping of the naturally crimped collagen fibrils. Once these fibers are straight, the curve enters the linear elastic phase.

Question 33

Which of the following construct modifications will most effectively decrease construct stiffness and promote secondary bone healing via callus formation in a comminuted diaphyseal fracture treated with a locked plate?

Explanation

Using a titanium plate decreases construct stiffness compared to stainless steel due to titanium's lower modulus of elasticity. Decreasing the working length or adding more screws near the fracture site would inappropriately increase construct stiffness.

Question 34

During the pathogenesis of a prosthetic joint infection, Staphylococcus epidermidis secures itself to the implant surface by producing a protective extracellular polymeric substance. Which of the following is the primary component of this biofilm matrix?

Explanation

Biofilm formation by S. epidermidis is highly dependent on the production of polysaccharide intercellular adhesin (PIA). This matrix allows the bacteria to adhere to the implant and evade host immune responses and antibiotics.

Question 35

Following a severe crush injury to the sciatic nerve, Wallerian degeneration occurs distal to the injury site. Which cell type is primarily responsible for clearing myelin debris in the peripheral nervous system to facilitate axonal regeneration?

Explanation

In the peripheral nervous system, Schwann cells dedifferentiate and, alongside recruited macrophages, phagocytize myelin and axonal debris. This clears the pathway and creates Büngner bands to guide regenerating axons.

Question 36

A 14-year-old boy presents with a diaphyseal femur lesion showing an "onion skin" periosteal reaction. Biopsy reveals small, round blue cells. Which chromosomal translocation is most characteristic of this malignancy?

Explanation

Ewing sarcoma is classically associated with the t(11;22)(q24;q12) translocation, resulting in the EWS-FLI1 fusion protein. Synovial sarcoma exhibits t(X;18), and alveolar rhabdomyosarcoma shows t(2;13).

Question 37

A patient with chronic kidney disease presents with severe renal osteodystrophy. Which of the following laboratory profiles is most characteristic of secondary hyperparathyroidism in this clinical setting?

Explanation

In chronic kidney disease, impaired phosphate excretion and decreased 1-alpha-hydroxylase activity lead to hyperphosphatemia and hypocalcemia. The low serum calcium stimulates the parathyroid glands, resulting in high PTH levels.

Question 38

Aseptic loosening is a leading cause for late revision in total hip arthroplasty. The osteolytic cascade is primarily initiated by the macrophage phagocytosis of which specific type of wear particle?

Explanation

Macrophage phagocytosis of ultra-high-molecular-weight polyethylene (UHMWPE) wear particles in the 0.1 to 1.0 micrometer range triggers the release of pro-inflammatory cytokines like IL-1 and TNF-alpha. This inflammatory cascade directly leads to osteoclast activation and periprosthetic osteolysis.

Question 39

Recombinant human BMP-2 (rhBMP-2) is frequently utilized in spine surgery to enhance bone fusion. Through which intracellular signaling cascade do bone morphogenetic proteins primarily exert their osteoinductive effects?

Explanation

Bone morphogenetic proteins (BMPs) bind to cell surface serine/threonine kinase receptors, which phosphorylate intracellular Smad proteins. The activated Smad complex translocates to the nucleus to regulate target gene transcription for osteoblast differentiation.

Question 40

Which of the following biomechanical conditions is an absolute prerequisite for primary (direct) bone healing to occur without the formation of an intermediate cartilaginous callus?

Explanation

Primary bone healing relies on direct Haversian remodeling across the fracture gap. This requires absolute stability with an interfragmentary strain of less than 2% and a minimal gap distance (less than 0.1 mm).

Question 41

When evaluating muscle fiber types in an elite long-distance runner, one would expect a higher proportion of Type I fibers compared to a sprinter. Which of the following is a key physiological characteristic of Type I muscle fibers?

Explanation

Type I (slow-twitch) muscle fibers are highly oxidative, fatigue-resistant, and possess a high density of mitochondria, myoglobin, and capillaries. They primarily rely on aerobic metabolism, making them ideal for sustained endurance activities.

Question 42

Demineralized bone matrix (DBM) is commonly used as a graft substitute in orthopedic surgery. Which of the following biologic properties does DBM possess that synthetic calcium phosphate ceramics lack?

Explanation

DBM is produced by removing the mineral content of allograft bone, exposing the underlying native bone morphogenetic proteins (BMPs). This provides DBM with osteoinductive properties, whereas synthetic ceramics are purely osteoconductive.

Question 43

Tranexamic acid (TXA) has become the standard of care to reduce intraoperative blood loss in total joint arthroplasty. What is the specific mechanism of action of TXA?

Explanation

Tranexamic acid is a synthetic lysine analog that competitively inhibits the binding of plasminogen to fibrin. This prevents the activation of plasminogen to plasmin, thereby inhibiting fibrinolysis and maintaining clot stability.

Question 44

A 55-year-old female with osteoporosis is started on a new treatment regimen. Normal bone homeostasis relies on the balance between osteoblastic and osteoclastic activity. Osteoprotegerin (OPG) functions to inhibit bone resorption by directly binding to which of the following molecules?

Explanation

Osteoprotegerin (OPG) is a decoy receptor produced by osteoblasts that binds to RANKL. This prevents RANKL from interacting with RANK on osteoclast precursors, thereby inhibiting osteoclast differentiation and bone resorption.

Question 45

A revision total hip arthroplasty is performed for a failed implant. Intraoperatively, significant black debris and tissue necrosis are noted around the modular head-neck junction. Which of the following types of corrosion is most likely responsible for this phenomenon at a modular junction of dissimilar metals?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in electrical contact within an electrolytic solution (like body fluids). The less noble metal undergoes accelerated anodic dissolution, leading to implant degradation and adverse local tissue reactions.

Question 46

A synthetic polymer is being evaluated for use as an intervertebral disc replacement. When a constant sustained load is applied to the material over a prolonged period, it continues to deform progressively over time. What biomechanical property does this behavior describe?

Explanation

Creep is a viscoelastic property where a material undergoes time-dependent progressive deformation under a constant load. In contrast, stress relaxation is the decrease in stress over time when a material is held at a constant strain.

Question 47

In articular cartilage, compressive resilience and the ability to withstand physiological loads are primarily provided by the interaction between interstitial fluid and which of the following extracellular matrix components?

Explanation

Aggrecan is the most abundant proteoglycan in articular cartilage and possesses a high negative charge density due to its glycosaminoglycan chains. This creates a high osmotic pressure that draws water into the tissue, providing resistance to compressive forces.

Question 48

Following a peripheral nerve transection injury, Wallerian degeneration occurs in the distal stump to clear myelin and axonal debris. Which of the following structures is formed by proliferating Schwann cells to guide regenerating axonal sprouts across the injury site?

Explanation

During Wallerian degeneration, Schwann cells proliferate and align longitudinally to form the Bands of Büngner within the endoneurial tubes. These bands provide a supportive pathway and secrete neurotrophic factors to guide regenerating axons.

Question 49

During the remodeling phase of tendon healing, the biomechanical strength of the repaired tendon increases significantly. This improvement in tensile strength is primarily driven by which of the following physiological processes?

Explanation

The remodeling phase of tendon healing is characterized by the replacement of immature Type III collagen with mature Type I collagen. Furthermore, these collagen fibers reorganize and align parallel to the direction of mechanical stress, significantly increasing tensile strength.

Question 50

According to Perren's strain theory, what is the maximum amount of interfragmentary strain that can be tolerated to allow for primary bone healing (osteonal reconstruction) without the formation of a visible fracture callus?

Explanation

Perren's strain theory dictates that primary bone healing (direct osteonal remodeling without callus formation) requires absolute stability with an interfragmentary strain of less than 2%. Strains between 2% and 10% result in secondary bone healing via endochondral ossification.

Question 51

The introduction of highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) in total hip arthroplasty has significantly reduced volumetric wear rates. This improvement in wear resistance is typically achieved at the expense of decreasing which of the following mechanical properties?

Explanation

While high-dose irradiation and cross-linking of UHMWPE significantly improve wear resistance, these processes alter the polymer chain structure. This results in a reduction of mechanical properties such as fracture toughness, ductility, and fatigue crack propagation resistance.

Question 52

A 14-year-old boy presents with a painful, destructive diaphyseal lesion in his femur with an associated soft tissue mass. Biopsy reveals uniform small round blue cells. Molecular testing of the tumor tissue is most likely to reveal which of the following chromosomal translocations?

Explanation

The clinical and histologic picture describes Ewing sarcoma. The hallmark genetic abnormality in the vast majority of Ewing sarcoma cases is the t(11;22) chromosomal translocation, resulting in the EWS-FLI1 fusion protein.

Question 53

Titanium alloys (e.g., Ti-6Al-4V) are frequently used in orthopedic implants due to their excellent biocompatibility and fatigue strength. Which of the following characteristics best explains the superior in vivo corrosion resistance of titanium compared to stainless steel?

Explanation

Titanium alloys exhibit excellent corrosion resistance due to the spontaneous formation of a highly stable, self-healing passive titanium dioxide (TiO2) layer on the implant surface when exposed to oxygen in body fluids.

Question 54

Staphylococcus aureus is a leading cause of periprosthetic joint infection. The persistence of these infections is largely attributed to the formation of a biofilm. Which of the following is the primary constituent of the extracellular polymeric substance (EPS) matrix in a mature S. aureus biofilm?

Explanation

The extracellular polymeric substance (EPS) matrix of a S. aureus biofilm is primarily composed of polysaccharides, specifically poly-N-acetylglucosamine (PNAG). This matrix also contains extracellular DNA and proteins, which collectively protect the bacteria from immune cells and antibiotics.

Question 55

A 68-year-old female presents with diffuse bone pain and proximal muscle weakness. Laboratory evaluation reveals hypocalcemia, hypophosphatemia, and elevated alkaline phosphatase. A transiliac bone biopsy with double tetracycline labeling would most likely demonstrate which of the following?

Explanation

The patient's clinical and laboratory findings are consistent with osteomalacia. Histologically, osteomalacia is characterized by an increased width of unmineralized osteoid seams and an extended mineralization lag time, as evidenced by abnormal tetracycline labeling.

Question 56

During normal human walking, the articular cartilage of the knee is subjected to varying loads and sliding speeds. Which lubrication mechanism is most predominant during the swing phase of the gait cycle, when the joint experiences high sliding speeds and relatively low loads?

Explanation

Fluid-film lubrication is the primary mechanism reducing friction when the joint surfaces are moving rapidly under low loads, such as the swing phase. A thick layer of synovial fluid completely separates the articulating cartilage surfaces.

Question 57

Bupivacaine is commonly used for regional anesthesia and postoperative pain control in orthopedics. Its primary mechanism of action involves the blockade of which of the following neuronal ion channels?

Explanation

Local anesthetics like bupivacaine exert their effect by reversibly binding to the intracellular portion of voltage-gated sodium channels. This prevents sodium influx, thereby inhibiting the initiation and propagation of action potentials.

Question 58

Bone morphogenetic proteins (BMPs) are signaling molecules belonging to the TGF-beta superfamily that promote bone formation. Which of the following BMPs is an FDA-approved osteoinductive agent commonly used in acute open tibial shaft fractures?

Explanation

Recombinant human BMP-2 (rhBMP-2) is an FDA-approved osteoinductive biologic used to enhance fracture healing, particularly in acute open tibial shaft fractures treated with an intramedullary nail. BMP-7 (osteogenic protein-1) has also been used for fracture nonunions.

Question 59

During a heavy eccentric contraction, such as the lowering phase of a bicep curl, the muscle generates active tension while lengthening. Which of the following describes the fundamental structural and functional unit of the muscle fiber that lengthens during this process?

Explanation

The sarcomere is the basic contractile unit of a muscle fiber, defined as the segment between two adjacent Z-lines. During an eccentric contraction, the sarcomere lengthens as the actin and myosin filaments are pulled apart while actively generating tension.

Question 60

A 4-year-old boy presents with a history of multiple recurrent long bone fractures following minimal trauma, blue sclerae, and dentinogenesis imperfecta. This condition is most commonly caused by a genetic mutation affecting the synthesis of which of the following structural proteins?

Explanation

Osteogenesis imperfecta is a genetic disorder characterized by brittle bones and is most frequently caused by mutations in the COL1A1 or COL1A2 genes. These genes are responsible for the synthesis of Type I collagen, the major structural protein of bone.

Question 61

The anterior cruciate ligament (ACL) of the knee is primarily composed of water and a highly organized extracellular matrix of collagen. Which of the following cell types is predominantly responsible for the synthesis, maintenance, and repair of this extracellular matrix?

Explanation

Fibroblasts are the primary cellular components of ligaments and tendons. They synthesize and secrete the extracellular matrix proteins, predominantly Type I collagen, and maintain the structural integrity of the tissue.

Question 62

A patient undergoing a total knee arthroplasty is prescribed oral rivaroxaban for postoperative venous thromboembolism (VTE) prophylaxis. This medication exerts its anticoagulant effect by directly and reversibly inhibiting which of the following factors in the coagulation cascade?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that work by specifically and reversibly binding to and inhibiting Factor Xa. This interrupts the common pathway of the coagulation cascade, preventing the conversion of prothrombin to thrombin.

Question 63

Cefazolin is a first-generation cephalosporin routinely utilized for surgical site infection prophylaxis in clean orthopedic procedures. It exerts its bactericidal effect against susceptible organisms through which of the following mechanisms?

Explanation

Cephalosporins, including cefazolin, are beta-lactam antibiotics that are bactericidal. They work by binding to penicillin-binding proteins (PBPs), thereby inhibiting the cross-linking of peptidoglycan chains during bacterial cell wall synthesis.

Question 64

A 45-year-old male sustains a nonunion of the tibial shaft. A treatment plan involving demineralized bone matrix (DBM) is chosen. Which of the following best describes the osteogenic potential and primary mechanism of bone formation associated with DBM?

Explanation

DBM is osteoinductive and osteoconductive but completely lacks osteogenic properties as it contains no live cells. Its osteoinductive ability is derived from retained bone morphogenetic proteins (BMPs) that recruit and differentiate mesenchymal stem cells.