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Infection, Pharmacology & VTE MCQs

Practice Set 62 of 68

This practice set contains high-yield board review questions covering key concepts in Infection, Pharmacology & VTE. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 1221

Topic: Infection, Pharmacology & VTE

A 24-year-old sexually active male presents with an acute, atraumatic swollen knee. Arthrocentesis yields 65,000 WBCs/mcL with 90% polymorphonuclear cells. Pending culture results, what is the most appropriate empiric antibiotic therapy?

. Vancomycin alone
. Ceftriaxone alone
. Vancomycin and Ceftriaxone
. Cefazolin and Gentamicin
. Linezolid and Piperacillin-tazobactam

Correct Answer & Explanation

. Vancomycin and Ceftriaxone

Explanation

In a young, sexually active patient with suspected septic arthritis, both Neisseria gonorrhoeae and Staphylococcus aureus must be covered. Empiric therapy with Vancomycin (for MRSA) and Ceftriaxone (for gonococcus) is the standard of care.

Question 1222

Topic: Infection, Pharmacology & VTE
A 45-year-old farmer sustains a highly contaminated Grade IIIB open tibial shaft fracture after being trapped under a tractor. Which of the following intravenous antibiotic regimens is most appropriate upon presentation?
. Cefazolin alone
. Cefazolin and Gentamicin
. Cefazolin, Gentamicin, and Penicillin
. Ciprofloxacin alone
. Vancomycin and Piperacillin-Tazobactam

Correct Answer & Explanation

. Cefazolin, Gentamicin, and Penicillin

Explanation

Farm injuries with heavy soil contamination carry a high risk of Clostridium perfringens infection. The addition of high-dose Penicillin to a first-generation cephalosporin and an aminoglycoside is strongly recommended to cover this anaerobic threat.

Question 1223

Topic: Infection, Pharmacology & VTE
A 70-year-old patient scheduled for elective total knee arthroplasty takes rivaroxaban for atrial fibrillation. What is the precise mechanism of action of this medication?
. Direct thrombin inhibitor
. Vitamin K epoxide reductase inhibitor
. Direct Factor Xa inhibitor
. Antithrombin III activator
. ADP receptor antagonist

Correct Answer & Explanation

. Direct Factor Xa inhibitor

Explanation

Rivaroxaban and apixaban are direct Factor Xa inhibitors, which block the convergence point of the intrinsic and extrinsic coagulation pathways. Dabigatran, in contrast, is a direct thrombin (Factor IIa) inhibitor.

Question 1224

Topic: Infection, Pharmacology & VTE

Bacterial biofilms make the eradication of prosthetic joint infections highly challenging. Which initial phase of biofilm development involves the reversible attachment of planktonic bacteria to the implant surface?

. Maturation
. Dispersion
. Adhesion
. Microcolony formation
. Quorum sensing

Correct Answer & Explanation

. Adhesion

Explanation

Biofilm formation begins with adhesion, a reversible attachment of planktonic bacteria to a surface via van der Waals forces. This is followed by irreversible attachment, microcolony formation, maturation, and eventual dispersion.

Question 1225

Topic: Infection, Pharmacology & VTE
A 45-year-old male presents with a swollen, acutely painful knee. A joint aspiration is performed yielding cloudy, yellow fluid. The synovial fluid analysis reveals a white blood cell count of 45,000 cells/mm³ with 80% polymorphonuclear leukocytes. No crystals are visualized under polarized light. What is the most likely diagnostic category?
. Non-inflammatory (e.g., Osteoarthritis)
. Hemorrhagic (e.g., Traumatic hemarthrosis)
. Inflammatory (e.g., Rheumatoid arthritis)
. Septic (e.g., Bacterial arthritis)
. Crystalline (e.g., Gout)

Correct Answer & Explanation

. Inflammatory (e.g., Rheumatoid arthritis)

Explanation

Synovial fluid analysis categorizes joint effusions based on WBC count. Non-inflammatory (OA) fluid has <2,000 WBCs/mm³. Inflammatory fluid (such as in Rheumatoid Arthritis) typically ranges from 2,000 to 50,000 WBCs/mm³ with >50% PMNs. Septic arthritis generally presents with >50,000 WBCs/mm³ (frequently >100,000) and >90% PMNs. The absence of crystals makes gout or pseudogout unlikely.

Question 1226

Topic: Infection, Pharmacology & VTE

In periprosthetic joint infections, Staphylococcus epidermidis frequently evades the immune system and antibiotics by forming a complex biofilm. The crucial initial stage of biofilm formation is the primary, irreversible attachment of the bacteria to the synthetic polymer surface. Which bacterial component predominantly mediates this initial attachment?

. Exopolysaccharide matrix (slime)
. Protein A
. Fibronectin-binding protein
. Autolysin (AtlE)
. Polysaccharide intercellular adhesin (PIA)

Correct Answer & Explanation

. Autolysin (AtlE)

Explanation

The initial, irreversible attachment of S. epidermidis to hydrophobic synthetic polymer surfaces (implant materials) is heavily mediated by surface-associated proteins, particularly Autolysin (AtlE). Polysaccharide intercellular adhesin (PIA) is vital for the subsequent phase of biofilm development: intercellular adhesion, accumulation, and formation of the mature extracellular matrix 'slime'.

Question 1227

Topic: Infection, Pharmacology & VTE
Rivaroxaban is frequently prescribed for deep vein thrombosis (DVT) prophylaxis following lower extremity total joint arthroplasty. Which of the following describes the precise pharmacologic mechanism of action of this medication?
. Direct, reversible inhibition of thrombin (Factor IIa)
. Inhibition of Vitamin K epoxide reductase
. Direct, highly selective inhibition of Factor Xa
. Indirect inhibition of Factor Xa via activation of antithrombin III
. Irreversible antagonism of the platelet P2Y12 ADP receptor

Correct Answer & Explanation

. Direct, highly selective inhibition of Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that function as direct, highly selective, and reversible inhibitors of free and clot-bound Factor Xa. They do not require antithrombin III to exert their effect (unlike indirect inhibitors such as fondaparinux or low-molecular-weight heparins). Dabigatran is a direct thrombin inhibitor.

Question 1228

Topic: Infection, Pharmacology & VTE

A 40-year-old male presents with an acutely swollen and painful knee. Aspiration yields opaque synovial fluid. Which of the following synovial fluid cell count profiles is most diagnostic of a bacterial septic arthritis?

. WBC 15,000 cells/mm3 with 40% PMNs
. WBC 35,000 cells/mm3 with 60% PMNs
. WBC 60,000 cells/mm3 with 95% PMNs
. WBC 10,000 cells/mm3 with 20% PMNs and positive birefringence
. WBC 2,000 cells/mm3 with 10% PMNs

Correct Answer & Explanation

. WBC 60,000 cells/mm3 with 95% PMNs

Explanation

Septic arthritis is typically characterized by a synovial fluid white blood cell (WBC) count greater than 50,000 cells/mm3 with a polymorphonuclear (PMN) cell predominance of greater than 75% to 90%. Counts above 50,000 with a severe left shift mandate urgent treatment for joint sepsis.

Question 1229

Topic: Infection, Pharmacology & VTE

Staphylococcus epidermidis is a frequent pathogen in periprosthetic joint infections. Which of its virulence factors is primarily responsible for its ability to evade systemic antibiotics and host immune responses?

. Production of Panton-Valentine leukocidin (PVL)
. Formation of an exopolysaccharide glycocalyx (biofilm)
. Secretion of powerful endotoxins
. Intracellular survival within macrophages
. Rapid mutation of penicillin-binding proteins

Correct Answer & Explanation

. Formation of an exopolysaccharide glycocalyx (biofilm)

Explanation

Staphylococcus epidermidis is an opportunistic pathogen known for adhering to orthopedic implants and producing a thick glycocalyx biofilm. This biofilm physically blocks immune cells and prevents antibiotics from reaching the bacteria.

Question 1230

Topic: Infection, Pharmacology & VTE

A patient develops a postoperative surgical site infection. Cultures grow Methicillin-resistant Staphylococcus aureus (MRSA). The genetic basis for this resistance is primarily mediated by the mecA gene, which encodes for:

. A highly efficient multi-drug efflux pump
. An enzyme that directly cleaves the beta-lactam ring
. An altered penicillin-binding protein (PBP2a) with low affinity for beta-lactams
. A ribosomal methylation factor protecting against protein synthesis inhibition
. Down-regulation of outer membrane porin channels

Correct Answer & Explanation

. An altered penicillin-binding protein (PBP2a) with low affinity for beta-lactams

Explanation

MRSA resistance is conferred by the acquisition of the mecA gene on a mobile genetic element. This gene encodes an altered penicillin-binding protein (PBP2a) that has very low binding affinity for beta-lactam antibiotics.

Question 1231

Topic: Infection, Pharmacology & VTE

A 35-year-old farmer sustains a severe open tibia fracture after being run over by agricultural machinery. In addition to standard broad-spectrum coverage, which antibiotic is classically recommended to specifically cover for Clostridium species due to soil contamination?

. Vancomycin
. Clindamycin
. Penicillin
. Linezolid
. Daptomycin

Correct Answer & Explanation

. Penicillin

Explanation

Farm or soil-contaminated injuries have a high risk of clostridial infection (gas gangrene). High-dose intravenous penicillin is traditionally added to the antibiotic regimen to target Clostridium perfringens.

Question 1232

Topic: Infection, Pharmacology & VTE

Which of the following organisms is most notoriously associated with early and robust biofilm formation on orthopedic implants, frequently resulting in indolent, chronic periprosthetic joint infections?

. Streptococcus pneumoniae
. Neisseria gonorrhoeae
. Staphylococcus epidermidis
. Haemophilus influenzae
. Mycobacterium tuberculosis

Correct Answer & Explanation

. Staphylococcus epidermidis

Explanation

Staphylococcus epidermidis, a coagulase-negative staphylococcus, is a major colonizer of orthopedic hardware. It produces a thick glycocalyx biofilm that acts as a physical barrier against both systemic antibiotics and host immune cells.

Question 1233

Topic: Infection, Pharmacology & VTE

Venous thromboembolism (VTE) prophylaxis is a standard of care following major joint arthroplasty. Which of the following oral anticoagulants exerts its effect by acting as a direct thrombin inhibitor?

. Rivaroxaban
. Apixaban
. Dabigatran
. Fondaparinux
. Warfarin

Correct Answer & Explanation

. Dabigatran

Explanation

Dabigatran is a direct thrombin (Factor IIa) inhibitor used for oral VTE prophylaxis. Rivaroxaban and apixaban are direct Factor Xa inhibitors, whereas warfarin inhibits Vitamin K epoxide reductase.

Question 1234

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 1235

Topic: Infection, Pharmacology & VTE
A patient is prescribed rivaroxaban for deep vein thrombosis (DVT) prophylaxis following a total knee arthroplasty. What is the exact mechanism of action of this pharmacological agent?
. Direct inhibition of thrombin (Factor IIa)
. Indirect inhibition of Factor Xa via antithrombin III
. Direct inhibition of Factor Xa
. Inhibition of Vitamin K epoxide reductase
. Irreversible inhibition of cyclooxygenase

Correct Answer & Explanation

. Direct inhibition of Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants that selectively and directly inhibit Factor Xa. Low molecular weight heparins inhibit Factor Xa indirectly via antithrombin III. Dabigatran directly inhibits thrombin (Factor IIa).

Question 1236

Topic: Infection, Pharmacology & VTE

In the pathogenesis of periprosthetic joint infection, Staphylococcus epidermidis relies on a specific structural matrix to adhere to metallic implants, which subsequently shields the bacteria from host immune defenses and antibiotics. What is the primary constituent of this protective biofilm?

. Peptidoglycan
. Lipopolysaccharide
. Exopolysaccharide (glycocalyx)
. Teichoic acid
. Protein A

Correct Answer & Explanation

. Exopolysaccharide (glycocalyx)

Explanation

Staphylococcus epidermidis is notorious for forming biofilms on medical implants. The biofilm matrix is primarily composed of an exopolysaccharide (also known as glycocalyx or polysaccharide intercellular adhesin, PIA), which firmly anchors the colony and resists antimicrobial penetration.

Question 1237

Topic: Infection, Pharmacology & VTE

A 12-year-old boy from Connecticut presents with an acute, massive, painless effusion of his right knee. He denies trauma. He had a brief febrile illness with an erythematous rash a few months ago. Joint aspiration yields 40,000 WBCs/mcL (mostly neutrophils). What is the most appropriate initial diagnostic test?

. Gram stain and bacterial culture of synovial fluid
. Crystal analysis under polarized light
. Serum Lyme enzyme-linked immunosorbent assay (ELISA)
. Serum Western blot for Borrelia burgdorferi
. Polymerase chain reaction (PCR) of synovial fluid for Neisseria gonorrhoeae

Correct Answer & Explanation

. Serum Lyme enzyme-linked immunosorbent assay (ELISA)

Explanation

Lyme arthritis commonly presents as a monoarticular large joint effusion (classically the knee) with inflammatory fluid, often with minimal pain compared to septic arthritis. The standard two-tiered diagnostic approach begins with a screening serum ELISA. If positive or equivocal, it is followed by a confirmatory Western blot.

Question 1238

Topic: Infection, Pharmacology & VTE

A 55-year-old diabetic male presents with an acutely painful, erythematous, and swollen knee. Arthrocentesis is performed. Which of the following synovial fluid profiles is most specific and diagnostic for a bacterial septic arthritis?

. WBC count of 2,000/mm^3 with 30% polymorphonuclear cells
. WBC count of 15,000/mm^3 with 50% polymorphonuclear cells
. WBC count of 30,000/mm^3 with 60% polymorphonuclear cells
. WBC count of 85,000/mm^3 with 95% polymorphonuclear cells
. WBC count of 12,000/mm^3 with positively birefringent crystals

Correct Answer & Explanation

. WBC count of 85,000/mm^3 with 95% polymorphonuclear cells

Explanation

Synovial fluid indicating septic arthritis typically shows a markedly elevated white blood cell count, usually greater than 50,000/mm^3 (often >100,000), with a severe neutrophilic predominance (>90% polymorphonuclear leukocytes).

Question 1239

Topic: Infection, Pharmacology & VTE
Following total hip arthroplasty, a patient is prescribed oral rivaroxaban for chemical venous thromboembolism (VTE) prophylaxis. Which of the following accurately describes the precise mechanism of action of this medication?
. Irreversible inhibition of cyclooxygenase-1 (COX-1)
. Activation of antithrombin III, which secondarily inhibits Factor Xa
. Direct, selective inhibition of Factor Xa
. Direct inhibition of thrombin (Factor IIa)
. Inhibition of Vitamin K epoxide reductase

Correct Answer & Explanation

. Direct, selective inhibition of Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that work by directly and reversibly binding to and inhibiting free and clot-bound Factor Xa. Unlike Low Molecular Weight Heparins, they do not require antithrombin III to exert their effect.

Question 1240

Topic: Infection, Pharmacology & VTE

In periprosthetic joint infections, Staphylococcus epidermidis evades the host immune response and resists antimicrobial therapy primarily through the production of a biofilm. Which of the following is the main structural component of this biofilm matrix?

. Peptidoglycan
. Lipopolysaccharide
. Polysaccharide intercellular adhesin
. Hyaluronic acid
. Teichoic acid

Correct Answer & Explanation

. Polysaccharide intercellular adhesin

Explanation

Staphylococcus epidermidis produces a biofilm (glycocalyx) that is predominantly composed of polysaccharide intercellular adhesin (PIA). This protective slime layer defends the bacteria against phagocytosis and blocks antibiotic penetration.

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