Pharmacology Antimicrobial Therapy Principles 3 — Questions and Answers
Question 1: A patient with a creatinine clearance of 15 mL/min requires antibiotic therapy. Which agent requires the most significant dose reduction?
- Azithromycin
- Doxycycline
- Vancomycin (Correct answer)
- Clindamycin
Correct answer: Vancomycin
Vancomycin is primarily renally eliminated and requires substantial dose adjustment (extended intervals or reduced doses) in severe renal impairment.
Question 2: Bacteriostatic antibiotics are generally preferred over bactericidal agents in which clinical scenario?
- Bacterial endocarditis
- Meningitis
- Mild community-acquired pneumonia in an immunocompetent host (Correct answer)
- Neutropenic fever
Correct answer: Mild community-acquired pneumonia in an immunocompetent host
In immunocompetent patients with mild infections, bacteriostatic antibiotics are sufficient because the host immune system can clear growth-inhibited bacteria.
Question 3: Which antibiotic is considered the drug of choice for treating Legionella pneumophila pneumonia?
- Penicillin G
- Azithromycin (Correct answer)
- Ampicillin-sulbactam
- Ceftriaxone
Correct answer: Azithromycin
Azithromycin (or other macrolides/fluoroquinolones) is first-line for Legionella because the organism is an intracellular pathogen and beta-lactams have no efficacy.
Question 4: Combining a beta-lactam with a beta-lactamase inhibitor (e.g., amoxicillin-clavulanate) expands coverage primarily against which organisms?
- MRSA and VRE
- Beta-lactamase-producing Haemophilus influenzae and anaerobes (Correct answer)
- Pseudomonas aeruginosa
- Atypical pathogens like Mycoplasma
Correct answer: Beta-lactamase-producing Haemophilus influenzae and anaerobes
Beta-lactamase inhibitors protect the beta-lactam from hydrolysis, restoring activity against organisms that produce beta-lactamase enzymes.
Question 5: A patient is treated with trimethoprim-sulfamethoxazole (TMP-SMX). Which nutrient synthesis pathway does this combination block?
- Peptidoglycan synthesis
- Folate synthesis (Correct answer)
- Ergosterol synthesis
- RNA polymerase activity
Correct answer: Folate synthesis
TMP inhibits dihydrofolate reductase and SMX inhibits dihydropteroate synthase, together blocking sequential steps in folate synthesis.
Question 6: Which term describes the lowest concentration of antibiotic that prevents visible bacterial growth after 18–24 hours of incubation?
- Minimum bactericidal concentration (MBC)
- Minimum inhibitory concentration (MIC) (Correct answer)
- Breakpoint concentration
- Area under the inhibitory curve (AUIC)
Correct answer: Minimum inhibitory concentration (MIC)
The MIC is the lowest antibiotic concentration that inhibits visible bacterial growth under standardized incubation conditions.
Question 7: A patient on warfarin starts metronidazole for C. difficile. What is the expected drug interaction?
- Decreased warfarin levels due to CYP induction
- Increased INR due to CYP2C9 inhibition by metronidazole (Correct answer)
- Decreased metronidazole efficacy
- No clinically significant interaction
Correct answer: Increased INR due to CYP2C9 inhibition by metronidazole
Metronidazole inhibits CYP2C9, reducing warfarin metabolism and increasing INR, raising the risk of bleeding.
Pharmacology Antimicrobial Therapy Principles 3
A patient with a creatinine clearance of 15 mL/min requires antibiotic therapy.
Which agent requires the most significant dose reduction?