MCCQE - Medical Council of Canada Qualifying Examination Antimicrobial Pharmacology Questions and Answers 1 — Questions and Answers
Question 1: A 58-year-old male is receiving intravenous vancomycin for MRSA osteomyelitis. Partway through the infusion, he develops intense flushing of his face and neck, pruritus, and an erythematous rash on his upper torso. His vital signs are stable. What is the most likely mechanism responsible for this reaction?
- IgE-mediated type I hypersensitivity
- Non-immunologic mast cell degranulation (Correct answer)
- Immune complex deposition (Type III)
- T-cell mediated delayed hypersensitivity (Type IV)
Correct answer: Non-immunologic mast cell degranulation
The clinical presentation is classic for Vancomycin Infusion Reaction, also known as Red Man Syndrome. This is an anaphylactoid reaction, not a true allergy, caused by the rapid infusion of vancomycin leading to direct, non-immunologic degranulation of mast cells and basophils, which releases histamine. [3, 5, 12] It is rate-dependent and can be managed by slowing the infusion and administering antihistamines. [20]
Question 2: A physician prescribes an antibiotic that acts by inhibiting bacterial DNA gyrase and topoisomerase IV, thereby preventing DNA replication and repair. Which of the following antibiotic classes utilizes this mechanism of action?
- Beta-lactams
- Aminoglycosides
- Fluoroquinolones (Correct answer)
- Macrolides
Correct answer: Fluoroquinolones
Fluoroquinolones (e.g., ciprofloxacin, levofloxacin) exert their bactericidal effect by inhibiting bacterial DNA gyrase (topoisomerase II) and topoisomerase IV. [4, 11] These enzymes are essential for bacterial DNA replication, transcription, and repair, and their inhibition leads to bacterial cell death. [23]
Question 3: A 68-year-old man presents with fever, productive cough, and a chest X-ray confirming community-acquired pneumonia (CAP). He has a documented history of anaphylaxis to amoxicillin. According to current guidelines for outpatient management, which of the following is an appropriate monotherapy?
- Amoxicillin-clavulanate
- Ceftriaxone
- Trimethoprim-sulfamethoxazole
- Doxycycline (Correct answer)
Correct answer: Doxycycline
In a patient with a history of anaphylaxis to a penicillin, all beta-lactam antibiotics (including cephalosporins like ceftriaxone) should be avoided. Doxycycline is a recommended first-line alternative monotherapy for outpatient CAP as it provides coverage against common typical and atypical pathogens. [14, 26] Macrolides are also an option, but resistance can be an issue. [14]
Question 4: A 72-year-old female with atrial fibrillation, managed with warfarin, is diagnosed with a urinary tract infection. Which of the following antibiotics, if prescribed, would most significantly increase her risk of bleeding by potentiating the effect of warfarin?
- Nitrofurantoin
- Cephalexin
- Ciprofloxacin (Correct answer)
- Amoxicillin
Correct answer: Ciprofloxacin
Ciprofloxacin is a known inhibitor of the cytochrome P450 enzyme system (specifically CYP1A2 and CYP3A4), which is involved in the metabolism of warfarin. [1, 16] By inhibiting its metabolism, ciprofloxacin can lead to elevated serum levels of warfarin, increasing the INR and the risk of significant bleeding. [6, 18]
Question 5: A 45-year-old male develops profuse, watery diarrhea and abdominal cramping ten days after completing a course of clindamycin. A stool assay is positive for *Clostridioides difficile* toxin. For an initial, non-severe episode of *C. difficile* infection, which of the following is the recommended first-line treatment according to current guidelines?
- Metronidazole
- Rifaximin
- Vancomycin (oral)
- Fidaxomicin (Correct answer)
Correct answer: Fidaxomicin
Current infectious disease guidelines (IDSA 2021) recommend fidaxomicin as the preferred first-line agent for an initial episode of *C. difficile* infection. [8, 25] This is due to its superior efficacy in achieving a sustained clinical response and lower rates of recurrence compared to vancomycin. [8] Oral vancomycin is a recommended alternative, while metronidazole is no longer considered first-line for most initial episodes. [15, 25]
Question 6: A 65-year-old patient with enterococcal endocarditis is started on a parenteral antibiotic regimen that includes gentamicin for synergy. Due to the known toxicities of this medication, which of the following parameters must be closely monitored throughout the course of therapy?
- Prothrombin time (PT/INR)
- Complete blood count with differential
- Serum creatinine and drug trough levels (Correct answer)
- Liver function tests (ALT, AST)
Correct answer: Serum creatinine and drug trough levels
Gentamicin is an aminoglycoside antibiotic, a class of drugs well-known for causing nephrotoxicity (kidney damage) and ototoxicity (hearing and vestibular damage). [7, 19] Monitoring renal function via serum creatinine and ensuring appropriate drug clearance by measuring trough levels are essential to minimize the risk of these serious, dose-dependent adverse effects. [24, 29]
MCCQE - Medical Council of Canada Qualifying Examination Antimicrobial Pharmacology Questions and Answers 1
A 58-year-old male is receiving intravenous vancomycin for MRSA osteomyelitis.
Partway through the infusion, he develops intense flushing of his face and neck, pruritus, and an erythematous rash on his upper torso.
His vital signs are stable.
What is the most likely mechanism responsible for this reaction?