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1. General Principles & Basic Science MCQs

Practice Set 409 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 8161

Topic: Biomechanics & Biomaterials

Corrosion of metallic orthopaedic implants can lead to catastrophic implant failure and adverse local tissue reactions. Which of the following scenarios best describes the mechanism underlying galvanic corrosion?

. Repeated cyclic loading of a single implant in a corrosive physiological environment
. Frictional micromotion between two modular implant components that disrupts the passivation layer
. Electrochemical dissolution occurring when two dissimilar metals are placed in direct physical contact within an electrolytic medium
. Degradation of polyethylene components leading to macrophage-mediated osteolysis
. Preferential loss of metal ions due to exposure to an alternating magnetic field

Correct Answer & Explanation

. Electrochemical dissolution occurring when two dissimilar metals are placed in direct physical contact within an electrolytic medium

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials (e.g., a stainless steel wire around a titanium rod) are placed in direct contact within an electrolytic solution (such as human body fluid). The less noble metal acts as an anode and undergoes accelerated corrosion. Fretting corrosion, in contrast, results from micromotion at modular interfaces (such as a head-neck taper) that repetitively strips the protective oxide (passivation) layer. Corrosion fatigue combines cyclic mechanical loading with a corrosive environment.

Question 8162

Topic: Physiology & Rehabilitation

During the active cross-bridge cycle of skeletal muscle contraction, specific molecular interactions dictate the binding and unbinding of actin and myosin. What is the direct biophysical consequence of a new molecule of adenosine triphosphate (ATP) binding to the myosin head?

. Exposure of the myosin-binding sites on the actin filament
. Detachment of the myosin head from the actin filament
. Hydrolysis of ATP that triggers the immediate 'power stroke'
. Release of inorganic phosphate (Pi) to stabilize the cross-bridge
. Active transport of calcium ions back into the sarcoplasmic reticulum

Correct Answer & Explanation

. Detachment of the myosin head from the actin filament

Explanation

In the sliding filament mechanism of muscle contraction, the binding of a new ATP molecule to the myosin head decreases myosin's affinity for actin, causing it to detach from the actin filament. Subsequent hydrolysis of ATP into ADP and inorganic phosphate (Pi) provides the energy to 'cock' the myosin head into a high-energy state. The actual 'power stroke' is triggered later when the inorganic phosphate is released from the myosin head. Calcium binding to troponin C is responsible for exposing the myosin-binding sites on actin.

Question 8163

Topic: Biology, Genetics & Bone Healing
Secondary fracture healing occurs primarily through endochondral ossification and progresses through distinct biological phases. During the formation of the soft callus (cartilaginous phase), which collagen type is predominantly synthesized by the local cellular infiltrate?
. Type I collagen
. Type II collagen
. Type III collagen
. Type IX collagen
. Type X collagen

Correct Answer & Explanation

. Type II collagen

Explanation

Secondary bone healing relies on endochondral ossification, which features an intermediate cartilaginous soft callus. During this phase, proliferating chondrocytes predominantly synthesize Type II collagen, the major structural protein of hyaline cartilage. As the process progresses to the hard callus phase, the cartilage is resorbed and replaced by woven bone, which is predominantly composed of Type I collagen. Type X collagen is uniquely secreted by hypertrophic chondrocytes during the late stages of endochondral ossification, facilitating mineralization.

Question 8164

Topic: Infection, Pharmacology & VTE
Rivaroxaban is frequently prescribed for extended venous thromboembolism (VTE) prophylaxis following total joint arthroplasty. What is the specific molecular mechanism of action by which rivaroxaban exerts its anticoagulant effect?
. Direct inhibition of thrombin (Factor IIa)
. Activation of antithrombin III
. Vitamin K antagonism via inhibition of epoxide reductase
. Direct inhibition of Factor Xa
. Irreversible inhibition of cyclooxygenase-1 (COX-1)

Correct Answer & Explanation

. Direct inhibition of Factor Xa

Explanation

Rivaroxaban, along with apixaban, belongs to the class of direct oral anticoagulants (DOACs) that function by directly and selectively inhibiting Factor Xa, preventing the conversion of prothrombin to thrombin. Dabigatran is an example of a direct thrombin (Factor IIa) inhibitor. Warfarin acts as a Vitamin K antagonist by inhibiting epoxide reductase. Heparins (including low-molecular-weight heparins like enoxaparin) exert their effect by binding to and potentiating antithrombin III. Aspirin irreversibly inhibits COX-1, affecting platelet aggregation.

Question 8165

Topic: Biomechanics & Biomaterials
What is the primary biomechanical tradeoff of heating highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) above its melting point (remelting) during processing to quench residual free radicals?
. Increased long-term wear rate
. Decreased ultimate tensile strength and fatigue resistance
. Increased potential for in vivo oxidation
. Increased elastic modulus and brittleness
. Decreased adhesive wear resistance

Correct Answer & Explanation

. Decreased ultimate tensile strength and fatigue resistance

Explanation

Highly cross-linked polyethylene is irradiated to improve wear resistance, but this creates free radicals that can lead to oxidation and degradation in vivo. To eliminate these free radicals, the material is either annealed (heated below the melting point) or remelted (heated above the melting point). Remelting effectively eliminates all free radicals, preventing oxidation, but the structural changes associated with melting significantly decrease the ultimate tensile strength, yield strength, and fatigue resistance of the material. Annealing preserves mechanical strength but leaves residual free radicals.

Question 8166

Topic: 1. General Principles & Basic Science

In the microscopic architecture of normal articular cartilage, which zone is characterized by the highest concentration of proteoglycans, the lowest water content, and type II collagen fibers oriented primarily perpendicular to the joint surface?

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

Correct Answer & Explanation

. Deep (radial) zone

Explanation

The deep (radial) zone of articular cartilage provides the greatest resistance to compressive loads. It contains the highest concentration of proteoglycans and the lowest concentration of water. The collagen fibers (primarily type II) in this zone are thickest and are arranged vertically (perpendicular to the articular surface), anchoring the cartilage to the underlying calcified zone. The superficial zone has the highest water content and collagen fibers parallel to the joint surface to resist shear stress.

Question 8167

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, the formation of primary lamellar bone requires a tissue strain environment of less than what percentage?

. < 2%
. 2% to 10%
. 10% to 30%
. 30% to 100%
. > 100%

Correct Answer & Explanation

. < 2%

Explanation

Perren's strain theory dictates the type of tissue that can form in a fracture gap based on the relative deformation (strain) of that tissue. Granulation tissue can tolerate up to 100% strain. Fibrocartilage and woven bone can form in environments with 2% to 10% strain. Primary lamellar bone formation, however, is highly sensitive to deformation and can only occur when the strain across the fracture gap is less than 2%.

Question 8168

Topic: Infection, Pharmacology & VTE

Which of the following best describes the primary phenotypic change in bacteria during the maturation phase of a biofilm in a periprosthetic joint infection, making eradication with standard systemic antibiotics exceedingly difficult?

. Increased replication and metabolic rate
. Shift to a metabolically dormant (sessile) state
. Downregulation of exopolysaccharide matrix production
. Upregulation of penicillin-binding proteins
. Enhanced flagellar motility for tissue invasion

Correct Answer & Explanation

. Shift to a metabolically dormant (sessile) state

Explanation

Biofilm formation involves initial attachment, maturation, and detachment (dispersion). During the maturation phase, bacteria encase themselves in a protective extracellular polymeric substance (EPS) matrix and undergo a phenotypic shift from a highly metabolic, free-floating (planktonic) state to a metabolically dormant (sessile) state. Standard antibiotics target active cellular processes (like cell wall synthesis and protein translation); therefore, these metabolically inactive sessile bacteria are highly recalcitrant to standard antimicrobial therapy.

Question 8169

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with progressive bowing of his right tibia, a localized increase in skin temperature over the leg, and elevated serum alkaline phosphatase levels with normal calcium and phosphorus. Which of the following best describes the primary cellular abnormality in this condition?

. Decreased osteoprotegerin (OPG) production by osteoblasts
. Defective mineralization of osteoid by osteoblasts
. Hyperactive, multinucleated osteoclasts with viral-like inclusion bodies
. Monoclonal proliferation of plasma cells stimulating osteoclast activating factor
. Parathyroid hormone-related peptide (PTHrP) driven osteoclastogenesis

Correct Answer & Explanation

. Hyperactive, multinucleated osteoclasts with viral-like inclusion bodies

Explanation

The patient's presentation is classic for Paget disease of bone (osteitis deformans). The primary cellular abnormality in Paget disease is an intense, focal increase in bone resorption by massive, hyperactive, multinucleated osteoclasts. These abnormal osteoclasts frequently contain viral-like inclusion bodies, and the disease process is often linked to SQSTM1 gene mutations and potential paramyxovirus infection. The intense osteoclastic phase is followed by disorganized osteoblastic woven bone formation.

Question 8170

Topic: 1. General Principles & Basic Science

During skeletal muscle contraction via the sliding filament theory, calcium ions released from the sarcoplasmic reticulum bind directly to which of the following proteins to initiate the exposure of myosin-binding sites on actin?

. Tropomyosin
. Troponin C
. Troponin I
. Troponin T
. Calmodulin

Correct Answer & Explanation

. Troponin C

Explanation

Muscle contraction is initiated when an action potential triggers the release of calcium ions from the sarcoplasmic reticulum. These calcium ions directly bind to Troponin C on the thin actin filaments. This binding causes a conformational change in the troponin complex (which includes Troponin C, I, and T), causing tropomyosin to shift away from the myosin-binding sites on actin, allowing the myosin heads to bind and cross-bridge cycling to begin.

Question 8171

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman is diagnosed with an unresectable giant cell tumor of the sacrum. Her multidisciplinary tumor board decides to initiate systemic medical therapy to help consolidate the lesion before attempting any possible surgical intervention. Which of the following best describes the mechanism of action of the most appropriate medication prescribed for this patient?

. Inhibition of vascular endothelial growth factor (VEGF)
. Binding to RANK ligand (RANKL), preventing osteoclast activation
. Selective estrogen receptor modulation
. Inhibition of the tyrosine kinase signaling pathway
. Alkylating agent causing DNA cross-linking

Correct Answer & Explanation

. Binding to RANK ligand (RANKL), preventing osteoclast activation

Explanation

Denosumab is a human monoclonal antibody that binds directly to RANKL, preventing it from binding to the RANK receptor on the surface of osteoclasts and their precursors. This effectively inhibits osteoclast formation, function, and survival, leading to decreased bone resorption. It is FDA-approved for the treatment of giant cell tumor of bone (GCTB) that is unresectable or where surgical resection is likely to result in severe morbidity. The neoplastic stromal cells in GCTB express high levels of RANKL, which recruit and activate the reactive multinucleated giant osteoclast-like cells responsible for aggressive bone destruction.

Question 8172

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman is evaluated for severe osteoporosis and started on an oral nitrogen-containing bisphosphonate. At the cellular level, these medications primarily inhibit osteoclastic bone resorption through which of the following mechanisms?

. Inhibiting the enzyme farnesyl pyrophosphate (FPP) synthase in the mevalonate pathway
. Antagonizing the parathyroid hormone (PTH) receptor
. Stimulating osteoblast production of osteoprotegerin (OPG)
. Inducing premature apoptosis of osteoblasts
. Binding directly to RANKL to prevent osteoclastogenesis

Correct Answer & Explanation

. Inhibiting the enzyme farnesyl pyrophosphate (FPP) synthase in the mevalonate pathway

Explanation

Nitrogen-containing bisphosphonates (e.g., alendronate, risedronate, zoledronic acid) act by inhibiting the enzyme farnesyl pyrophosphate (FPP) synthase within the mevalonate pathway. This inhibition prevents the prenylation of small GTPase proteins (such as Ras, Rho, and Rac) that are essential for osteoclast ruffled border formation, function, and survival, ultimately leading to osteoclast apoptosis and significantly decreased bone resorption. Non-nitrogenous bisphosphonates act via a different mechanism, becoming incorporated into non-hydrolyzable ATP analogues.

Question 8173

Topic: Biology, Genetics & Bone Healing

Bone Morphogenetic Proteins (BMPs) play a crucial role in osteoinduction and bone healing. Following the binding of a BMP to its specific cell surface serine/threonine kinase receptor, which of the following intracellular signaling molecules is directly phosphorylated to propagate the signal to the nucleus?

. Janus kinase (JAK)
. Signal Transducer and Activator of Transcription (STAT)
. Smad 1, 5, and 8
. Smad 2 and 3
. beta-catenin

Correct Answer & Explanation

. Smad 1, 5, and 8

Explanation

BMPs signal through highly specific cell surface serine/threonine kinase receptors. Upon ligand binding, the type II receptor phosphorylates the type I receptor, which subsequently phosphorylates receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These phosphorylated R-Smads then form a complex with the common-partner Smad (Co-Smad), Smad 4, and translocate to the nucleus to regulate the transcription of target osteogenic genes. In contrast, Smad 2 and 3 are typically activated by TGF-beta and Activin signaling.

Question 8174

Topic: 1. General Principles & Basic Science

Articular cartilage is structurally organized into distinct zones, each with unique biomechanical properties. Which zone is characterized by the highest concentration of proteoglycans, the lowest concentration of water, and collagen fibers that are oriented perpendicular to the articular surface?

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

Correct Answer & Explanation

. Deep (radial) zone

Explanation

The deep (radial) zone of normal articular cartilage contains the highest concentration of proteoglycans and the lowest water content. The collagen fibers (primarily type II) in this zone are oriented perpendicular to the articular surface to provide high compressive strength and securely anchor the uncalcified cartilage to the underlying calcified zone via the tidemark. Conversely, the superficial zone has the highest water content, the lowest proteoglycan content, and collagen fibers arranged parallel to the joint surface to resist shear stress.

Question 8175

Topic: 1. General Principles & Basic Science
A 55-year-old man presents with an acutely painful, erythematous, and swollen right knee. A diagnostic joint aspiration is performed, yielding opaque synovial fluid. Analysis reveals a white blood cell count of 85,000 cells/mm³ with 90% polymorphonuclear leukocytes, and a fluid glucose level of 25 mg/dL (serum glucose is 100 mg/dL). Which of the following is the most appropriate next step in management?
. Intra-articular corticosteroid injection
. Nonsteroidal anti-inflammatory drugs (NSAIDs) and observation
. Intravenous antibiotics and urgent surgical debridement of the knee
. Initiation of disease-modifying antirheumatic drugs (DMARDs)
. Physical therapy and weight-bearing as tolerated

Correct Answer & Explanation

. Intravenous antibiotics and urgent surgical debridement of the knee

Explanation

The synovial fluid analysis strongly suggests a septic joint. Fluid that is opaque, has >50,000 WBCs/mm³ (often >80,000) with a predominance of polymorphonuclear leukocytes (>75%), and significantly decreased glucose compared to serum is highly diagnostic of bacterial septic arthritis. The definitive standard of care is urgent surgical debridement and irrigation (arthroscopic or open) combined with empiric intravenous antibiotics, which should be tailored once culture and sensitivity results are available.

Question 8176

Topic: Biology, Genetics & Bone Healing

Perren's strain theory dictates the type of bone healing that occurs based on the interfragmentary strain present at the fracture site. According to this biomechanical theory, primary (direct) bone healing without the formation of an external callus occurs when the interfragmentary strain is maintained below what threshold?

. 2%
. 10%
. 15%
. 25%
. 50%

Correct Answer & Explanation

. 2%

Explanation

According to Perren's strain theory, primary (direct or osteonal) bone healing occurs when absolute stability is achieved, resulting in an interfragmentary strain of less than 2%. Under these rigid conditions, osteoclasts create cutting cones across the fracture, followed by osteoblasts depositing lamellar bone, without intermediate callus formation. Secondary bone healing with robust callus formation occurs under conditions of relative stability, where interfragmentary strain is between 2% and 10%. If strain exceeds 10%, bone cannot form, often resulting in fibrous tissue or nonunion.

Question 8177

Topic: Infection, Pharmacology & VTE
Rivaroxaban and apixaban are frequently used for effective venous thromboembolism (VTE) prophylaxis following total joint arthroplasty. At which specific point in the coagulation cascade do these oral medications exert their primary mechanism of action?
. Direct inhibition of thrombin (Factor IIa)
. Indirect inhibition of Factor Xa via antithrombin III
. Direct inhibition of Factor Xa
. Inhibition of vitamin K epoxide reductase
. Irreversible inhibition of cyclooxygenase-1 (COX-1)

Correct Answer & Explanation

. Direct inhibition of Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that specifically and directly inhibit Factor Xa (FXa) in the common pathway of the coagulation cascade. Dabigatran directly inhibits thrombin (Factor IIa). Fondaparinux and low-molecular-weight heparins (LMWH) indirectly inhibit FXa by potentiating the action of antithrombin III. Warfarin acts by inhibiting vitamin K epoxide reductase, preventing the synthesis of vitamin K-dependent factors (II, VII, IX, X). Aspirin irreversibly inhibits COX-1, preventing platelet aggregation.

Question 8178

Topic: Biomechanics & Biomaterials

Among the following solid metallic materials commonly used in orthopedic implants, which possesses the lowest modulus of elasticity (Young's modulus), thereby theoretically minimizing the risk of stress shielding when utilized for diaphyseal fixation?

. 316L Stainless steel
. Cobalt-chromium-molybdenum alloy
. Titanium alloy (Ti-6Al-4V)
. Alumina ceramic
. Zirconia ceramic

Correct Answer & Explanation

. Titanium alloy (Ti-6Al-4V)

Explanation

The modulus of elasticity (Young's modulus) represents the intrinsic stiffness of a material. Cortical bone has a modulus of approximately 15-20 GPa. Titanium alloys (e.g., Ti-6Al-4V) have a modulus of about 100-110 GPa, which is roughly half the stiffness of stainless steel (approx. 200 GPa) and cobalt-chromium alloys (approx. 210-230 GPa). Although titanium is still significantly stiffer than bone, its relatively lower elastic modulus compared to other common solid surgical metals translates to improved load-sharing and theoretically less severe stress shielding around intramedullary stems and plates.

Question 8179

Topic: Biomechanics & Biomaterials

In an orthopedic biomechanics laboratory, a researcher is testing the viscoelastic properties of a human anterior cruciate ligament (ACL) graft. A constant, unchanging tensile load is applied to the graft, and the researcher observes a gradual, time-dependent increase in the deformation (length) of the tissue. Which of the following biomechanical phenomena does this observation describe?

. Stress relaxation
. Creep
. Hysteresis
. Fatigue failure
. Anisotropy

Correct Answer & Explanation

. Creep

Explanation

Creep is the progressive deformation of a viscoelastic material when subjected to a constant load over time. Stress relaxation, another viscoelastic property, is the decrease in stress (force) within a material over time when it is held at a constant deformation or length. Hysteresis represents the energy lost (usually as heat) during the loading and unloading cycles of a material. Anisotropy refers to a material possessing different mechanical properties depending on the direction of the applied load.

Question 8180

Topic: Biology, Genetics & Bone Healing

Recombinant human bone morphogenetic proteins (rhBMP-2 and rhBMP-7) are potent osteoinductive agents frequently utilized to augment bone healing and spinal fusion. BMPs belong to the transforming growth factor-beta (TGF-beta) superfamily. Through which of the following intracellular signaling pathways do BMPs predominantly exert their osteoinductive effects?

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

Correct Answer & Explanation

. Smad pathway

Explanation

Bone morphogenetic proteins (BMPs) initiate intracellular signaling primarily through the Smad pathway. Upon binding to specific serine/threonine kinase surface receptors, BMPs cause the phosphorylation of receptor-regulated Smads (R-Smads: 1, 5, and 8). These phosphorylated R-Smads then complex with a common-partner Smad (Co-Smad 4) and translocate into the nucleus, where they act as transcription factors to upregulate osteogenic genes such as Runx2.

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