Infectious Disease — USMLE Step 3 Practice
Infectious disease is one of the broadest and highest-yield areas on USMLE Step 3. Topics include sepsis management, HIV/AIDS, opportunistic infections, antimicrobial selection, and emerging pathogens.
Frequently Asked Questions
What is the Hour-1 Bundle for sepsis on USMLE Step 3?
Within 1 hour of sepsis/septic shock recognition: measure lactate, obtain blood cultures (before antibiotics), administer broad-spectrum antibiotics, give 30 mL/kg IV crystalloid for hypotension or lactate >4 mmol/L, apply vasopressors if refractory hypotension (norepinephrine first-line).
What CD4 count triggers which opportunistic infections in HIV?
CD4 <500: oral thrush (Candida), Kaposi sarcoma, cervical cancer. CD4 <200: PCP (Pneumocystis jirovecii pneumonia), toxoplasmosis prophylaxis starts at <100. CD4 <50: MAC (Mycobacterium avium complex), CMV retinitis. TMP-SMX is prophylaxis for PCP and toxoplasmosis.
How do you choose antibiotics for community-acquired pneumonia on Step 3?
Outpatient, no comorbidities: amoxicillin or azithromycin. Outpatient with comorbidities: respiratory fluoroquinolone (levofloxacin, moxifloxacin) OR beta-lactam + macrolide. Inpatient, non-ICU: beta-lactam + macrolide. ICU: beta-lactam + azithromycin OR respiratory fluoroquinolone. Add MRSA coverage (vancomycin) if risk factors present.
What is the treatment for Clostridioides difficile infection?
Non-severe CDI: oral vancomycin 125 mg QID x 10 days (preferred) or fidaxomicin. Metronidazole is no longer first-line. Severe CDI (leukocytosis >15,000, Cr >1.5): oral vancomycin. Fulminant CDI (hypotension, ileus, megacolon): oral vancomycin + IV metronidazole, consider surgical consult. Fecal microbiota transplant for recurrent CDI.
Which vaccines are contraindicated in immunocompromised patients on Step 3?
Live vaccines are contraindicated in immunocompromised patients: MMR, varicella/zoster (Zostavax), yellow fever, intranasal influenza, oral typhoid. Safe alternatives: recombinant zoster vaccine (Shingrix — preferred), inactivated influenza, injectable typhoid. Pneumococcal, Hep B, and meningococcal vaccines are safe and recommended.
Related Articles
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Master infectious disease for USMLE Step 3 — sepsis bundles, HIV opportunistic infections, antimicrobial selection, and vaccine contraindications.
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