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1. General Principles & Basic Science MCQs

Practice Set 407 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 8121

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with worsening knee pain. Radiographs reveal an eccentric, expansile, lytic lesion in the epiphysis of the distal femur extending to the subchondral bone. Biopsy confirms a giant cell tumor of bone. She is initiated on a systemic monoclonal antibody treatment prior to surgical intervention to consolidate the lesion. What is the primary cellular target of this medication?

. Vascular Endothelial Growth Factor (VEGF)
. Receptor Activator of Nuclear Factor Kappa-B (RANK)
. Receptor Activator of Nuclear Factor Kappa-B Ligand (RANKL)
. Osteoprotegerin (OPG)
. Tyrosine Kinase Receptors

Correct Answer & Explanation

. Receptor Activator of Nuclear Factor Kappa-B Ligand (RANKL)

Explanation

Denosumab is often used in the treatment of locally advanced or difficult-to-resect Giant Cell Tumors (GCT) of bone. It is a human monoclonal antibody that binds to and neutralizes RANK Ligand (RANKL). RANKL is overexpressed by the neoplastic stromal cells in GCT, which drives the recruitment and activation of the multinucleated giant cells (osteoclast-like cells) responsible for the aggressive osteolysis. By inhibiting RANKL, Denosumab halts osteoclastogenesis.

Question 8122

Topic: Biomechanics & Biomaterials

When designing an intramedullary nail for a diaphyseal long-bone fracture, minimizing stress shielding while maintaining adequate fatigue strength is a primary biomechanical goal. Which of the following orthopaedic implant materials has a modulus of elasticity closest to that of human cortical bone?

. Stainless steel 316L
. Cobalt-chromium-molybdenum alloy
. Polymethylmethacrylate (PMMA)
. Titanium-6Aluminum-4Vanadium alloy
. Alumina ceramic

Correct Answer & Explanation

. Titanium-6Aluminum-4Vanadium alloy

Explanation

Human cortical bone has a modulus of elasticity (Young's modulus) of approximately 15-20 GPa. Titanium alloys (e.g., Ti-6Al-4V) have a modulus of about 110 GPa, which is the closest metallic implant material to cortical bone. Stainless steel is approximately 200 GPa, and Cobalt-chromium alloys are around 220-240 GPa. The lower modulus of elasticity of Titanium reduces the degree of stress shielding compared to stiffer metals.

Question 8123

Topic: 1. General Principles & Basic Science

Articular cartilage is a highly specialized tissue designed to resist compressive and shear forces within the joint. In which zone of articular cartilage are the collagen fibers oriented perpendicular to the articular surface, and which collagen type primarily constitutes these fibers?

. Superficial zone; Type I
. Superficial zone; Type II
. Middle (transitional) zone; Type II
. Deep (radial) zone; Type I
. Deep (radial) zone; Type II

Correct Answer & Explanation

. Deep (radial) zone; Type II

Explanation

Articular (hyaline) cartilage is primarily composed of Type II collagen. In the deep (radial) zone, the collagen fibers are oriented perpendicularly (vertically) to the articular surface to provide resistance to compressive loads. In the superficial (tangential) zone, collagen fibers are oriented parallel to the joint surface to resist shear forces.

Question 8124

Topic: 1. General Principles & Basic Science
A 68-year-old woman undergoes an uncomplicated total hip arthroplasty. To minimize perioperative blood loss, intravenous tranexamic acid (TXA) is administered. What is the specific mechanism of action of this agent?
. Irreversible inhibition of platelet cyclooxygenase-1 (COX-1)
. Direct inhibition of Factor Xa in the coagulation cascade
. Reversible competitive inhibition of plasminogen activation
. Potentiation of antithrombin III activity
. Inhibition of the von Willebrand factor binding to platelet glycoprotein Ib

Correct Answer & Explanation

. Reversible competitive inhibition of plasminogen activation

Explanation

Tranexamic acid (TXA) is a synthetic derivative of the amino acid lysine. It acts as an antifibrinolytic by reversibly binding to the lysine-binding sites on plasminogen. This competitively inhibits the activation of plasminogen to plasmin, thereby preventing the enzymatic degradation of fibrin blood clots.

Question 8125

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, the local mechanical strain environment dictates the specific type of tissue that forms at the fracture site. What is the maximum tissue strain percentage that permits the formation of lamellar bone directly without an intermediate cartilaginous phase (primary bone healing)?

. Less than 2%
. Between 2% and 10%
. Between 10% and 30%
. Between 30% and 100%
. Greater than 100%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory postulates that tissues can only form if the local strain is less than the specific tissue's elongation at failure. Lamellar bone can only form and survive in environments with less than 2% strain (primary/direct bone healing). Fibrocartilage and woven bone form at moderate strains (2-10%), while granulation tissue can tolerate high strains (up to 100%).

Question 8126

Topic: Biology, Genetics & Bone Healing

A 30-year-old woman presents with worsening right knee pain. Radiographs reveal an eccentric, lytic epiphyseal lesion of the distal femur that extends to the subchondral bone. A core biopsy confirms the diagnosis of a giant cell tumor of bone. The multidisciplinary tumor board recommends neoadjuvant treatment with denosumab prior to surgical intervention. What is the precise mechanism of action of this medication?

. Binds directly to the RANK receptor on osteoclasts
. Inhibits farnesyl pyrophosphate synthase in the mevalonate pathway
. Binds to RANKL, preventing its interaction with RANK
. Stimulates osteoprotegerin (OPG) production by osteoblasts
. Induces apoptosis of the neoplastic mononuclear cells directly

Correct Answer & Explanation

. Binds to RANKL, preventing its interaction with RANK

Explanation

Denosumab is a fully human monoclonal antibody that binds directly to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By binding to RANKL, it prevents this ligand from interacting with the RANK receptor on the surface of osteoclasts and osteoclast precursors. This prevents osteoclast formation, function, and survival, thereby halting bone resorption. In Giant Cell Tumor of Bone (GCTB), the true neoplastic cells are the mononuclear cells which secrete RANKL, driving the recruitment of reactive, destructive osteoclast-like giant cells. Denosumab targets the microenvironment signaling, not the neoplastic cells directly.

Question 8127

Topic: 1. General Principles & Basic Science

A 65-year-old woman with metastatic breast cancer presents with diffuse skeletal pain and is found to have multiple osteolytic bone lesions. To prevent skeletal-related events, she is started on intravenous zoledronic acid. Which of the following best describes the intracellular pharmacodynamic mechanism of action of this drug class?

. Inhibition of osteoblast apoptosis
. Direct binding to hydroxyapatite crystals
. Neutralization of parathyroid hormone-related peptide (PTHrP)
. Activation of the Wnt/beta-catenin signaling pathway
. Inhibition of farnesyl pyrophosphate synthase

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase

Explanation

Zoledronic acid is a potent nitrogen-containing bisphosphonate. While all bisphosphonates bind with high affinity to hydroxyapatite crystals in bone (pharmacokinetics), their intracellular mechanism of action (pharmacodynamics) differs based on their structure. Nitrogen-containing bisphosphonates work by inhibiting farnesyl pyrophosphate synthase, a key enzyme in the mevalonate pathway. This prevents the prenylation (attachment of lipid tails) of essential small GTP-binding proteins (e.g., Ras, Rho, Rac) in osteoclasts, leading to osteoclast dysfunction and apoptosis. Non-nitrogen-containing bisphosphonates (like clodronate) are metabolized into toxic ATP analogs that induce apoptosis.

Question 8128

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with worsening knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femoral metaphysis extending into the epiphysis up to the subchondral bone, lacking a sclerotic margin. Histology shows mononuclear stromal cells and numerous uniformly distributed multinucleated osteoclast-like giant cells. The stromal cells demonstrate a mutation in the H3F3A gene. What is the molecular target of the preferred medical therapy for unresectable disease?

. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Tyrosine kinase
. Vascular endothelial growth factor (VEGF)
. Macrophage colony-stimulating factor (M-CSF)
. Fibroblast growth factor receptor (FGFR)

Correct Answer & Explanation

. Receptor activator of nuclear factor kappa-B ligand (RANKL)

Explanation

The diagnosis is a Giant Cell Tumor (GCT) of bone, supported by the epiphyseal/metaphyseal location, lack of sclerotic rim, characteristic histology, and H3F3A mutation. In GCT, the neoplastic mononuclear stromal cells express RANKL, which recruits and activates the reactive osteoclast-like giant cells. Denosumab, a monoclonal antibody targeting RANKL, is highly effective for locally advanced, unresectable, or recurrent GCTs.

Question 8129

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy demonstrates mononuclear stromal cells interspersed with numerous multinucleated giant cells. The patient is prescribed a targeted biologic agent to reduce the tumor size prior to curettage. This agent exerts its effect by directly binding to which of the following?

. RANK receptor
. Osteoprotegerin (OPG)
. Vascular endothelial growth factor (VEGF)
. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Macrophage colony-stimulating factor (M-CSF)

Correct Answer & Explanation

. Receptor activator of nuclear factor kappa-B ligand (RANKL)

Explanation

The patient has a Giant Cell Tumor (GCT) of bone, and the targeted biologic agent described is Denosumab. Denosumab is a fully human monoclonal antibody that specifically binds to RANKL. In GCT, the neoplastic mononuclear stromal cells overexpress RANKL, which recruits and stimulates the differentiation of osteoclast precursors into reactive, bone-resorbing multinucleated giant cells. By binding to and inhibiting RANKL, denosumab prevents the activation of the RANK receptor on osteoclasts, halting osteoclastogenesis and significantly reducing tumor-associated osteolysis.

Question 8130

Topic: Biomechanics & Biomaterials

What is the primary mechanism of corrosion at the modular taper junction (head-neck interface) of a total hip arthroplasty?

. Galvanic corrosion
. Mechanically assisted crevice corrosion
. Intergranular corrosion
. Pitting corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Mechanically assisted crevice corrosion

Explanation

Mechanically assisted crevice corrosion (MACC), also known as fretting corrosion, is the primary mode of failure at modular head-neck taper junctions in total hip arthroplasty. The cyclic loading causes mechanical disruption of the protective oxide layer (fretting), and the fluid in the confined modular space becomes acidic and depleted of oxygen (crevice), leading to synergistic corrosion.

Question 8131

Topic: Biology, Genetics & Bone Healing

A 68-year-old man presents with progressive bowing of his right tibia and dull, aching pain. Laboratory studies reveal an isolated elevation in serum alkaline phosphatase. Which of the following is the primary cellular defect in the early pathogenesis of this disease?

. Increased activity of structurally abnormal osteoclasts
. Defective mineralization of osteoid by osteoblasts
. Mutation in the type I collagen gene
. Defective fibroblast growth factor receptor 3 (FGFR3)
. Overproduction of parathyroid hormone-related peptide (PTHrP)

Correct Answer & Explanation

. Increased activity of structurally abnormal osteoclasts

Explanation

The patient's presentation is classic for Paget's disease of bone. The primary pathogenetic defect lies in the osteoclast. The osteoclasts are increased in number, abnormally large, and possess many more nuclei than normal osteoclasts. This leads to an initial phase of intense bone resorption, which is followed by a robust, haphazard compensatory bone formation by osteoblasts, resulting in structurally weak woven bone.

Question 8132

Topic: 1. General Principles & Basic Science

Which of the following modifications to a cortical screw design will most significantly increase its pullout strength in diaphyseal bone?

. Decreasing the outer diameter
. Increasing the inner (root) diameter
. Decreasing the thread pitch
. Increasing the thread pitch
. Decreasing the thread depth

Correct Answer & Explanation

. Decreasing the thread pitch

Explanation

Screw pullout strength is primarily determined by the outer diameter of the screw, thread pitch, thread depth, and the quality of the bone. Pullout strength is directly proportional to the outer diameter and thread depth, and inversely proportional to the thread pitch. Decreasing the thread pitch (meaning more threads per unit of length) increases the amount of bone engaged by the screw threads, thereby increasing pullout strength.

Question 8133

Topic: Biology, Genetics & Bone Healing

Which of the following statements best describes the process of intramembranous ossification during fracture healing?

. It requires a cartilaginous template prior to bone formation
. It occurs primarily through the action of chondrocytes
. It is the primary mechanism of healing in rigidly fixed fractures with absolute stability
. It depends on the influx of undifferentiated mesenchymal cells differentiating into chondroblasts
. It is clinically characterized by the formation of a prominent external fracture callus

Correct Answer & Explanation

. It is the primary mechanism of healing in rigidly fixed fractures with absolute stability

Explanation

Intramembranous ossification involves the direct differentiation of mesenchymal stem cells into osteoblasts without a cartilaginous intermediate. It is the primary mechanism of primary bone healing (contact healing) in rigidly fixed fractures (e.g., with compression plating) where strain is minimized to absolute stability. Secondary bone healing (callus formation) relies on endochondral ossification, which requires a cartilaginous template.

Question 8134

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy confirms a giant cell tumor of bone. If medical management is initiated, denosumab works by targeting which of the following?

. RANK ligand (RANKL) expressed by the multinucleated giant cells
. RANK receptor expressed by the mononuclear stromal cells
. RANK ligand (RANKL) expressed by the mononuclear stromal cells
. Osteoprotegerin (OPG) expressed by the multinucleated giant cells
. Vascular endothelial growth factor (VEGF) expressed by the tumor cells

Correct Answer & Explanation

. RANK ligand (RANKL) expressed by the mononuclear stromal cells

Explanation

In Giant Cell Tumor (GCT) of bone, the actual neoplastic cells are the mononuclear spindle-shaped stromal cells, which express high levels of RANK ligand (RANKL). This RANKL binds to RANK receptors on the surface of normal osteoclast precursors, leading to their differentiation into reactive, bone-resorbing multinucleated giant cells. Denosumab is a monoclonal antibody that binds directly to RANKL on the stromal cells, preventing its interaction with RANK and inhibiting osteoclast-like giant cell formation.

Question 8135

Topic: 1. General Principles & Basic Science

Tranexamic acid (TXA) is frequently used in orthopedic surgery to reduce perioperative blood loss. Which of the following correctly describes its primary mechanism of action?

. It inhibits the conversion of fibrinogen to fibrin by blocking thrombin
. It competitively blocks the lysine-binding sites on plasminogen, preventing its binding to fibrin
. It promotes the release of von Willebrand factor from endothelial cells
. It directly inhibits factor Xa in the coagulation cascade
. It irreversibly inhibits cyclooxygenase, preventing platelet aggregation

Correct Answer & Explanation

. It competitively blocks the lysine-binding sites on plasminogen, preventing its binding to fibrin

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It acts as an antifibrinolytic agent by competitively inhibiting the activation of plasminogen to plasmin. It achieves this by binding to the lysine-binding sites on plasminogen, thereby blocking the binding of plasminogen to fibrin. This prevents the degradation of fibrin clots, effectively reducing surgical bleeding.

Question 8136

Topic: Infection, Pharmacology & VTE

A 70-year-old man develops a chronic prosthetic joint infection 2 years following a total knee arthroplasty. Which of the following represents the primary component of the biofilm matrix that protects the bacteria from host immune clearance and antibiotics?

. Type I collagen
. Fibronectin
. Extracellular polymeric substances (polysaccharides)
. Peptidoglycan
. Lipopolysaccharide

Correct Answer & Explanation

. Extracellular polymeric substances (polysaccharides)

Explanation

Biofilms are structured communities of bacteria encased in a self-produced polymeric matrix adhered to an implant surface. This glycocalyx matrix is primarily composed of extracellular polymeric substances (EPS), which include heavily hydrated polysaccharides, proteins, and extracellular DNA (eDNA). This matrix acts as a physical and chemical barrier against host immune cells and prevents the penetration of many antimicrobial agents.

Question 8137

Topic: Biology, Genetics & Bone Healing

A 55-year-old man undergoes an anterior lumbar interbody fusion. Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized to augment the fusion. What is the primary intracellular signaling pathway through which rhBMP-2 exerts its osteoinductive effect?

. Wnt/beta-catenin pathway
. Smad 1/5/8 pathway
. MAPK/ERK pathway
. JAK/STAT pathway
. Notch signaling pathway

Correct Answer & Explanation

. Smad 1/5/8 pathway

Explanation

BMPs, including rhBMP-2, bind to serine/threonine kinase receptors on the cell surface. Upon activation, they primarily signal through the phosphorylation of receptor-regulated Smad proteins (Smads 1, 5, and 8). These phosphorylated Smads form a complex with the common-partner Smad 4, which then translocates to the nucleus to regulate transcription of osteogenic target genes, including Runx2, thereby promoting osteoblast differentiation.

Question 8138

Topic: Biomechanics & Biomaterials

A 30-year-old athlete sustains a suspected Achilles tendon injury. During the physiological loading of the tendon, the initial 'toe region' of the stress-strain curve represents which specific microscopic change?

. Microscopic failure of individual collagen fibrils
. Uncrimping of the naturally wavy collagen fibers
. Rupture of intermolecular collagen cross-links
. Plastic deformation of the extracellular matrix
. Macroscopic failure of the tendon fascicles

Correct Answer & Explanation

. Uncrimping of the naturally wavy collagen fibers

Explanation

The stress-strain curve for ligaments and tendons begins with a non-linear 'toe region.' This region corresponds to the straightening out (uncrimping) of the naturally crimped collagen fibers as initial tension is applied. Once uncrimped, the curve becomes linear (the elastic region), where the fibers are stretched until yielding occurs.

Question 8139

Topic: 1. General Principles & Basic Science

Articular cartilage relies on its unique biochemical and biomechanical properties for load bearing. Which specific zone of articular cartilage contains the highest concentration of proteoglycans and is primarily responsible for resisting compressive forces?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified cartilage zone
. Tidemark

Correct Answer & Explanation

. Deep (radial) zone

Explanation

The deep (radial) zone of articular cartilage possesses the highest concentration of proteoglycans (aggrecan) and the lowest water content. The collagen fibers (primarily type II) in this zone are oriented perpendicularly to the articular surface. This composition makes the deep zone primarily responsible for resisting significant compressive forces. The superficial zone has the highest water and collagen content, which functions primarily to resist shear forces.

Question 8140

Topic: 1. General Principles & Basic Science
A 65-year-old man presents with an acutely swollen, erythematous, and severely painful right knee. Joint aspiration yields synovial fluid with 55,000 WBC/mm³, 85% polymorphonuclear leukocytes, and strongly negatively birefringent needle-shaped crystals under polarized light microscopy. Which of the following best describes the pathogenesis of the crystal formation in this patient?
. Underexcretion of uric acid by the kidneys
. Defect in inorganic pyrophosphate metabolism
. Deposition of basic calcium phosphate
. Autoimmune complex deposition in the synovium
. Precipitation of cholesterol crystals

Correct Answer & Explanation

. Underexcretion of uric acid by the kidneys

Explanation

The patient has an acute gout flare, characterized by monosodium urate crystals that are needle-shaped and strongly negatively birefringent under polarized light. Gout results from hyperuricemia, which in the vast majority of cases (approximately 90%) is due to the underexcretion of uric acid by the kidneys, rather than overproduction. Pseudogout (CPPD) involves a defect in inorganic pyrophosphate metabolism, resulting in rhomboid, weakly positively birefringent calcium pyrophosphate crystals.

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