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Infection, Pharmacology & VTE MCQs

Practice Set 32 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 621

Topic: Infection, Pharmacology & VTE

A 9-year-old boy presents with mild shoulder pain after throwing a baseball. Radiographs reveal a centrally located, completely lytic, expansile lesion in the proximal humerus metaphysis that does not breach the cortex. A subtle 'fallen leaf' sign is noted. What biochemical substance is typically found in high concentrations within the fluid of this lesion?

. Lactic acid
. Prostaglandins
. Histamine
. Vasoactive intestinal peptide
. Alkaline phosphatase

Correct Answer & Explanation

. Prostaglandins

Explanation

The classic presentation of a central, purely lytic lesion in the proximal humerus of a child with a 'fallen leaf' or 'fallen fragment' sign (representing a piece of fractured cortex resting at the bottom of the cyst) is highly characteristic of a unicameral (simple) bone cyst. The cyst fluid is clear and serous, and it has been shown to contain high concentrations of prostaglandins (specifically PGE2), interleukins, and other bone-resorbing factors.

Question 622

Topic: Infection, Pharmacology & VTE

A 4-year-old girl presents with acute onset of right hip pain and a limp. Which of the following parameters is NOT included in the original Kocher criteria for differentiating septic arthritis from transient synovitis?

. Non-weight-bearing on the affected side
. Erythrocyte sedimentation rate (ESR) > 40 mm/hr
. C-reactive protein (CRP) > 20 mg/L
. Temperature > 38.5 C (101.3 F)
. White blood cell (WBC) count > 12,000/mm3

Correct Answer & Explanation

. C-reactive protein (CRP) > 20 mg/L

Explanation

The classic Kocher criteria include history of non-weight-bearing, ESR > 40, temperature > 38.5 C, and WBC > 12,000. Although CRP is highly predictive and used in modified criteria, it was not part of the original four-part Kocher criteria.

Question 623

Topic: Infection, Pharmacology & VTE
An 8-year-old boy presents with a 2-day history of right knee pain, limp, and a fever of 38.8°C. Labs show a WBC of 16,000/mm³, ESR of 65 mm/hr, and CRP of 5.2 mg/dL. Knee aspiration yields turbid fluid with 85,000 WBCs/mm³ (>90% polymorphonuclear cells). Which of the following is the most appropriate definitive management?
. Intravenous antibiotics and splinting
. Serial needle aspirations of the knee
. Urgent arthrotomy and irrigation of the knee joint
. MRI of the knee with and without contrast
. Outpatient oral antibiotics and follow-up in 48 hours

Correct Answer & Explanation

. Urgent arthrotomy and irrigation of the knee joint

Explanation

The clinical presentation and aspirate findings (>50,000 WBCs with >75% PMNs) are highly diagnostic of septic arthritis. This is a surgical emergency requiring urgent joint irrigation and debridement to prevent irreversible cartilage destruction.

Question 624

Topic: Infection, Pharmacology & VTE

A 4-year-old boy presents with a 2-day history of right hip pain. His temperature is 38.6 C (101.5 F), WBC count is 14,000/mm3, ESR is 45 mm/hr, and he refuses to bear weight. According to the Kocher criteria, what is the approximate probability that this child has septic arthritis of the hip?

. 3%
. 40%
. 73%
. 93%
. 99%

Correct Answer & Explanation

. 99%

Explanation

The Kocher criteria include history of fever (>38.5 C), non-weight bearing, ESR >40 mm/hr, and WBC >12,000/mm3. Having all four predictors yields a >99% probability of septic arthritis.

Question 625

Topic: Infection, Pharmacology & VTE

According to the Kocher criteria, which of the following is NOT one of the four classic predictors used to differentiate septic arthritis of the hip from transient synovitis in children?

. Non-weight-bearing on the affected side
. Erythrocyte sedimentation rate > 40 mm/hr
. C-reactive protein > 2.0 mg/dL
. White blood cell count > 12,000 cells/mm3
. Fever > 38.5 degrees C (101.3 degrees F)

Correct Answer & Explanation

. C-reactive protein > 2.0 mg/dL

Explanation

The original Kocher criteria include non-weight-bearing status, ESR > 40, WBC > 12,000, and Temp > 38.5C. CRP > 2.0 was later added by Caird to improve the predictive model but is not one of the original four.

Question 626

Topic: Infection, Pharmacology & VTE

An 8-year-old boy presents with a limp, fever, and refusal to bear weight on his right leg. Blood cultures are drawn and he is taken for aspiration and debridement of suspected acute hematogenous osteomyelitis. Which of the following organisms is the most common cause of this condition?

. Kingella kingae
. Streptococcus pyogenes
. Staphylococcus aureus
. Haemophilus influenzae
. Pseudomonas aeruginosa

Correct Answer & Explanation

. Staphylococcus aureus

Explanation

Staphylococcus aureus is the most common causative organism for acute hematogenous osteomyelitis across all pediatric age groups.

Question 627

Topic: Infection, Pharmacology & VTE

A 4-year-old boy presents with a 2-day history of right hip pain and a limp. He refuses to bear weight. His temperature is 38.8 C (101.8 F), ESR is 45 mm/hr, WBC count is 13,000/mm3, and serum CRP is 2.5 mg/dL. According to the classic Kocher criteria, what is the approximate probability that this child has septic arthritis?

. 3%
. 40%
. 71%
. 93%
. 99%

Correct Answer & Explanation

. 93%

Explanation

The child meets three of the four classic Kocher criteria (fever > 38.5 C, non-weight-bearing, ESR > 40, WBC > 12,000). The probability of septic arthritis with 3 criteria is approximately 93%.

Question 628

Topic: Infection, Pharmacology & VTE

A 3-year-old boy presents with an inability to bear weight on his right leg, a temperature of 38.8 C, a WBC count of 14,000/mm3, and an ESR of 55 mm/hr. According to the Kocher criteria, what is the probability that this child has septic arthritis of the hip?

. 40%
. 71%
. 83%
. 93%
. 99%

Correct Answer & Explanation

. 99%

Explanation

The patient meets all four Kocher criteria: non-weight-bearing, temperature >38.5 C, WBC >12,000, and ESR >40. The presence of all four criteria carries a 99% probability of septic arthritis, necessitating urgent joint aspiration.

Question 629

Topic: Infection, Pharmacology & VTE

A patient with a severe penicillin allergy (anaphylaxis) is undergoing THA. Current guidelines recommend which of the following single agents as the best alternative for surgical prophylaxis?

. Cefazolin
. Vancomycin
. Erythromycin
. Ciprofloxacin
. Aztreonam

Correct Answer & Explanation

. Vancomycin

Explanation

For patients with a severe, IgE-mediated penicillin allergy (e.g., anaphylaxis), Vancomycin or Clindamycin are the recommended single-agent alternatives for surgical prophylaxis in total joint arthroplasty.

Question 630

Topic: Infection, Pharmacology & VTE

A 60-year-old man develops a chronic prosthetic joint infection 2 years after a total knee arthroplasty. Intraoperative cultures grow Staphylococcus epidermidis. The organism's ability to persist on the implant surface despite appropriate systemic antibiotic therapy is primarily due to:

. Intracellular sequestration within osteoblasts
. Inherent resistance to beta-lactam antibiotics via mecA gene expression
. Formation of a glycocalyx biofilm protecting the bacteria from the host immune system
. Production of neutralizing enzymes that degrade locally applied antibiotics
. Rapid acquisition of plasmid-mediated resistance from the local microbiome

Correct Answer & Explanation

. Formation of a glycocalyx biofilm protecting the bacteria from the host immune system

Explanation

Staphylococcus epidermidis and S. aureus produce a polysaccharide intercellular adhesin (PIA) that forms an extracellular polymeric matrix, known as a glycocalyx or biofilm, on the surface of orthopedic implants. This biofilm severely limits the penetration of antibiotics and host immune cells. Furthermore, the bacteria deep within the biofilm become metabolically inactive 'persister cells,' rendering traditional bactericidal antibiotics ineffective. For chronic biofilm infections, surgical removal of the implant is usually required.

Question 631

Topic: Infection, Pharmacology & VTE
A 72-year-old woman is scheduled for an elective total hip arthroplasty. Her surgeon prescribes apixaban for postoperative deep vein thrombosis (DVT) prophylaxis. By which precise mechanism does apixaban achieve its primary anticoagulant effect?
. Irreversible inhibition of cyclooxygenase-1 in platelets
. Direct inhibition of factor Xa
. Direct inhibition of thrombin (factor IIa)
. Potentiation of antithrombin III activity against factors IIa and Xa
. Vitamin K antagonism via inhibition of epoxide reductase

Correct Answer & Explanation

. Direct inhibition of factor Xa

Explanation

Apixaban and rivaroxaban are direct, oral, highly selective inhibitors of Factor Xa (DOACs). They bind directly to the active site of factor Xa, preventing the conversion of prothrombin to thrombin in the coagulation cascade. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin is a vitamin K antagonist. Low-molecular-weight heparins primarily potentiate antithrombin III. Aspirin irreversibly inhibits cyclooxygenase-1 in platelets.

Question 632

Topic: Infection, Pharmacology & VTE
A 70-year-old patient is scheduled for a total knee arthroplasty. The patient has a history of atrial fibrillation and takes rivaroxaban daily. What is the precise mechanism of action of this anticoagulant?
. Direct inhibition of thrombin (Factor IIa)
. Direct inhibition of Factor Xa
. Vitamin K antagonism via epoxide reductase inhibition
. Activation of antithrombin III
. Inhibition of ADP-induced platelet aggregation

Correct Answer & Explanation

. Direct inhibition of Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that exert their effect by directly binding to and inhibiting Factor Xa, blocking the conversion of prothrombin to thrombin. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin is a vitamin K antagonist. Heparin and low molecular weight heparins (like enoxaparin) act by enhancing the activity of antithrombin III.

Question 633

Topic: Infection, Pharmacology & VTE

Following total joint arthroplasty, periprosthetic joint infections are notoriously difficult to eradicate due to bacterial biofilm formation. Which of the following represents the critical first step in the formation of a biofilm on an orthopedic implant?

. Quorum sensing and colony dispersion
. Secretion of extracellular polymeric substance (EPS)
. Reversible bacterial adherence to a host conditioning film
. Maturation of water channels within the colony
. Irreversible adherence via species-specific adhesins

Correct Answer & Explanation

. Reversible bacterial adherence to a host conditioning film

Explanation

The formation of a biofilm on an implant occurs in a stepwise fashion: 1) Within minutes of implantation, a 'conditioning film' of host proteins (fibronectin, fibrinogen) coats the implant; 2) Planktonic bacteria undergo reversible adherence to this film via non-specific physical forces; 3) Adherence becomes irreversible via specific bacterial adhesins (e.g., MSCRAMMs); 4) Bacteria proliferate and secrete an extracellular polymeric substance (EPS) to form the mature biofilm matrix; 5) The biofilm matures and later uses quorum sensing to coordinate dispersion.

Question 634

Topic: Infection, Pharmacology & VTE

A 65-year-old man presents with an acutely swollen, erythematous, and exquisitely painful right knee. Aspiration of the knee yields cloudy synovial fluid with 45,000 WBCs/mm^3 (85% polymorphonuclear leukocytes). Polarized light microscopy reveals negatively birefringent, needle-shaped crystals. What is the composition of the crystals?

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

Correct Answer & Explanation

. Monosodium urate

Explanation

The clinical presentation and synovial fluid analysis are diagnostic for an acute gout attack, which is characterized by the deposition of monosodium urate crystals. These crystals appear needle-shaped and exhibit strong negative birefringence under polarized light microscopy. In contrast, calcium pyrophosphate dihydrate (CPPD) crystals, seen in pseudogout, are typically rhomboid-shaped and weakly positively birefringent.

Question 635

Topic: Infection, Pharmacology & VTE
Which of the following pharmacologic agents used for venous thromboembolism (VTE) prophylaxis acts by directly and reversibly inhibiting Factor Xa?
. Warfarin
. Enoxaparin
. Rivaroxaban
. Dabigatran
. Fondaparinux

Correct Answer & Explanation

. Rivaroxaban

Explanation

Rivaroxaban and apixaban are direct Factor Xa inhibitors. Enoxaparin (a low-molecular-weight heparin) binds antithrombin III, which then indirectly inhibits Factor Xa and IIa. Fondaparinux also binds antithrombin III to indirectly but selectively inhibit Factor Xa. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin inhibits vitamin K-dependent factors (II, VII, IX, X).

Question 636

Topic: Infection, Pharmacology & VTE

A 65-year-old woman presents with an acutely infected total knee arthroplasty. Cultures from the joint aspiration grow Methicillin-resistant Staphylococcus aureus (MRSA). Which of the following mechanisms is primarily responsible for this organism's methicillin resistance?

. Alteration of bacterial DNA gyrase
. Hyperproduction of beta-lactamase enzyme
. Modification of penicillin-binding protein via the mecA gene (PBP2a)
. Upregulation of transmembrane efflux pumps
. Ribosomal RNA methylation protecting the drug target

Correct Answer & Explanation

. Modification of penicillin-binding protein via the mecA gene (PBP2a)

Explanation

MRSA resistance to beta-lactam antibiotics is primarily mediated by the acquisition of the staphylococcal cassette chromosome mec (SCCmec) containing the mecA gene. This gene encodes for a novel penicillin-binding protein, PBP2a, which has a significantly lower binding affinity for beta-lactam antibiotics, allowing cell wall synthesis to continue even in the presence of these drugs.

Question 637

Topic: Infection, Pharmacology & VTE
A 70-year-old patient is scheduled for a primary total hip arthroplasty and will be placed on oral Apixaban for deep vein thrombosis prophylaxis postoperatively. Which of the following best describes the specific pharmacological mechanism of action of this medication?
. Direct inhibition of Thrombin (Factor IIa)
. Direct inhibition of Factor Xa
. Irreversible inhibition of cyclooxygenase-1 (COX-1)
. Vitamin K epoxide reductase inhibition
. Activation of antithrombin III

Correct Answer & Explanation

. Direct inhibition of Factor Xa

Explanation

Apixaban and Rivaroxaban are direct, oral Factor Xa inhibitors. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin acts by inhibiting Vitamin K epoxide reductase. Aspirin irreversibly inhibits COX-1. Low molecular weight heparins (like enoxaparin) work by binding to and activating antithrombin III, which subsequently inactivates Factor Xa and, to a lesser extent, Factor IIa.

Question 638

Topic: Infection, Pharmacology & VTE



Biofilm formation is a critical and devastating step in the pathogenesis of periprosthetic joint infection. Which extracellular component makes up the vast majority of the biofilm's volume and protects the sessile bacteria from host defenses?

. Fibrinogen networks
. Extracellular DNA matrices
. Peptidoglycan layers
. Polysaccharide glycocalyx
. Lipopolysaccharide envelopes

Correct Answer & Explanation

. Polysaccharide glycocalyx

Explanation

The biofilm matrix is predominantly composed of a highly hydrated polysaccharide glycocalyx. This thick slime layer shields the embedded sessile bacteria from antibiotics, antibodies, and immune cell phagocytosis.

Question 639

Topic: Infection, Pharmacology & VTE
Rivaroxaban is commonly prescribed for deep vein thrombosis prophylaxis following total joint arthroplasty. Which step in the coagulation cascade is directly inhibited by this medication?
. Conversion of fibrinogen to fibrin
. Factor Xa
. Antithrombin III
. Factor IIa (Thrombin)
. Vitamin K epoxide reductase

Correct Answer & Explanation

. Factor Xa

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants that specifically inhibit Factor Xa. Dabigatran directly inhibits thrombin (Factor IIa), while warfarin inhibits Vitamin K epoxide reductase.

Question 640

Topic: Infection, Pharmacology & VTE

In the pathogenesis of periprosthetic joint infection, bacteria commonly form a biofilm on the implant surface. Which phase of biofilm development involves the irreversible attachment of bacteria and synthesis of an extracellular polymeric substance (EPS) matrix?

. Planktonic phase
. Initial reversible adhesion
. Maturation phase
. Dispersion phase
. Lag phase

Correct Answer & Explanation

. Maturation phase

Explanation

The maturation phase of biofilm formation is characterized by irreversible bacterial adhesion and robust production of the extracellular polymeric substance (EPS) matrix. This creates a highly antibiotic-resistant environment.

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