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1. General Principles & Basic Science MCQs

Practice Set 574 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 11461

Topic: Infection, Pharmacology & VTE
A patient is prescribed Rivaroxaban for DVT prophylaxis following a total knee arthroplasty. What is the specific mechanism of action of this medication?
. Direct thrombin (Factor IIa) inhibitor
. Vitamin K epoxide reductase antagonist
. Direct Factor Xa inhibitor
. Antithrombin III activator
. ADP receptor antagonist

Correct Answer & Explanation

. Direct Factor Xa inhibitor

Explanation

Rivaroxaban and Apixaban are oral direct Factor Xa inhibitors (indicated by the 'xa' in their names). Dabigatran is a direct thrombin inhibitor. Warfarin is a Vitamin K antagonist. Heparin and LMWH exert their effect by binding to and activating antithrombin III.

Question 11462

Topic: 1. General Principles & Basic Science

Which of the following alterations to a standard cortical screw design would most significantly increase its pullout strength when inserted into diaphyseal bone?

. Decreasing the outer (major) diameter
. Increasing the inner (minor/root) diameter
. Decreasing the thread pitch
. Decreasing the thread depth
. Increasing the screw head diameter

Correct Answer & Explanation

. Decreasing the outer (major) diameter

Explanation

Pullout strength of a screw depends on outer diameter, length of engagement, shear strength of the bone, and thread pitch. Decreasing the thread pitch increases the number of threads engaged in a given length of bone, thereby increasing pullout strength. Increasing outer diameter also increases pullout strength, but option A says 'decreasing'. Increasing root diameter (Option B) decreases thread depth, which would lower pullout strength.

Question 11463

Topic: Biomechanics & Biomaterials

Normal synovial fluid exhibits non-Newtonian flow characteristics. Which of the following properties describes the ability of synovial fluid to decrease in viscosity under high shear rates, thereby facilitating joint lubrication during rapid movement?

. Thixotropy
. Elasticity
. Galvanism
. Creep
. Hysteresis

Correct Answer & Explanation

. Thixotropy

Explanation

Thixotropy is the property of certain non-Newtonian fluids to show a time-dependent change in viscosity; specifically, they become less viscous (thinner) when subjected to shear stress. Synovial fluid is thixotropic due to hyaluronic acid, allowing it to act as a viscous shock absorber at rest and a highly fluid lubricant during rapid joint motion.

Question 11464

Topic: 1. General Principles & Basic Science

Polymethylmethacrylate (PMMA) bone cement provides mechanical fixation for implants in total joint arthroplasty primarily through which of the following mechanisms?

. Chemical bonding directly to the metallic prosthesis
. Osteointegration and biological in-growth with the host bone
. Micro-interlocking with the trabeculae of cancellous bone
. Covalent cross-linking to host collagen fibrils
. Piezoelectric stimulation of local osteoblasts

Correct Answer & Explanation

. Chemical bonding directly to the metallic prosthesis

Explanation

PMMA bone cement acts as a grout rather than an adhesive. It does not chemically bond to bone or the implant. Instead, it provides immediate fixation by mechanically interlocking into the micro-interstices of the cancellous bone bed and conforming to the macroscopic shape of the prosthesis.

Question 11465

Topic: Biology, Genetics & Bone Healing

A 2-year-old child presents with frequent fractures, anemia, and cranial nerve palsies. Radiographs demonstrate a generalized increase in bone density with a "bone-within-bone" appearance. What is the primary cellular defect responsible for this condition?

. Impaired differentiation of osteoblasts
. Defective mineralization of osteoid matrix
. Failure of osteoclasts to form a ruffled border
. Abnormal type I collagen synthesis
. Overproduction of fibroblast growth factor receptor 3

Correct Answer & Explanation

. Impaired differentiation of osteoblasts

Explanation

Osteopetrosis is caused by defective osteoclast function, specifically the failure to form a ruffled border and properly acidify the resorption pit. This is often due to mutations in genes such as TCIRG1 or carbonic anhydrase II (CAII). This leads to dense, brittle bones and obliteration of the medullary space.

Question 11466

Topic: 1. General Principles & Basic Science

A surgeon attempts to anesthetize an infected superficial abscess with local lidocaine injection. The clinical effect is significantly diminished compared to normal tissue. What is the primary mechanism explaining the failure of the local anesthetic in this setting?

. Increased tissue vascularity clears the drug rapidly
. Acidic environment keeps the drug heavily ionized, preventing cellular entry
. Basic environment prevents receptor binding
. Inflammatory mediators competitively bind to sodium channels
. Upregulation of potassium channels overcomes the sodium channel blockade

Correct Answer & Explanation

. Increased tissue vascularity clears the drug rapidly

Explanation

Local anesthetics are weak bases. They must cross the lipid bilayer in an un-ionized form to block the voltage-gated sodium channels from the intracellular side. Infected tissue has an acidic pH, which causes the anesthetic to become highly ionized (protonated), drastically reducing its ability to penetrate the nerve cell membrane.

Question 11467

Topic: Biomechanics & Biomaterials

During revision of a total hip arthroplasty, examination of the explanted femoral head reveals multiple deep scratches. Histology of the surrounding tissue shows acrylic bone cement particles embedded within the articular surface of the polyethylene liner. This phenomenon represents which specific wear mechanism?

. Adhesive wear
. Abrasive wear
. Third-body wear
. Fretting wear
. Galvanic corrosion

Correct Answer & Explanation

. Adhesive wear

Explanation

Third-body wear occurs when hard particles (such as bone cement, metal debris, or bone fragments) become trapped between two articulating surfaces. They embed into the softer surface (polyethylene) and act like sandpaper, scratching the harder surface (metal or ceramic head) and severely accelerating wear rates.

Question 11468

Topic: Infection, Pharmacology & VTE
A 65-year-old patient undergoing a total knee arthroplasty is prescribed oral Rivaroxaban for deep vein thrombosis prophylaxis. What is the specific mechanism of action of this medication?
. Direct thrombin inhibitor
. Direct Factor Xa inhibitor
. Vitamin K epoxide reductase inhibitor
. Enhances antithrombin III activity
. Inhibits ADP-induced platelet aggregation

Correct Answer & Explanation

. Direct Factor Xa inhibitor

Explanation

Rivaroxaban and Apixaban are direct, competitive inhibitors of Factor Xa. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin inhibits Vitamin K epoxide reductase. Low molecular weight heparins (like enoxaparin) act by enhancing antithrombin III.

Question 11469

Topic: Biomechanics & Biomaterials

In articular cartilage biomechanics, boundary lubrication is essential for reducing friction under high-load, low-speed conditions. Which molecule is primarily responsible for boundary lubrication at the articular surface?

. Hyaluronic acid
. Lubricin (PRG4)
. Aggrecan
. Type II collagen
. Chondroitin sulfate

Correct Answer & Explanation

. Hyaluronic acid

Explanation

Boundary lubrication prevents wear in the articular cartilage under high-load, low-speed conditions. It is mediated by lubricin (proteoglycan 4 or PRG4), a glycoprotein secreted by superficial zone chondrocytes and synovial fibroblasts. Elastohydrodynamic lubrication is more relevant at higher speeds.

Question 11470

Topic: Biomechanics & Biomaterials

Analysis of a synovial fluid aspirate from an acutely swollen, painful knee reveals rhomboid-shaped crystals that exhibit weak positive birefringence under polarized light microscopy. What is the chemical composition of these crystals?

. Monosodium urate
. Calcium pyrophosphate dihydrate
. Calcium hydroxyapatite
. Cholesterol
. Basic calcium phosphate

Correct Answer & Explanation

. Monosodium urate

Explanation

Calcium pyrophosphate dihydrate (CPPD) crystals are the hallmark of pseudogout. They appear rhomboid-shaped and exhibit weak positive birefringence. Monosodium urate crystals, seen in gout, are needle-shaped and show strong negative birefringence.

Question 11471

Topic: Surgical Anatomy & Approaches

A 35-year-old patient presents with a radial nerve palsy following a closed humeral shaft fracture. The physician elects to observe the injury but orders an electromyography (EMG) study to evaluate for axonal loss. What is the minimum time after the injury that fibrillation potentials will reliably appear on an EMG?

. 24 to 48 hours
. 3 to 5 days
. 10 to 14 days
. 3 to 4 weeks
. 6 to 8 weeks

Correct Answer & Explanation

. 24 to 48 hours

Explanation

Fibrillation potentials and positive sharp waves on EMG are signs of active denervation and muscle membrane instability following Wallerian degeneration. It takes approximately 3 to 4 weeks for these findings to reliably appear after an acute peripheral nerve injury.

Question 11472

Topic: Biology, Genetics & Bone Healing

A 4-year-old child presents with severe lower extremity bowing and waddling gait. Laboratory studies reveal a normal serum calcium, markedly low serum phosphate, normal parathyroid hormone (PTH) level, and normal 25-hydroxyvitamin D. What is the underlying pathophysiology of this patient's condition?

. Mutation in the PHEX gene causing excess FGF23 production
. Nutritional deficiency of Vitamin D
. Defective 1-alpha-hydroxylase enzyme activity in the kidneys
. End-organ resistance to calcitriol
. Mutation in the Type I collagen alpha-1 chain

Correct Answer & Explanation

. Mutation in the PHEX gene causing excess FGF23 production

Explanation

The clinical and lab picture describes X-linked hypophosphatemic (XLH) rickets. It is caused by an inactivating mutation in the PHEX gene, which normally degrades Fibroblast Growth Factor 23 (FGF23). The resulting excess FGF23 causes profound renal phosphate wasting and downregulates 1-alpha-hydroxylase, preventing calcitriol synthesis despite low phosphate.

Question 11473

Topic: 1. General Principles & Basic Science

During skeletal muscle contraction, the cross-bridge cycle is initiated following the release of intracellular calcium from the sarcoplasmic reticulum. To which specific structural protein does calcium bind to allow this process to begin?

. Tropomyosin
. Troponin C
. Troponin I
. Troponin T
. Myosin light chain

Correct Answer & Explanation

. Tropomyosin

Explanation

Calcium ions bind directly to Troponin C. This binding induces a conformational change that shifts the position of Tropomyosin, exposing the myosin-binding sites on the actin filament and allowing the myosin head to attach and initiate the power stroke.

Question 11474

Topic: Biology, Genetics & Bone Healing

Denosumab is an effective medical treatment for recurrent or unresectable giant cell tumors of bone. What is the specific molecular target of this monoclonal antibody?

. Vascular endothelial growth factor (VEGF)
. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Tumor necrosis factor-alpha (TNF-a)
. Interleukin-1 (IL-1)
. Fibroblast growth factor receptor 3 (FGFR3)

Correct Answer & Explanation

. Vascular endothelial growth factor (VEGF)

Explanation

Denosumab is a monoclonal antibody that specifically binds to and inhibits RANKL. In giant cell tumors of bone, the neoplastic mononuclear stromal cells secrete massive amounts of RANKL, which recruits and activates the destructive multinucleated giant cells (osteoclast-like cells). Denosumab halts this process.

Question 11475

Topic: Biomechanics & Biomaterials

When analyzing the biomechanical properties of a tendon or ligament on a standard stress-strain curve, the initial nonlinear phase known as the "toe region" represents which physiological phenomenon?

. Microscopic failure of individual collagen fibers
. Macroscopic failure of the ligament structure
. Uncrimping of the resting wavy collagen fibers
. Elastic deformation of the bone-ligament interface
. The yield point transitioning to plastic deformation

Correct Answer & Explanation

. Microscopic failure of individual collagen fibers

Explanation

The initial 'toe region' of the stress-strain curve for ligaments and tendons is characterized by a low amount of stress resulting in a relatively large amount of strain. This corresponds structurally to the un-crimping, or straightening, of the naturally wavy collagen fibers.

Question 11476

Topic: Infection, Pharmacology & VTE

Orthopedic implant-associated infections are notoriously difficult to eradicate with systemic antibiotics alone. This resistance is primarily attributed to which structural or physiological characteristic of bacterial biofilms?

. Rapid and unchecked exponential division of bacteria within the biofilm
. Presence of a self-produced protective extracellular polymeric substance (EPS) matrix
. High oxygen tension at the implant-tissue interface
. Upregulation of generalized bacterial efflux pumps across all species
. Direct covalent binding of antibiotics to titanium and cobalt-chrome surfaces

Correct Answer & Explanation

. Rapid and unchecked exponential division of bacteria within the biofilm

Explanation

Biofilms confer extreme resistance to antibiotics and the host immune system because the bacteria encase themselves in a protective extracellular polymeric substance (EPS) matrix (glycocalyx). Within this matrix, deep layers of bacteria exist in a sessile, metabolically dormant state, rendering cell-wall-active antibiotics largely ineffective.

Question 11477

Topic: 1. General Principles & Basic Science

During a regional nerve block, inadvertent intravascular injection of local anesthetic occurs. Which of the following local anesthetics is associated with the highest risk of severe, refractory cardiotoxicity, including fatal ventricular arrhythmias?

. Lidocaine
. Mepivacaine
. Ropivacaine
. Bupivacaine
. Prilocaine

Correct Answer & Explanation

. Lidocaine

Explanation

Bupivacaine is highly lipid-soluble and binds strongly to cardiac sodium channels. Unlike lidocaine, it dissociates extremely slowly during diastole, which can lead to profound, difficult-to-resuscitate cardiac arrhythmias and cardiovascular collapse if injected intravascularly. Ropivacaine was developed to offer similar duration but with less cardiotoxicity.

Question 11478

Topic: Biomechanics & Biomaterials
During total joint arthroplasty, ultra-high molecular weight polyethylene (UHMWPE) is used. Over time, constant load leads to progressive, permanent deformation of the polyethylene without fracture. What is this biomechanical property called?
. Fatigue failure
. Creep
. Stress shielding
. Hysteresis
. Galvanic corrosion

Correct Answer & Explanation

. Creep

Explanation

Creep is the progressive, time-dependent permanent deformation of a material under a constant load below its yield strength. It is a classic characteristic of viscoelastic materials like UHMWPE used in arthroplasty.

Question 11479

Topic: Biology, Genetics & Bone Healing

A patient with multiple myeloma presents with hypercalcemia and lytic bone lesions. The primary mechanism of bone resorption in this condition involves the overexpression of which of the following molecules by myeloma cells or surrounding stromal cells?

. Osteoprotegerin (OPG)
. Wnt antagonists (e.g., Dkk-1) and RANKL
. Bone morphogenetic protein-2 (BMP-2)
. Transforming growth factor-beta (TGF-b)
. Runx2 transcription factor

Correct Answer & Explanation

. Osteoprotegerin (OPG)

Explanation

Myeloma cells secrete RANKL and Wnt signaling antagonists like DKK-1. This shifts the balance toward osteoclastogenesis (via RANKL) and inhibits osteoblast function (via DKK-1 blocking Wnt), leading to lytic lesions.

Question 11480

Topic: Biomechanics & Biomaterials
When selecting an intramedullary nail for a tibial shaft fracture, a surgeon considers the material's modulus of elasticity. Which of the following lists materials in order of DECREASING modulus of elasticity (from stiffest to most flexible)?
. Cortical bone, Titanium, Stainless Steel, Cobalt-Chromium
. Cobalt-Chromium, Stainless Steel, Titanium, Cortical bone
. Stainless Steel, Cobalt-Chromium, Titanium, Cortical bone
. Titanium, Stainless Steel, Cobalt-Chromium, Cortical bone
. Cortical bone, Cobalt-Chromium, Stainless Steel, Titanium

Correct Answer & Explanation

. Cobalt-Chromium, Stainless Steel, Titanium, Cortical bone

Explanation

Cobalt-Chromium is the stiffest (approx 220 GPa), followed by Stainless Steel (approx 200 GPa), Titanium (approx 100 GPa), and Cortical bone (approx 15-20 GPa). Matching the modulus closer to bone reduces stress shielding.

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