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1. General Principles & Basic Science MCQs

Practice Set 410 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 8181

Topic: Infection, Pharmacology & VTE

During the pathogenesis of periprosthetic joint infection (PJI), Staphylococcus aureus adheres to the orthopedic implant surface and forms a biofilm, making the infection highly recalcitrant to host immune responses and systemic antibiotics. Which of the following phases of biofilm formation is specifically characterized by the production of a complex extracellular polymeric substance (EPS) matrix and active quorum sensing?

. Initial reversible attachment
. Irreversible attachment
. Maturation
. Dispersion
. Planktonic phase

Correct Answer & Explanation

. Maturation

Explanation

Biofilm formation occurs in sequential stages: (1) Initial reversible attachment of planktonic bacteria to the implant surface; (2) Irreversible attachment mediated by specific adhesins like MSCRAMMs; (3) Maturation, characterized by cellular multiplication, complex architectural development, synthesis of the extracellular polymeric substance (EPS) matrix, and quorum sensing (cell-to-cell communication); and (4) Dispersion or detachment, where bacteria escape the biofilm to colonize new sites.

Question 8182

Topic: Biomechanics & Biomaterials

Viscoelastic materials, such as articular cartilage and tendons, exhibit time-dependent mechanical behaviors. Which of the following best describes the phenomenon of 'stress relaxation'?

. Increasing deformation over time under a constant load
. Decreasing stress over time under a constant deformation
. Energy loss between loading and unloading cycles
. Stiffening of the material with increased strain rate
. Microfracture accumulation under cyclic submaximal loading

Correct Answer & Explanation

. Decreasing stress over time under a constant deformation

Explanation

Viscoelastic materials display time-dependent responses to loading. Stress relaxation is the decrease in internal stress over time when the material is held at a constant deformation (strain). Creep is the gradual increase in deformation (strain) over time under a constant load (stress). Hysteresis is the energy lost (as heat) during the loading and unloading cycle. Strain-rate dependency means the material becomes stiffer when loaded at a faster rate.

Question 8183

Topic: Biology, Genetics & Bone Healing

A 35-year-old man undergoes open reduction and internal fixation of a transverse radial shaft fracture using a dynamic compression plate (DCP) that provides absolute stability. By which of the following mechanisms will the fracture heal?

. Endochondral ossification
. Intramembranous ossification
. Intramembranous ossification followed by endochondral ossification
. Direct cutting cone remodeling across the fracture site
. Callus formation without a cartilage intermediate

Correct Answer & Explanation

. Direct cutting cone remodeling across the fracture site

Explanation

When absolute stability is achieved with interfragmentary compression (e.g., lag screw and plating), primary bone healing occurs. This process lacks callus formation and occurs via direct osteonal remodeling (cutting cones) crossing the fracture site. Secondary bone healing involves callus formation (via endochondral and intramembranous ossification) and occurs when there is relative stability (e.g., intramedullary nailing, external fixation, casting).

Question 8184

Topic: Biomechanics & Biomaterials
In total hip arthroplasty, the use of highly cross-linked polyethylene (HXLPE) has significantly reduced wear rates compared to conventional ultra-high molecular weight polyethylene (UHMWPE). Which of the following trade-offs is most commonly associated with increasing the radiation dose to maximize cross-linking in HXLPE?
. Increased ultimate tensile strength
. Decreased fatigue and fracture resistance
. Decreased oxidation resistance during shelf storage
. Increased generation of submicron wear debris
. Enhanced resistance to third-body wear

Correct Answer & Explanation

. Decreased fatigue and fracture resistance

Explanation

Cross-linking of polyethylene by irradiation improves its wear resistance significantly. However, increasing the radiation dose leads to decreased mechanical properties, specifically reducing ultimate tensile strength, elongation to failure, and fracture/fatigue resistance. To prevent in vivo oxidation caused by free radicals generated during irradiation, the polyethylene is usually melted or annealed, or infused with an antioxidant like Vitamin E.

Question 8185

Topic: 1. General Principles & Basic Science

Articular cartilage is highly specialized to resist compressive and shear forces. In which zone of articular cartilage are the collagen fibers oriented parallel to the joint surface, providing the highest tensile strength?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

Articular cartilage is divided into zones. In the superficial (tangential) zone, collagen fibers (mostly Type II) are oriented parallel to the articular surface to resist shear forces and provide high tensile strength. In the middle (transitional) zone, they are obliquely arranged. In the deep (radial) zone, they are oriented vertically (perpendicular to the joint surface) to resist compressive forces. The tidemark separates the deep zone from the calcified cartilage.

Question 8186

Topic: Infection, Pharmacology & VTE
Low-molecular-weight heparin (LMWH) is commonly prescribed for deep vein thrombosis (DVT) prophylaxis following total knee arthroplasty. LMWH exerts its primary anticoagulant effect through the selective inhibition of which of the following coagulation cascade components?
. Thrombin (Factor IIa) via direct binding
. Factor Xa via antithrombin III activation
. Factor VIIa via tissue factor pathway inhibitor (TFPI)
. Factor IXa via direct inhibition
. Vitamin K epoxide reductase

Correct Answer & Explanation

. Factor Xa via antithrombin III activation

Explanation

Low-molecular-weight heparins (e.g., enoxaparin) have a high ratio of anti-Factor Xa to anti-Factor IIa activity. They bind to and activate antithrombin III (ATIII), which then predominantly inhibits Factor Xa. Unfractionated heparin inhibits both Factor Xa and Thrombin (Factor IIa) more equally. Direct thrombin inhibitors (like dabigatran or argatroban) directly bind Factor IIa. Warfarin inhibits Vitamin K epoxide reductase.

Question 8187

Topic: Infection, Pharmacology & VTE

Staphylococcus epidermidis is a common pathogen in periprosthetic joint infections due to its ability to form a robust biofilm on orthopedic implants. Which of the following is the primary constituent of the extracellular polymeric substance (EPS) that protects the bacteria in this biofilm?

. Hyaluronic acid
. Polysaccharide intercellular adhesin (PIA)
. Peptidoglycan cross-linkages
. Fibronectin-binding protein A (FnBPA)
. Protein A

Correct Answer & Explanation

. Polysaccharide intercellular adhesin (PIA)

Explanation

Staphylococcus epidermidis forms a biofilm by producing an extracellular polymeric substance (EPS). A major component of this slime layer in staphylococci is the polysaccharide intercellular adhesin (PIA), which is synthesized by products of the icaADBC operon. PIA protects the bacteria from host immune responses and antibiotic penetration. Fibronectin-binding proteins (e.g., FnBPA) are important for initial bacterial adherence to host proteins coating the implant but are not the primary structural component of the mature EPS slime layer.

Question 8188

Topic: Biology, Genetics & Bone Healing
A 25-year-old woman presents with multiple recurrent fragility fractures and a history of delayed dental eruption. Radiographs reveal generalized, diffusely sclerotic, 'bone-within-bone' appearance. This condition is most likely caused by a defect in which of the following cellular mechanisms?
. Deficient Type I collagen synthesis by osteoblasts
. Defective mineralization of the osteoid matrix
. Impaired acidification of the osteoclast ruffled border resorptive pit
. Hyperactive osteoprotegerin (OPG) secretion
. Mutation in the FGFR3 gene affecting chondrocyte proliferation

Correct Answer & Explanation

. Impaired acidification of the osteoclast ruffled border resorptive pit

Explanation

The clinical and radiographic presentation is classic for osteopetrosis (Albers-Schönberg disease), which is characterized by dense but brittle bones ('marble bone disease'). The primary defect is in osteoclast function, most commonly due to impaired acidification of the extracellular resorptive pit (e.g., mutations in carbonic anhydrase II, TCIRG1, or CLCN7). Without an acidic environment, osteoclasts cannot dissolve hydroxyapatite. Deficient Type I collagen is seen in osteogenesis imperfecta. Defective mineralization causes rickets/osteomalacia.

Question 8189

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with severe lower back pain and fatigue. Radiographs of the spine and pelvis reveal multiple punched-out lytic lesions without reactive sclerosis. Laboratory evaluation is notable for hypercalcemia, anemia, and an elevated serum creatinine. A bone marrow biopsy is expected to show an abnormal proliferation of which cell type?

. Osteoclasts
. Plasma cells
. Histiocytes
. Lymphoblasts
. Chondrocytes

Correct Answer & Explanation

. Plasma cells

Explanation

The presentation of punched-out lytic lesions, bone pain, hypercalcemia, anemia, and renal insufficiency (CRAB criteria) in an older adult is highly characteristic of multiple myeloma. Multiple myeloma is a hematologic malignancy characterized by the monoclonal proliferation of abnormal plasma cells in the bone marrow. These plasma cells secrete factors (like MIP-1 alpha and RANKL) that stimulate osteoclasts and inhibit osteoblasts, leading to pure lytic lesions without reactive new bone formation.

Question 8190

Topic: Biology, Genetics & Bone Healing

A 32-year-old woman presents with persistent knee pain. Radiographs demonstrate an eccentric, lytic, epiphyseal lesion in the proximal tibia extending to the subchondral bone without a sclerotic margin. Biopsy reveals multinucleated giant cells intermixed with mononuclear stromal cells. Denosumab is considered for adjuvant treatment prior to surgery. What is the mechanism of action of this medication?

. Inhibition of vascular endothelial growth factor (VEGF)
. Monoclonal antibody against RANKL
. Tyrosine kinase inhibitor targeting c-KIT
. Selective estrogen receptor modulator
. Monoclonal antibody against CD20

Correct Answer & Explanation

. Monoclonal antibody against RANKL

Explanation

Giant cell tumor (GCT) of bone contains neoplastic mononuclear cells that express RANKL, which recruits and activates the reactive osteoclast-like giant cells. Denosumab is a monoclonal antibody that binds to RANKL, preventing its interaction with the RANK receptor on the surface of giant cells and their precursors. This inhibits bone resorption and halts tumor progression.

Question 8191

Topic: 1. General Principles & Basic Science

Which of the following biochemical changes in articular cartilage is most characteristic of the earliest stage of osteoarthritis?

. Increased proteoglycan concentration
. Decreased water content
. Increased water content
. Decreased chondrocyte proliferation
. Increased type II collagen synthesis

Correct Answer & Explanation

. Increased water content

Explanation

The earliest biochemical change in osteoarthritis is an increase in the water content of the articular cartilage. This is due to the breakdown of the collagen network (damage to type II collagen framework), allowing the hydrophilic proteoglycans to swell and draw in more water. The overall proteoglycan concentration actually decreases, and initially, chondrocytes may increase synthesis of both collagen and proteoglycans in an attempt to repair the damage.

Question 8192

Topic: Biology, Genetics & Bone Healing

During endochondral ossification in fracture healing, which of the following transcription factors is essential for the differentiation of mesenchymal stem cells into chondrocytes?

. Runx2 / Cbfa1
. SOX9
. Osterix
. Sclerostin
. RANKL

Correct Answer & Explanation

. SOX9

Explanation

SOX9 is the master transcription factor for chondrogenesis, directing mesenchymal stem cells to differentiate into chondrocytes. Runx2 (Cbfa1) is the key transcription factor for osteoblast differentiation. Osterix functions downstream of Runx2 in osteoblast differentiation. Sclerostin is an inhibitor of the Wnt signaling pathway, produced by osteocytes to inhibit bone formation.

Question 8193

Topic: Biomechanics & Biomaterials

Galvanic corrosion is a mode of implant failure that involves the electrochemical destruction of metal. It is most likely to occur in vivo when which of the following two metal alloys are placed in direct contact?

. Titanium alloy and pure titanium
. Cobalt-chromium and titanium alloy
. Stainless steel and titanium alloy
. Cobalt-chromium and pure titanium
. Stainless steel and stainless steel

Correct Answer & Explanation

. Stainless steel and titanium alloy

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in an electrolytic solution (such as bodily fluids). The risk of galvanic corrosion is highest when there is a large difference in the anodic index between the two metals. Stainless steel and titanium have significantly different electrochemical potentials, making their combination highly susceptible to galvanic corrosion. Cobalt-chromium and titanium are frequently used together because their electrochemical potentials are similar.

Question 8194

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy is evaluated for short stature and bowing of the legs. Laboratory studies show normal serum calcium, significantly decreased serum phosphate, elevated alkaline phosphatase, and normal parathyroid hormone levels. Genetic testing confirms an X-linked dominant mutation in the PHEX gene. Which of the following is the primary pathophysiologic mechanism of this disease?

. Defective renal 1-alpha-hydroxylase activity
. Overproduction of Fibroblast Growth Factor 23 (FGF23)
. Defective intestinal calcium absorption
. Inactivating mutation in the calcium-sensing receptor (CaSR)
. Defective mineralization of osteoid due to vitamin C deficiency

Correct Answer & Explanation

. Overproduction of Fibroblast Growth Factor 23 (FGF23)

Explanation

The patient has X-linked hypophosphatemic rickets (XLH), the most common heritable form of rickets. It is caused by an inactivating mutation in the PHEX gene, which leads to reduced degradation and thus elevated levels of FGF23. Excess FGF23 causes renal phosphate wasting by downregulating sodium-phosphate cotransporters in the proximal tubule and inhibits 1-alpha-hydroxylase, preventing the synthesis of active Vitamin D. This results in normal calcium but very low phosphate levels.

Question 8195

Topic: Biology, Genetics & Bone Healing
A 65-year-old man presents with generalized bone pain, fatigue, and a recent pathological fracture of his left humerus. Laboratory tests reveal anemia, hypercalcemia, and elevated creatinine. A skeletal survey demonstrates multiple 'punched-out' lytic lesions in the skull, spine, and pelvis. A bone marrow biopsy shows >10% clonal plasma cells. Which of the following pathways is most directly responsible for uncoupling bone remodeling and causing the lytic lesions in this disease?
. Increased osteoprotegerin (OPG) secretion by tumor cells
. Direct differentiation of tumor cells into osteoclasts
. Secretion of Dickkopf-1 (DKK-1) and increased RANKL expression
. Systemic overproduction of parathyroid hormone (PTH)
. Tumor-induced inhibition of macrophage colony-stimulating factor (M-CSF)

Correct Answer & Explanation

. Secretion of Dickkopf-1 (DKK-1) and increased RANKL expression

Explanation

In multiple myeloma, clonal plasma cells secrete factors such as Dickkopf-1 (DKK-1), which inhibits Wnt signaling, thereby strongly suppressing osteoblast differentiation and bone formation. Additionally, there is an upregulation of RANKL and downregulation of osteoprotegerin (OPG) in the marrow microenvironment, which stimulates osteoclastogenesis and bone resorption. This uncoupled bone remodeling leads to the characteristic lytic bone lesions.

Question 8196

Topic: 1. General Principles & Basic Science

To maximize the pullout strength of a cortical screw inserted into diaphyseal bone, which of the following design modifications is most effective?

. Increasing the outer diameter of the screw
. Decreasing the outer diameter of the screw
. Increasing the inner (core) diameter of the screw
. Increasing the thread pitch
. Decreasing the thread depth

Correct Answer & Explanation

. Increasing the outer diameter of the screw

Explanation

The pullout strength of a screw is directly proportional to its outer diameter, the length of thread engagement, and the thread depth (which is the difference between the outer diameter and inner/core diameter). It is inversely proportional to the thread pitch. Therefore, increasing the outer diameter will significantly increase the pullout strength. Increasing the inner (core) diameter without changing the outer diameter decreases the thread depth and lowers pullout strength, although it increases the bending strength of the screw.

Question 8197

Topic: 1. General Principles & Basic Science

Which zone of articular cartilage is characterized by having the highest concentration of proteoglycans and the lowest concentration of water?

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

Correct Answer & Explanation

. Deep (radial) zone

Explanation

Articular cartilage is divided into distinct zones. The deep (radial) zone contains the highest concentration of proteoglycans and the lowest concentration of water. The collagen fibers in this zone are oriented perpendicular to the articular surface to resist compressive loads. Conversely, the superficial (tangential) zone has the highest water content and the lowest proteoglycan concentration, with collagen fibers oriented parallel to the joint surface to resist shear stress.

Question 8198

Topic: Biology, Genetics & Bone Healing

Which of the following bone graft substitutes acts primarily via an osteoinductive mechanism?

. Calcium sulfate
. Calcium phosphate cement
. Cancellous allograft
. Demineralized bone matrix (DBM)
. Tricalcium phosphate

Correct Answer & Explanation

. Demineralized bone matrix (DBM)

Explanation

Demineralized bone matrix (DBM) is an allograft product that has been treated to remove the mineralized component, leaving behind the collagen matrix and native growth factors, specifically bone morphogenetic proteins (BMPs). These BMPs provide an osteoinductive signal that stimulates host mesenchymal stem cells to differentiate into osteoblasts. Calcium sulfate, calcium phosphate, and tricalcium phosphate are purely osteoconductive. Cancellous allograft is primarily osteoconductive, as the sterilization and processing methods typically destroy osteoinductive proteins and native cells.

Question 8199

Topic: 1. General Principles & Basic Science
A 55-year-old man presents with acute knee pain and swelling. Synovial fluid analysis shows a leukocyte count of 55,000 cells/mm³ with 85% polymorphonuclear leukocytes. Microscopic evaluation demonstrates positively birefringent, rhomboid-shaped crystals. What is the most likely composition of these crystals?
. Monosodium urate
. Calcium pyrophosphate dihydrate
. Basic calcium phosphate
. Cholesterol
. Hydroxyapatite

Correct Answer & Explanation

. Calcium pyrophosphate dihydrate

Explanation

The patient presents with pseudogout, which is characterized by the deposition of calcium pyrophosphate dihydrate (CPPD) crystals in the joint. Under polarized light microscopy, CPPD crystals appear rhomboid-shaped and are weakly positively birefringent (blue when parallel to the compensator axis). In contrast, monosodium urate crystals, seen in gout, are needle-shaped and strongly negatively birefringent (yellow when parallel to the compensator axis).

Question 8200

Topic: Physiology & Rehabilitation

During the sliding filament mechanism of skeletal muscle contraction, which of the following events directly follows the binding of adenosine triphosphate (ATP) to the myosin head?

. The power stroke occurs
. The myosin head detaches from the actin filament
. Calcium is released from the sarcoplasmic reticulum
. Tropomyosin shifts to expose the binding site
. ATP is hydrolyzed to ADP and inorganic phosphate

Correct Answer & Explanation

. The myosin head detaches from the actin filament

Explanation

In the muscle contraction cycle, the binding of ATP to the myosin head causes a conformational change that decreases the affinity of myosin for actin, resulting in the detachment of the myosin head from the actin filament. Subsequent hydrolysis of ATP to ADP and inorganic phosphate cocks the myosin head into a high-energy state. When the myosin binds a new actin site and releases the inorganic phosphate, the power stroke occurs.

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