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1. General Principles & Basic Science MCQs

Practice Set 726 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 14501

Topic: Surgical Anatomy & Approaches

A 30-year-old male sustains a closed midshaft humerus fracture and presents concurrently with an isolated wrist drop. Initial observation is elected. At 12 weeks post-injury, there is no clinical or electromyographic (EMG) evidence of radial nerve recovery. What is the most appropriate next step in management?

. Continued observation for an additional 12 weeks
. Tendon transfers for wrist and finger extension
. Surgical exploration of the radial nerve
. Open reduction and internal fixation of the humerus only
. Corticosteroid injection at the spiral groove

Correct Answer & Explanation

. Surgical exploration of the radial nerve

Explanation

While initial observation is appropriate for a primary radial nerve palsy associated with a closed humeral shaft fracture, the lack of clinical or EMG signs of reinnervation by 12 weeks mandates surgical exploration of the radial nerve.

Question 14502

Topic: Biology, Genetics & Bone Healing

Denosumab is utilized as an adjuvant or neoadjuvant treatment for advanced Giant Cell Tumor of Bone (GCTB). At the cellular level, denosumab exerts its therapeutic effect by directly targeting and binding to which entity within the tumor microenvironment?

. RANK receptor on the multinucleated giant cells
. RANKL produced by the neoplastic mononuclear stromal cells
. Osteoprotegerin (OPG) produced by osteoblasts
. Vascular Endothelial Growth Factor (VEGF) in the tumor matrix
. The tartrate-resistant acid phosphatase (TRAP) enzyme

Correct Answer & Explanation

. RANKL produced by the neoplastic mononuclear stromal cells

Explanation

In Giant Cell Tumor of Bone, the mononuclear stromal cells are the actual neoplastic elements that oversecrete RANKL. Denosumab, a monoclonal antibody, binds directly to this RANKL, preventing it from activating the non-neoplastic, bone-resorbing multinucleated giant cells.

Question 14503

Topic: Biology, Genetics & Bone Healing

During distraction osteogenesis using an Ilizarov frame, bone healing occurs primarily through which of the following processes?

. Endochondral ossification
. Intramembranous ossification
. Creeping substitution
. Appositional bone growth
. Cutting cone remodeling

Correct Answer & Explanation

. Intramembranous ossification

Explanation

Distraction osteogenesis promotes new bone formation primarily via intramembranous ossification, bypassing the cartilaginous intermediate phase. This occurs under conditions of stable fixation and controlled, gradual mechanical tension.

Question 14504

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, secondary bone healing (callus formation) optimally occurs within which of the following ranges of mechanical strain at the fracture site?

. Less than 2%
. 2% to 10%
. 10% to 30%
. 30% to 50%
. Greater than 50%

Correct Answer & Explanation

. 2% to 10%

Explanation

According to Perren's strain theory, primary bone healing occurs when strain is less than 2%. Secondary bone healing (which includes endochondral ossification and callus formation) occurs in the presence of moderate strain, typically between 2% and 10%. Granulation tissue can tolerate up to 100% strain, cartilage up to 10%, and lamellar bone only 2%.

Question 14505

Topic: 1. General Principles & Basic Science

Articular cartilage is structurally divided into four zones. Which zone is responsible for the highest resistance to shear forces and contains type II collagen fibers oriented parallel to the joint surface?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone makes up 10-20% of articular cartilage thickness. It has the highest concentration of water and collagen (oriented parallel to the joint surface) and the lowest concentration of proteoglycans. This structure provides the highest resistance to shear stress.

Question 14506

Topic: Biomechanics & Biomaterials
In total joint arthroplasty, the use of highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) has significantly reduced volumetric wear. However, the cross-linking and subsequent melting processes negatively affect which of the following mechanical properties?
. Oxidation resistance
. Elastic modulus
. Ultimate tensile strength and fatigue resistance
. Creep resistance
. Biocompatibility

Correct Answer & Explanation

. Ultimate tensile strength and fatigue resistance

Explanation

Highly cross-linked polyethylene significantly decreases adhesive and abrasive wear. However, the radiation and subsequent thermal treatment (remelting or annealing) to eliminate free radicals lead to a reduction in ultimate tensile strength, fatigue resistance, and fracture toughness, which can increase the risk of mechanical failure (e.g., rim fracture).

Question 14507

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (e.g., alendronate, zoledronic acid) decrease osteoclastic bone resorption by inhibiting which of the following enzymes in the mevalonate pathway?

. Cathepsin K
. HMG-CoA reductase
. Farnesyl pyrophosphate (FPP) synthase
. Carbonic anhydrase II
. Tartrate-resistant acid phosphatase (TRAP)

Correct Answer & Explanation

. Farnesyl pyrophosphate (FPP) synthase

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate (FPP) synthase in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (like Rab, Rho, Rac) essential for osteoclast function, ruffled border formation, and cell survival, eventually leading to osteoclast apoptosis.

Question 14508

Topic: Biomechanics & Biomaterials

A ligament is stretched to a constant specific length. Over time, the amount of force required to maintain this length progressively decreases. This biomechanical phenomenon is known as:

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

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a viscoelastic property where the stress (force) decreases over time when a material is held at a constant strain (length). Creep is the opposite: an increase in strain (deformation) over time when subjected to a constant stress (load).

Question 14509

Topic: Biology, Genetics & Bone Healing

Denosumab is an antiresorptive medication used in the treatment of osteoporosis. What is its exact mechanism of action?

. Binds directly to the RANK receptor on osteoclasts
. Binds to RANKL, preventing its interaction with RANK
. Stimulates osteoprotegerin (OPG) production by osteoblasts
. Inhibits the Wnt/beta-catenin signaling pathway
. Directly stimulates osteoblast differentiation via PTH1R

Correct Answer & Explanation

. Binds to RANKL, preventing its interaction with RANK

Explanation

Denosumab is a fully human monoclonal antibody that binds to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By binding to RANKL, it prevents RANKL from binding to the RANK receptor on osteoclast precursors and mature osteoclasts, thus inhibiting their differentiation, activation, and survival.

Question 14510

Topic: Biology, Genetics & Bone Healing

During secondary bone remodeling, a 'cutting cone' travels through cortical bone. Which of the following accurately describes the spatial arrangement of cells within this basic multicellular unit (BMU)?

. Osteoblasts are at the leading edge, followed by osteocytes
. Osteoclasts are at the leading edge, followed by a reversal zone and trailing osteoblasts
. Chondrocytes are at the leading edge, followed by osteoblasts
. Macrophages are at the leading edge, directly followed by osteoclasts
. Osteoblasts and osteoclasts act simultaneously at the exact same geographic leading edge

Correct Answer & Explanation

. Osteoclasts are at the leading edge, followed by a reversal zone and trailing osteoblasts

Explanation

In cortical bone remodeling, the basic multicellular unit (BMU) forms a 'cutting cone'. Osteoclasts are situated at the leading edge (the head of the cone) to resorb bone. They are followed by a reversal zone containing mononuclear cells, and then trailing osteoblasts that lay down new osteoid (the closing cone).

Question 14511

Topic: 1. General Principles & Basic Science

Tranexamic acid (TXA) is frequently used in orthopedic surgery to reduce perioperative blood loss. What is the mechanism of action of TXA?

. Directly inhibits Factor Xa in the coagulation cascade
. Competitively inhibits the activation of plasminogen to plasmin
. Irreversibly binds to the ADP receptor (P2Y12) on platelets
. Promotes the release of von Willebrand factor from endothelial cells
. Inhibits cyclooxygenase-1 (COX-1) to reduce thromboxane A2

Correct Answer & Explanation

. Competitively inhibits the activation of plasminogen to plasmin

Explanation

Tranexamic acid is a synthetic derivative of the amino acid lysine. It competitively inhibits the activation of plasminogen to plasmin, a molecule responsible for fibrin degradation. By inhibiting fibrinolysis, it stabilizes the existing clot and reduces bleeding.

Question 14512

Topic: Biology, Genetics & Bone Healing

Parathyroid hormone (PTH) plays a critical role in calcium homeostasis and bone remodeling. Through which primary cellular mechanism does PTH stimulate osteoclast activity?

. Binding directly to PTH receptors on the osteoclast cell membrane
. Binding to PTH1R on osteoblasts, which upregulates RANKL expression
. Inhibiting the release of macrophage colony-stimulating factor (M-CSF)
. Stimulating calcitonin release from the thyroid gland
. Directly increasing intestinal calcium absorption independent of Vitamin D

Correct Answer & Explanation

. Binding to PTH1R on osteoblasts, which upregulates RANKL expression

Explanation

Osteoclasts do not possess receptors for Parathyroid Hormone (PTH). PTH binds to the PTH1R receptor on osteoblasts and osteocytes, which then increases their expression of RANKL and decreases OPG production. The increased RANKL binds to RANK on osteoclast precursors, stimulating osteoclastogenesis and bone resorption.

Question 14513

Topic: 1. General Principles & Basic Science

According to the Sunderland classification of peripheral nerve injuries, a 3rd-degree injury is characterized by the disruption of the axon and which other neural structure, while leaving the perineurium and epineurium intact?

. Myelin sheath only
. Epineurium
. Endoneurium
. Perineurium
. Schwann cell basal lamina

Correct Answer & Explanation

. Perineurium

Explanation

Sunderland classification: 1st degree = neuropraxia (local myelin injury, axons intact). 2nd degree = axonotmesis (axon severed, endoneurium intact). 3rd degree = axon and endoneurium severed, but perineurium and epineurium intact. 4th degree = only epineurium intact. 5th degree = complete nerve transection.

Question 14514

Topic: 1. General Principles & Basic Science
A joint aspirate from a swollen, painful knee reveals a leukocyte count of 25,000 cells/mm³ with 60% neutrophils. Polarized light microscopy shows positively birefringent, rhomboid-shaped crystals. What is the composition of these crystals?
. Monosodium urate
. Calcium hydroxyapatite
. Basic calcium phosphate
. Calcium pyrophosphate dihydrate
. Cholesterol

Correct Answer & Explanation

. Calcium pyrophosphate dihydrate

Explanation

The clinical picture and synovial fluid analysis (inflammatory range but not overtly septic) with positively birefringent, rhomboid-shaped crystals are diagnostic of Pseudogout. The crystals in pseudogout are composed of Calcium Pyrophosphate Dihydrate (CPPD). Gout crystals are monosodium urate and are needle-shaped and negatively birefringent.

Question 14515

Topic: Biomechanics & Biomaterials

Titanium alloys (e.g., Ti-6Al-4V) are frequently used in orthopedic implants. Compared to 316L stainless steel, titanium alloy possesses which of the following mechanical advantages that reduces stress shielding in bone?

. A lower modulus of elasticity
. A higher ultimate tensile strength
. A greater risk of galvanic corrosion
. Increased ductility allowing intraoperative contouring
. Higher density and weight

Correct Answer & Explanation

. A lower modulus of elasticity

Explanation

Titanium alloys have a modulus of elasticity (stiffness) closer to that of cortical bone compared to stainless steel or cobalt-chromium alloys. This decreased stiffness allows for more physiological load sharing with the host bone, thereby reducing the phenomenon of stress shielding (which leads to osteopenia around the implant).

Question 14516

Topic: Infection, Pharmacology & VTE

Vancomycin is commonly included in orthopedic antibiotic cement spacers for the treatment of methicillin-resistant Staphylococcus aureus (MRSA) osteomyelitis. What is the mechanism of action of vancomycin?

. Inhibits DNA gyrase to prevent bacterial replication
. Binds to the 50S ribosomal subunit to inhibit protein synthesis
. Binds to the 30S ribosomal subunit causing mRNA misreading
. Inhibits cell wall synthesis by binding to the D-alanyl-D-alanine terminus of peptidoglycan precursors
. Disrupts the bacterial cell membrane by binding to lipopolysaccharides

Correct Answer & Explanation

. Inhibits cell wall synthesis by binding to the D-alanyl-D-alanine terminus of peptidoglycan precursors

Explanation

Vancomycin is a glycopeptide antibiotic that exerts its bactericidal effect by binding tightly to the D-alanyl-D-alanine terminus of cell wall precursor units. This prevents cross-linking of peptidoglycans, thereby inhibiting bacterial cell wall synthesis. It is highly effective against Gram-positive organisms, including MRSA.

Question 14517

Topic: Biomechanics & Biomaterials

A surgeon revises a failed total hip arthroplasty where a cobalt-chrome femoral head was placed on a titanium alloy stem. Intraoperatively, black debris and significant tissue staining are observed near the modular junction, without signs of infection. Which of the following best describes the primary mechanism of this specific implant degradation?

. Fretting corrosion
. Galvanic corrosion
. Crevice corrosion
. Pitting corrosion
. Fatigue failure

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., cobalt-chrome and titanium) are in electrical contact within an electrolytic environment (body fluid). The less noble metal undergoes accelerated corrosion. While fretting (micromotion) may also contribute, the interaction of dissimilar metals classically defines galvanic corrosion.

Question 14518

Topic: 1. General Principles & Basic Science

In normal articular cartilage, which structural layer is characterized by the highest concentration of proteoglycans, the lowest water content, and collagen fibers arranged strictly perpendicular to the articular surface?

. Superficial zone
. Transitional (middle) zone
. Deep (radial) zone
. Calcified zone
. Tidemark

Correct Answer & Explanation

. Deep (radial) zone

Explanation

The deep (radial) zone of articular cartilage contains the highest concentration of proteoglycans, the lowest water content, and thick collagen fibrils oriented vertically (perpendicular) to the joint surface to maximally resist compressive forces.

Question 14519

Topic: 1. General Principles & Basic Science
A patient sustains a peripheral nerve injury following a crush mechanism. Electromyography reveals Wallerian degeneration. Surgical exploration demonstrates that the epineurium and perineurium are completely intact, but the endoneurium is disrupted. What is the Sunderland classification of this nerve injury?
. Grade I
. Grade II
. Grade III
. Grade IV
. Grade V

Correct Answer & Explanation

. Grade III

Explanation

Sunderland Grade III injuries involve disruption of the axon, myelin sheath, and endoneurium, while the perineurium and epineurium remain intact. Wallerian degeneration occurs. Grade II leaves the endoneurium intact, and Grade IV disrupts the perineurium as well.

Question 14520

Topic: Biology, Genetics & Bone Healing

Which of the following bone graft substitutes inherently possesses osteoinductive properties but definitively lacks both osteoconductive and osteogenic properties?

. Cancellous autograft
. Demineralized bone matrix (DBM)
. Calcium phosphate cement
. Fresh frozen structural allograft
. Corticocancellous autograft

Correct Answer & Explanation

. Demineralized bone matrix (DBM)

Explanation

Demineralized bone matrix (DBM) is osteoinductive due to the release and preservation of bone morphogenetic proteins (BMPs) during the acid demineralization process. However, the process removes structural rigidity (lacking true osteoconduction) and devitalizes all cells (lacking osteogenesis).

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