PEBC MCQ study guide: Infectious Diseases & Antimicrobial Therapy

Providing Care – Clinical Care

Empiric therapy for common infections, antimicrobial stewardship, and key drug interactions.

Urinary Tract Infection

Uncomplicated cystitis (non-pregnant, premenopausal women): first-line options are nitrofurantoin 100 mg BID x5 days, trimethoprim-sulfamethoxazole (TMP-SMX) DS BID x3 days (if local resistance <20%), or fosfomycin 3 g single dose. Nitrofurantoin should be avoided at CrCl <30 mL/min (inadequate urinary concentration) and near term in pregnancy (theoretical neonatal hemolysis risk).

Pyelonephritis requires a broader/systemic-penetrating agent — fluoroquinolones (ciprofloxacin) or TMP-SMX for outpatient management, or IV therapy if admitted. Fluoroquinolones are generally reserved (not first-line for uncomplicated cystitis) due to resistance concerns and black-box warnings (tendon rupture, QT prolongation, CNS effects, aortic aneurysm risk).

Asymptomatic bacteriuria is NOT treated except in pregnancy or prior to an invasive urologic procedure — this is a frequently tested stewardship point.

Community-Acquired Pneumonia

Outpatient, previously healthy: amoxicillin or doxycycline first-line; a macrolide (azithromycin, clarithromycin) alone is now de-emphasized in many jurisdictions due to rising pneumococcal resistance, though still used in atypical-pneumonia-suspected or penicillin-allergic patients.

Outpatient with comorbidities (COPD, diabetes, renal/hepatic disease): amoxicillin-clavulanate or a respiratory fluoroquinolone (levofloxacin, moxifloxacin), OR a beta-lactam plus a macrolide in combination.

Key interaction: macrolides (clarithromycin, erythromycin) are strong/moderate CYP3A4 inhibitors — screen for statins (simvastatin/atorvastatin), warfarin, and other CYP3A4-metabolized drugs whenever a macrolide is dispensed.

Skin & Soft Tissue Infections

Non-purulent cellulitis (no abscess) is typically streptococcal — cephalexin or cloxacillin first-line. Purulent cellulitis/abscess (suspect MRSA) requires incision and drainage as primary therapy, with antibiotics (TMP-SMX, doxycycline, or clindamycin) reserved for systemic symptoms, extensive disease, or immunocompromise.

Antimicrobial Stewardship Principles

Core exam concepts: use the narrowest effective spectrum agent once a pathogen/susceptibility is known (de-escalation); match duration to evidence (many infections now have shorter validated courses than historically used — e.g., 5 days for uncomplicated CAP in a clinically improving patient); avoid treating colonization/contamination (asymptomatic bacteriuria, a single positive blood culture for a common skin contaminant without clinical correlation); and always confirm a true beta-lactam allergy history before defaulting to broader-spectrum alternatives — most patients labelled "penicillin allergic" can in fact tolerate cephalosporins/penicillins after proper risk assessment, and defaulting unnecessarily to fluoroquinolones or vancomycin drives resistance.

High-yield points

Nitrofurantoin: avoid at CrCl <30 mL/min and near term in pregnancy.
Asymptomatic bacteriuria is treated ONLY in pregnancy or before an invasive urologic procedure — otherwise, do not treat.
Macrolides + statin/warfarin = CYP3A4 interaction — always screen.
Purulent abscess = drain it; antibiotics are adjunctive, not primary therapy.
Confirm true penicillin allergy before jumping to broader-spectrum agents — most "allergic" patients tolerate cephalosporins.

Guideline references

Association of Medical Microbiology and Infectious Disease Canada (AMMI Canada)
Do Bugs Need Drugs? Antimicrobial Stewardship Program
Bugs & Drugs (BC Ministry of Health empiric therapy guide)