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1. General Principles & Basic Science MCQs

Practice Set 394 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 7861

Topic: Surgical Anatomy & Approaches

A patient sustains a closed midshaft humerus fracture and presents with a dense radial nerve palsy. Electromyography (EMG) performed at 4 weeks shows fibrillation potentials in the brachioradialis. High-resolution MRI reveals intact endoneurial and epineurial connective tissue architecture. According to the Seddon classification, this injury is best categorized as:

. Neuropraxia
. Axonotmesis
. Neurotmesis
. Sunderland Type IV injury
. Sunderland Type V injury

Correct Answer & Explanation

. Axonotmesis

Explanation

Axonotmesis (Seddon classification) involves disruption of the axon and myelin sheath, leading to Wallerian degeneration distal to the injury (hence fibrillation potentials on EMG after 3-4 weeks). However, the supporting connective tissue structures (endoneurium, perineurium, epineurium) remain intact, allowing for potential spontaneous recovery as the axon regenerates along the intact tubes. Neuropraxia is a temporary conduction block without axonal disruption (no fibrillations). Neurotmesis is complete nerve transection.

Question 7862

Topic: 1. General Principles & Basic Science
Tranexamic acid (TXA) is widely utilized in orthopedic surgery to reduce perioperative blood loss. Which of the following best describes its primary pharmacological mechanism?
. Direct inhibition of factor Xa
. Activation of antithrombin III
. Competitive inhibition of plasminogen activation
. Irreversible inhibition of cyclooxygenase
. Promotion of platelet aggregation via ADP receptors

Correct Answer & Explanation

. Competitive inhibition of plasminogen activation

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It acts as an antifibrinolytic agent by competitively and reversibly binding to the lysine-binding sites on plasminogen molecules. This prevents plasminogen from binding to fibrin, inhibiting its activation into plasmin and thereby preventing the degradation of fibrin clots (fibrinolysis).

Question 7863

Topic: Biomechanics & Biomaterials

During the biomechanical testing of a native human tendon, the initial portion of the stress-strain curve exhibits a non-linear, concave upward slope prior to entering the linear elastic region. This specific portion of the curve, known as the 'toe region,' is primarily due to:

. Microfailure of individual collagen fibrils
. Macroscopic tearing of the tendon substance
. The un-crimping of naturally wavy collagen fibers
. The transition from elastic to plastic deformation
. Expulsion of water from the proteoglycan matrix

Correct Answer & Explanation

. The un-crimping of naturally wavy collagen fibers

Explanation

The 'toe region' is the initial, non-linear segment of the stress-strain curve for ligaments and tendons where relatively little stress causes significant initial strain. Biologically, this corresponds to the 'un-crimping' or straightening of the naturally crimped (wavy) type I collagen fibers. Once the fibers are completely straight, the curve enters the linear elastic region. Microfailure occurs later, past the yield point in the plastic region.

Question 7864

Topic: Biomechanics & Biomaterials

A patient presents with a painful, swollen thigh 5 years after an open reduction and internal fixation of a femur fracture. Radiographs show localized radiolucency around a stainless steel screw that was inadvertently placed through a titanium alloy plate. What is the primary mechanism of implant failure in this scenario?

. Crevice corrosion
. Galvanic corrosion
. Fretting corrosion
. Pitting corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in direct physical contact within a conductive electrolytic fluid (such as body fluids). This creates an electrochemical potential difference, leading to an exchange of electrons and accelerated corrosion of the less noble metal (anode). This is why mixing dissimilar metals in a single construct is generally contraindicated.

Question 7865

Topic: 1. General Principles & Basic Science

Which zone of normal articular cartilage is characterized by the highest water content, the lowest proteoglycan concentration, and collagen fibers oriented parallel to the joint surface?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone of articular cartilage is the thinnest layer but is critical for resisting shear forces. It has the highest water content (approximately 80%), the lowest proteoglycan content, and densely packed type II collagen fibrils aligned parallel to the articular surface. Chondrocytes in this zone are flattened.

Question 7866

Topic: 1. General Principles & Basic Science

Tranexamic acid (TXA) is widely utilized in orthopedic surgery to minimize perioperative blood loss. Which of the following best describes its mechanism of action?

. Irreversible inhibition of cyclooxygenase-1
. Competitive inhibition of plasminogen activation
. Direct activation of the coagulation cascade via Factor VII
. Direct inhibition of thrombin generation
. Covalent cross-linking of fibrin monomers

Correct Answer & Explanation

. Competitive inhibition of plasminogen activation

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It exerts its antifibrinolytic effect by reversibly and competitively binding to the lysine receptor sites on plasminogen. This prevents plasminogen from binding to fibrin and being activated into plasmin, thereby inhibiting the degradation of fibrin clots (fibrinolysis).

Question 7867

Topic: Biomechanics & Biomaterials

During tensile mechanical testing of a normal, healthy ligament, a characteristic stress-strain curve is generated. What structural phenomenon is responsible for the initial 'toe region' of this curve?

. Microscopic tearing of individual collagen fibers
. Macroscopic failure of the tendon mid-substance
. Uncrimping of the naturally wavy collagen fibrils
. Plastic deformation of the extracellular matrix
. Elastic stretching of the completely straightened collagen fibers

Correct Answer & Explanation

. Uncrimping of the naturally wavy collagen fibrils

Explanation

The 'toe region' is the initial non-linear portion of the stress-strain curve for ligaments and tendons. It represents the elongation of the tissue with very little applied stress. This occurs due to the 'uncrimping' or straightening of the relaxed, naturally wavy collagen fibrils. Once the fibers are straight, the tissue enters the linear elastic region where stiffness increases.

Question 7868

Topic: 1. General Principles & Basic Science

Osteoclasts resorb the inorganic hydroxyapatite matrix of bone by secreting hydrogen ions into the isolated microenvironment of the ruffled border. Which intracellular enzyme is primarily responsible for generating these hydrogen ions?

. Cathepsin K
. Matrix metalloproteinase-9 (MMP-9)
. Tartrate-resistant acid phosphatase (TRAP)
. Carbonic anhydrase II
. Alkaline phosphatase

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

Carbonic anhydrase II catalyzes the hydration of carbon dioxide to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate. The hydrogen ions are actively pumped across the ruffled border by vacuolar H+-ATPases to create an acidic environment (pH ~4.5) that dissolves bone mineral. Cathepsin K and MMPs are responsible for degrading the organic matrix (e.g., Type I collagen) after demineralization.

Question 7869

Topic: Infection, Pharmacology & VTE

In the pathogenesis of an orthopedic implant-associated infection, what is the critical first step in the development of a bacterial biofilm?

. Synthesis of the extracellular polymeric substance (EPS)
. Quorum sensing and intercellular communication
. Reversible attachment of planktonic bacteria to the implant surface
. Irreversible adherence via species-specific adhesins
. Dispersion of bacteria to form secondary colonies

Correct Answer & Explanation

. Reversible attachment of planktonic bacteria to the implant surface

Explanation

Biofilm formation occurs in sequential stages. The initial step involves the reversible attachment of free-floating (planktonic) bacteria to the biomaterial surface via weak, non-specific forces (e.g., van der Waals forces, electrostatic interactions). This is followed by irreversible attachment mediated by specific adhesins (like MSCRAMMs), proliferation, EPS matrix production, maturation (involving quorum sensing), and finally dispersion.

Question 7870

Topic: Infection, Pharmacology & VTE
A 68-year-old patient is prescribed a postoperative oral anticoagulant following total hip arthroplasty. If the chosen medication acts as a direct, highly selective, reversible inhibitor of Factor Xa, which drug was prescribed?
. Warfarin
. Dabigatran
. Rivaroxaban
. Enoxaparin
. Fondaparinux

Correct Answer & Explanation

. Rivaroxaban

Explanation

Rivaroxaban (and apixaban) are direct, selective, and reversible inhibitors of Factor Xa. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin is a vitamin K antagonist affecting factors II, VII, IX, and X. Enoxaparin is a low-molecular-weight heparin that binds antithrombin III to inhibit Factor Xa indirectly. Fondaparinux is a synthetic pentasaccharide that also indirectly inhibits Factor Xa via antithrombin III.

Question 7871

Topic: Physiology & Rehabilitation

When comparing different skeletal muscle fiber types, fast-twitch glycolytic fibers (Type IIb) possess which of the following physiological and structural characteristics relative to slow-twitch oxidative fibers (Type I)?

. Higher density of mitochondria
. Heavy reliance on oxidative phosphorylation for energy
. High intracellular myoglobin content
. Rapid fatigue and high glycolytic enzyme capacity
. Smaller alpha motor neuron diameter

Correct Answer & Explanation

. Rapid fatigue and high glycolytic enzyme capacity

Explanation

Type IIb fibers (fast-twitch glycolytic) are specialized for rapid, high-force contractions but fatigue quickly. They have low myoglobin content (appearing 'white'), a low capillary density, and fewer mitochondria. They rely heavily on anaerobic glycolysis for ATP production, hence their high glycolytic enzyme capacity. Type I fibers are 'red', fatigue-resistant, mitochondria-rich, and rely on oxidative phosphorylation.

Question 7872

Topic: Biology, Genetics & Bone Healing
Sclerostin is an important regulator of bone metabolism and the target of the osteoporosis medication romosozumab. Which of the following best describes the primary mechanism of action of sclerostin?
. Stimulates RANKL expression by osteoblasts to increase osteoclastogenesis
. Binds to LRP5/6 receptors to inhibit the canonical Wnt/β-catenin signaling pathway
. Acts as a soluble decoy receptor for RANKL to prevent osteoclast activation
. Promotes the differentiation of mesenchymal stem cells into the chondrocytic lineage
. Cleaves osteopontin to facilitate initial osteoclast attachment to the bone surface

Correct Answer & Explanation

. Binds to LRP5/6 receptors to inhibit the canonical Wnt/β-catenin signaling pathway

Explanation

Sclerostin is a glycoprotein secreted primarily by mature osteocytes that acts as a potent negative regulator of bone formation. It functions by binding to the LRP5/6 co-receptors on the surface of osteoblasts, which competitively inhibits the canonical Wnt/β-catenin signaling pathway. This pathway is essential for osteoblast differentiation, proliferation, and survival. Inhibition of sclerostin (e.g., via the monoclonal antibody romosozumab) allows Wnt signaling to proceed, leading to increased bone formation.

Question 7873

Topic: Biomechanics & Biomaterials
Which of the following modifications to ultra-high molecular weight polyethylene (UHMWPE) used in total joint arthroplasty most effectively reduces adhesive and abrasive wear while preserving oxidation resistance?
. Gamma irradiation in the presence of an oxygen-rich environment
. Decreasing the overall degree of crystallinity of the polymer
. Highly cross-linking the polymer chains followed by remelting
. Increasing the free radical concentration within the polymer matrix
. Utilizing a non-cross-linked, non-irradiated pristine resin

Correct Answer & Explanation

. Highly cross-linking the polymer chains followed by remelting

Explanation

Highly cross-linked polyethylene (HXLPE) is manufactured using gamma or electron beam irradiation, which breaks polymer chains and facilitates cross-linking. This significantly improves wear resistance, reducing adhesive and abrasive wear. However, irradiation creates free radicals that can react with oxygen over time, leading to oxidative degradation and embrittlement. To combat this, the polyethylene is often remelted (heated above its melting point) to extinguish these free radicals, thereby preserving oxidation resistance, although remelting can cause a slight decrease in fatigue strength compared to annealing (heating below the melting point).

Question 7874

Topic: Infection, Pharmacology & VTE
A 65-year-old patient is scheduled for a total knee arthroplasty and will be prescribed rivaroxaban for postoperative venous thromboembolism (VTE) prophylaxis. At which specific point in the coagulation cascade does this medication primarily exert its effect?
. Direct, reversible inhibition of thrombin (Factor IIa)
. Antithrombin III-mediated indirect inhibition of Factor Xa
. Direct, competitive inhibition of free and clot-bound Factor Xa
. Inhibition of the vitamin K epoxide reductase complex
. Irreversible inhibition of the cyclooxygenase-1 (COX-1) enzyme

Correct Answer & Explanation

. Direct, competitive inhibition of free and clot-bound Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that function by directly and competitively inhibiting Factor Xa, interrupting both the intrinsic and extrinsic pathways of the blood coagulation cascade and preventing the conversion of prothrombin to thrombin. Dabigatran (Option A) is a direct thrombin inhibitor. Low molecular weight heparins like enoxaparin (Option B) act indirectly via antithrombin III. Warfarin (Option D) inhibits vitamin K epoxide reductase, and aspirin (Option E) irreversibly inhibits COX-1.

Question 7875

Topic: Biomechanics & Biomaterials

A surgeon is revising a nonunion of a femoral shaft fracture initially treated with a stainless steel dynamic compression plate. The surgeon plans to retain a broken stainless steel screw embedded in the far cortex but will place a new titanium intramedullary nail. What specific mode of corrosion is at greatest risk of occurring if these two distinct metals are in direct contact within the biologic environment?

. Crevice corrosion
. Fretting corrosion
. Galvanic corrosion
. Pitting corrosion
. Intergranular corrosion

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in direct physical contact within a conductive fluid medium (e.g., serum or body fluids). An electrochemical potential difference is established, which drives the accelerated corrosion of the less noble metal (anodic dissolution). For this reason, combining stainless steel and titanium implants in direct contact is generally contraindicated.

Question 7876

Topic: 1. General Principles & Basic Science

Articular cartilage is a highly specialized connective tissue designed for load-bearing and low-friction articulation. Under high-load physiological conditions, boundary lubrication prevents direct contact between the cartilage surfaces. Which of the following components is primarily responsible for boundary lubrication of the articular surface?

. Hyaluronic acid
. Aggrecan
. Type II collagen
. Proteoglycan 4 (Lubricin)
. Elastin

Correct Answer & Explanation

. Proteoglycan 4 (Lubricin)

Explanation

Lubricin (Proteoglycan 4, or PRG4) is a water-soluble glycoprotein synthesized by superficial zone chondrocytes and synoviocytes. It binds specifically to the articular surface, providing boundary lubrication, which drastically reduces friction under high compressive loads and prevents cell-cell adhesion. Hyaluronic acid contributes more to fluid-film lubrication and synovial fluid viscosity.

Question 7877

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory regarding secondary fracture healing, the differentiation of tissues within a fracture gap is dictated by the amount of local strain. What is the maximum local tissue strain that allows for the formation and persistence of lamellar (compact) bone without resulting in microfracture?

. Less than 2%
. Up to 10%
. Up to 15%
. Up to 30%
. Up to 100%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory dictates that a specific tissue can only form if the strain in the fracture gap is less than the tissue's tolerance before rupture. Granulation tissue can tolerate up to 100% strain; fibrous connective tissue and cartilage up to 10-15%; and woven bone up to 10%. Lamellar (compact) bone is highly brittle and requires a rigid mechanical environment with less than 2% strain to form and persist without microfracturing.

Question 7878

Topic: 1. General Principles & Basic Science
Tranexamic acid (TXA) is widely utilized in orthopedic surgery to mitigate perioperative blood loss. Which of the following best describes the precise mechanism of action of systemic tranexamic acid?
. Direct inhibition of Factor Xa
. Competitive inhibition of plasminogen activation to plasmin
. Irreversible acetylation of the cyclooxygenase-1 (COX-1) enzyme
. Reversible antagonism of the ADP (P2Y12) receptor on platelets
. Enhancement of antithrombin III activity

Correct Answer & Explanation

. Competitive inhibition of plasminogen activation to plasmin

Explanation

Tranexamic acid is a synthetic analog of the amino acid lysine. It acts as an antifibrinolytic agent by reversibly binding to the lysine-binding sites on plasminogen molecules. This competitively inhibits the activation of plasminogen into plasmin, thereby preventing the premature enzymatic degradation of established fibrin clots.

Question 7879

Topic: 1. General Principles & Basic Science

A 45-year-old patient with an upper trunk brachial plexus injury is scheduled for a nerve transfer. During the prolonged recovery phase, electromyography (EMG) is utilized to monitor reinnervation. Which of the following EMG findings represents the earliest reliable sign of muscle reinnervation?

. Fibrillation potentials
. Positive sharp waves
. Nascent motor unit action potentials (MUAPs)
. Complex repetitive discharges
. Giant motor unit action potentials

Correct Answer & Explanation

. Nascent motor unit action potentials (MUAPs)

Explanation

Fibrillation potentials and positive sharp waves are classic signs of active denervation, typically appearing 2-4 weeks after nerve injury. The earliest electromyographic indication of reinnervation is the appearance of nascent motor unit action potentials (MUAPs), which are characteristically low-amplitude, long-duration, highly polyphasic potentials. As reinnervation matures, collateral sprouting eventually organizes these into giant MUAPs.

Question 7880

Topic: Infection, Pharmacology & VTE

The success of surgical implant retention in acute periprosthetic joint infection strongly relies on early intervention before the establishment of a mature bacterial biofilm. The structural integrity, architecture, and antibiotic resistance of a mature Staphylococcus aureus biofilm are predominantly mediated by which of the following extracellular matrix components?

. Peptidoglycan cell wall
. Teichoic acids
. Polysaccharide intercellular adhesin (PIA) and extracellular DNA
. Lipopolysaccharide (LPS)
. Fibronectin-binding proteins

Correct Answer & Explanation

. Polysaccharide intercellular adhesin (PIA) and extracellular DNA

Explanation

The extracellular polymeric substance (EPS) or 'slime layer' that forms the mature structural matrix of a staphylococcal biofilm is composed primarily of Polysaccharide Intercellular Adhesin (PIA) (also known as poly-N-acetylglucosamine) and extracellular DNA (eDNA), along with various proteins. This EPS matrix physically shields the resident bacteria from host immune surveillance (phagocytes, antibodies) and prevents the penetration of systemic antibiotics.

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