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1. General Principles & Basic Science MCQs

Practice Set 596 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 11901

Topic: Biology, Genetics & Bone Healing

The Wnt/beta-catenin signaling pathway is a major regulator of bone mass. The medication Romosozumab targets this pathway to treat osteoporosis. What is the precise mechanism by which Romosozumab increases bone mineral density?

. It binds directly to RANKL, preventing activation of osteoclasts.
. It inhibits sclerostin, which unblocks the Wnt/beta-catenin pathway and promotes osteoblast activity.
. It directly activates the parathyroid hormone receptor to stimulate bone formation.
. It inhibits cathepsin K, preventing the breakdown of the collagen matrix.
. It acts as a decoy receptor for osteoprotegerin (OPG).

Correct Answer & Explanation

. It binds directly to RANKL, preventing activation of osteoclasts.

Explanation

Romosozumab is a humanized monoclonal antibody that binds to and inhibits sclerostin. Sclerostin is a glycoprotein secreted by osteocytes that normally inhibits the Wnt/beta-catenin pathway. By blocking sclerostin, Romosozumab "takes the brakes off" Wnt signaling, leading to robust osteoblast differentiation and marked bone formation, alongside a mild decrease in bone resorption.

Question 11902

Topic: Biomechanics & Biomaterials



Polymethylmethacrylate (PMMA) bone cement is routinely used for implant fixation. Adding an antibiotic powder or a radiopacifier (such as barium sulfate) to the PMMA prior to mixing has what primary biomechanical effect on the cured cement mantle?

. Increases the ultimate compressive strength
. Decreases the ultimate tensile strength and fatigue life
. Increases the fatigue life under tensile load
. Decreases the modulus of elasticity making it highly ductile
. Has no significant effect on the mechanical properties if added at less than 10% by weight

Correct Answer & Explanation

. Increases the ultimate compressive strength

Explanation

PMMA is a viscoelastic material that is strong in compression but relatively weak in tension and shear. The addition of powders, antibiotics, or radiopacifiers (like barium sulfate or zirconium dioxide) introduces impurities and microscopic stress risers into the cement mantle. This reliably decreases its ultimate tensile strength and reduces its fatigue life.

Question 11903

Topic: 1. General Principles & Basic Science

According to Sunderland's classification of peripheral nerve injuries, a third-degree nerve injury is characterized by the disruption of which specific structures?

. Myelin sheath only, with an intact axon
. Axon only, with intact endoneurium, perineurium, and epineurium
. Axon and endoneurium, with an intact perineurium and epineurium
. Axon, endoneurium, and perineurium, with an intact epineurium
. Complete transection of the nerve trunk including the epineurium

Correct Answer & Explanation

. Myelin sheath only, with an intact axon

Explanation

Sunderland Classification: 1st degree = neuropraxia (myelin injury, intact axon). 2nd degree = axonotmesis (axon disrupted, endoneurium intact). 3rd degree = axon and endoneurium are disrupted, but the perineurium remains intact (internal scarring occurs). 4th degree = axon, endoneurium, and perineurium disrupted, but the epineurium is intact. 5th degree = neurotmesis (complete transection).

Question 11904

Topic: 1. General Principles & Basic Science
The medial meniscus acts as an important secondary stabilizer to anterior tibial translation in an ACL-deficient knee. Which of the following best describes the predominant structural collagen orientation that provides the meniscus with its ability to resist hoop stresses during axial loading?
. Predominantly Type II collagen arranged in a radial pattern
. Predominantly Type I collagen arranged in longitudinal circumferential bundles
. Predominantly Type I collagen arranged radially from the periphery to the inner margin
. Predominantly Type II collagen arranged in longitudinal circumferential bundles
. Predominantly Type III collagen arranged in a disorganized woven pattern

Correct Answer & Explanation

. Predominantly Type I collagen arranged in longitudinal circumferential bundles

Explanation

The meniscus is a fibrocartilaginous structure composed primarily of Type I collagen (unlike articular hyaline cartilage, which is primarily Type II). The collagen fibers are highly organized in a longitudinal, circumferential pattern. This arrangement allows the meniscus to effectively dissipate axial compressive loads by converting them into tensile forces known as 'hoop stresses'. Less numerous radial 'tie fibers' help bind these circumferential bundles together.

Question 11905

Topic: Biomechanics & Biomaterials

Which combination of orthopedic metals has the highest risk of significant galvanic corrosion when placed in direct contact in a conductive physiologic environment?

. Titanium alloy (Ti-6Al-4V) and Cobalt-Chrome
. Stainless Steel 316L and Titanium alloy (Ti-6Al-4V)
. Cobalt-Chrome and Tantalum
. Titanium alloy (Ti-6Al-4V) and Tantalum
. Zirconium and Cobalt-Chrome

Correct Answer & Explanation

. Titanium alloy (Ti-6Al-4V) and Cobalt-Chrome

Explanation

Galvanic corrosion occurs when two dissimilar metals are in direct electrical contact within an electrolyte solution (such as body fluid). The severity of corrosion is dictated by the difference in their resting potentials on the galvanic series. Stainless steel and titanium alloys have a significant difference in their electrochemical potentials, leading to a high risk of galvanic corrosion of the less noble metal (stainless steel). Cobalt-chrome and titanium are close enough on the galvanic series that they are routinely mixed (e.g., a cobalt-chrome femoral head on a titanium stem) with negligible clinical galvanic corrosion.

Question 11906

Topic: Biomechanics & Biomaterials

A viscoelastic orthopedic implant material is subjected to a constant load over a prolonged period, resulting in a progressive increase in strain (deformation). What is this biomechanical phenomenon termed?

. Stress relaxation
. Hysteresis
. Creep
. Fatigue failure
. Isotropic behavior

Correct Answer & Explanation

. Stress relaxation

Explanation

Creep is defined as the progressive deformation (increasing strain) of a viscoelastic material when subjected to a constant load (stress) over time. In contrast, stress relaxation occurs when a material is held at a constant strain, and the stress within the material decreases over time. Hysteresis represents the energy lost (usually as heat) during the loading and unloading cycles of a viscoelastic material.

Question 11907

Topic: Biology, Genetics & Bone Healing

In normal adult articular cartilage, which zone is characterized by the highest concentration of water and collagen fibers oriented parallel to the articular surface?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone of articular cartilage contains the highest water content (approximately 80%), the lowest proteoglycan concentration, and collagen type II fibers that are oriented parallel to the joint surface to resist shear forces. The deep zone contains collagen fibers oriented perpendicular to the joint surface and has the highest concentration of proteoglycans with the lowest water content.

Question 11908

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) play a crucial role in osteogenesis by binding to cell surface receptors. Following binding, which intracellular proteins are phosphorylated to translocate to the nucleus and induce transcription of osteogenic genes?

. Beta-catenin
. Smad 1/5/8
. JAK2
. STAT3
. NF-kappa B

Correct Answer & Explanation

. Beta-catenin

Explanation

BMPs are members of the TGF-beta superfamily. They bind to serine/threonine kinase cell surface receptors. Once activated, these receptors phosphorylate intracellular receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These phosphorylated Smads then form a complex with the common-partner Smad (Smad 4) and translocate to the nucleus to regulate target gene transcription. Beta-catenin is involved in the Wnt signaling pathway.

Question 11909

Topic: Biology, Genetics & Bone Healing

Which of the following molecules acts as an antagonist to the Wnt/beta-catenin signaling pathway, thereby inhibiting osteoblastogenesis and reducing bone formation?

. Sclerostin
. Osteoprotegerin (OPG)
. Runx2
. Osterix
. Transforming growth factor-beta (TGF-beta)

Correct Answer & Explanation

. Sclerostin

Explanation

Sclerostin (produced primarily by osteocytes) binds to the LRP5/6 receptors on osteoblasts, competitively inhibiting the Wnt signaling pathway. This prevents the accumulation of beta-catenin, leading to decreased osteoblast differentiation and reduced bone formation. Monoclonal antibodies targeting sclerostin (e.g., romosozumab) are used therapeutically to increase bone mass.

Question 11910

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (such as alendronate and zoledronic acid) inhibit bone resorption primarily by inhibiting which of the following enzymes in the osteoclast?

. Cathepsin K
. Carbonic anhydrase II
. Farnesyl pyrophosphate synthase
. Tartrate-resistant acid phosphatase
. Matrix metalloproteinase-9

Correct Answer & Explanation

. Cathepsin K

Explanation

Nitrogen-containing bisphosphonates work by inhibiting farnesyl pyrophosphate synthase, an enzyme in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (like Ras, Rho, and Rac) that are essential for osteoclast function, ruffled border formation, and survival, ultimately leading to osteoclast apoptosis.

Question 11911

Topic: Physiology & Rehabilitation

During the normal human gait cycle, at which phase does the anterior tibialis muscle exhibit its peak electromyographic (EMG) activity?

. Initial contact (heel strike) to loading response
. Mid-stance
. Terminal stance
. Pre-swing (toe-off)
. Mid-swing

Correct Answer & Explanation

. Initial contact (heel strike) to loading response

Explanation

The anterior tibialis fires intensely during initial contact and the early loading response. Its primary role during this phase is eccentric contraction to smoothly decelerate plantarflexion of the foot, thereby preventing 'foot slap'. It also fires concentrically during the swing phase to ensure foot clearance, but its peak force/activity occurs eccentrically at heel strike.

Question 11912

Topic: 1. General Principles & Basic Science

Which of the following systemic factors most strongly upregulates the activity of 1-alpha-hydroxylase in the proximal tubule of the kidney, thus increasing the conversion of 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D?

. Elevated serum calcium levels
. Elevated serum phosphate levels
. Parathyroid hormone (PTH)
. Calcitonin
. Fibroblast growth factor 23 (FGF-23)

Correct Answer & Explanation

. Elevated serum calcium levels

Explanation

Parathyroid hormone (PTH) is secreted in response to low serum calcium. One of its primary actions in the kidney is to upregulate the enzyme 1-alpha-hydroxylase, which converts 25(OH)D to 1,25(OH)2D (calcitriol). Elevated calcium and FGF-23 inhibit this enzyme, whereas calcitriol itself exhibits negative feedback on 1-alpha-hydroxylase.

Question 11913

Topic: Biomechanics & Biomaterials

The in vivo elution of locally delivered antibiotics from polymethylmethacrylate (PMMA) bone cement is best characterized by which of the following release profiles?

. Zero-order kinetics (constant release rate over time)
. Biphasic, with an initial burst release followed by a prolonged low-level release
. Exponential continuous increase peaking at 6 weeks
. Delayed release beginning 14 days post-implantation
. Strictly dependent on the host's renal clearance rates

Correct Answer & Explanation

. Zero-order kinetics (constant release rate over time)

Explanation

Antibiotic elution from PMMA is famously biphasic. There is a high initial 'burst' release of antibiotic from the surface of the cement within the first 24 to 72 hours, producing high local concentrations. This is followed by a prolonged, exponentially decreasing, low-level release that can persist for weeks to months, often dropping below the minimum inhibitory concentration (MIC).

Question 11914

Topic: 1. General Principles & Basic Science

The direct insertion of a tendon to bone (enthesis) is structurally organized into four distinct histologic zones to minimize stress concentration. What is the correct order of these zones from the tendon toward the bone?

. Tendon, calcified fibrocartilage, uncalcified fibrocartilage, bone
. Tendon, uncalcified fibrocartilage, calcified fibrocartilage, bone
. Tendon, uncalcified fibrocartilage, tidemark, bone
. Tendon, loose areolar tissue, uncalcified fibrocartilage, bone
. Tendon, Sharpey's fibers, loose areolar tissue, bone

Correct Answer & Explanation

. Tendon, calcified fibrocartilage, uncalcified fibrocartilage, bone

Explanation

A direct (fibrocartilaginous) tendon insertion transitions through four gradual histologic zones to dissipate stress and prevent failure: 1) Tendon proper, 2) Uncalcified fibrocartilage, 3) Calcified fibrocartilage, and 4) Bone. A tidemark typically separates the uncalcified and calcified fibrocartilage layers.

Question 11915

Topic: 1. General Principles & Basic Science

Which of the following characteristics accurately differentiates Type I (slow-twitch) skeletal muscle fibers from Type II (fast-twitch) fibers?

. Lower mitochondrial density
. Higher reliance on anaerobic glycolytic metabolism
. Higher myoglobin content
. Faster contraction speed and fatiguability
. Larger cross-sectional area

Correct Answer & Explanation

. Lower mitochondrial density

Explanation

Type I fibers are 'slow-twitch' and rely on oxidative (aerobic) metabolism. They are rich in mitochondria, capillary density, and myoglobin, which gives them a red appearance. They are highly resistant to fatigue but have a slower contraction speed and lower peak force. Type II fibers are 'fast-twitch', rely more on glycolytic pathways, fatigue quickly, and have a larger cross-sectional area.

Question 11916

Topic: 1. General Principles & Basic Science
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is extensively used in total hip arthroplasty to reduce wear. What is the primary mechanical trade-off associated with increasing the radiation dose to maximize cross-linking?
. Increased adhesive wear volume
. Decreased ultimate tensile strength and reduced fatigue crack propagation resistance
. Increased risk of oxidative degradation without subsequent melting
. Decreased elasticity leading to brittle shattering during normal gait
. Increased creep (cold flow) deformation

Correct Answer & Explanation

. Decreased ultimate tensile strength and reduced fatigue crack propagation resistance

Explanation

While high doses of gamma or electron beam irradiation dramatically increase cross-linking (which vastly reduces adhesive/abrasive wear), it alters the mechanical properties of the UHMWPE. The primary trade-off is a decrease in ductility, ultimate tensile strength, yield strength, and fatigue crack propagation resistance. This is why highly cross-linked PE is used cautiously in highly stressed components like thin tibial trays, though it is standard for hip liners.

Question 11917

Topic: 1. General Principles & Basic Science

In the extracellular matrix of articular cartilage, aggrecan monomers attach to a central hyaluronic acid backbone to form massive proteoglycan aggregates. What specific molecule is required to stabilize this non-covalent interaction?

. Chondroitin sulfate
. Link protein
. Keratan sulfate
. Decorin
. Biglycan

Correct Answer & Explanation

. Chondroitin sulfate

Explanation

The massive proteoglycan aggregates in cartilage consist of aggrecan monomers (which themselves are core proteins decorated with chondroitin sulfate and keratan sulfate side chains) bound to a central linear strand of hyaluronic acid. This critical non-covalent binding is stabilized by a specific glycoprotein known as 'link protein'.

Question 11918

Topic: Biomechanics & Biomaterials

A loose fragment of polymethylmethacrylate (PMMA) cement becomes interposed between the femoral head and the polyethylene liner in a total hip arthroplasty, resulting in gouging and accelerated destruction of the liner. This is a classic example of which mode of wear?

. Adhesive wear
. Abrasive wear
. Third-body wear
. Fretting wear
. Corrosive wear

Correct Answer & Explanation

. Adhesive wear

Explanation

Third-body wear occurs when hard, unattached particles (such as PMMA cement debris, bone fragments, or loose metallic beads) become trapped between two articulating bearing surfaces. The trapped particles gouge and scratch the softer surface (like polyethylene), significantly accelerating wear. Adhesive wear involves microscopic bonding and tearing between the two primary surfaces, while abrasive wear is scratching caused by one primary surface being rougher and harder than the other.

Question 11919

Topic: Biology, Genetics & Bone Healing

Osteoprotegerin (OPG) is a critical regulator of bone remodeling that acts to inhibit osteoclast differentiation and activation. What is the precise molecular mechanism by which OPG exerts this effect?

. It directly degrades RANKL in the extracellular matrix via enzymatic cleavage
. It acts as a soluble decoy receptor, binding directly to RANKL
. It competitively binds to the RANK receptor on the osteoclast surface
. It phosphorylates intracellular Smad proteins within osteoclasts
. It inhibits farnesyl pyrophosphate synthase in mature osteoclasts

Correct Answer & Explanation

. It directly degrades RANKL in the extracellular matrix via enzymatic cleavage

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and acts as a soluble decoy receptor for Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By directly binding RANKL in the extracellular space, OPG prevents RANKL from interacting with its true receptor (RANK) on the surface of osteoclast precursors, thereby halting osteoclastogenesis and bone resorption.

Question 11920

Topic: Biology, Genetics & Bone Healing
Rigid internal fixation with an anatomically contoured dynamic compression plate results in absolute stability and minimal strain (< 2%) at the fracture site. This precise biomechanical environment forces the bone to heal via which primary mechanism?
. Primary intramembranous bone healing with robust external callus
. Secondary bone healing via endochondral ossification
. Primary bone healing via Haversian remodeling (cutting cones)
. Fibrocartilage intermediary healing
. Creeping substitution mediated by osteoprogenitor cells

Correct Answer & Explanation

. Primary bone healing via Haversian remodeling (cutting cones)

Explanation

Absolute stability (rigid fixation with interfragmentary compression, resulting in <2% strain) prevents the formation of a fibrocartilaginous callus. Instead, it leads to primary bone healing, which occurs via direct Haversian remodeling. Osteoclast 'cutting cones' traverse the fracture line, followed immediately by osteoblasts laying down new lamellar bone. Secondary healing (with a callus) occurs under conditions of relative stability (e.g., intramedullary nailing or cast immobilization).

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