Infectious Diseases
Core concepts
Focus on syndrome-based reasoning: sepsis (source control, early antibiotics, cultures before but not delaying therapy), meningitis (CSF profile interpretation, empiric therapy with vancomycin plus ceftriaxone plus ampicillin when Listeria is a concern), endocarditis (Duke criteria, blood cultures before antibiotics), and TB (latent vs active, IGRA vs tuberculin skin test, four-drug induction). Know the common organisms by site and host, antimicrobial mechanism and toxicity, and the principles of isolation and prophylaxis. HIV: CD4-based opportunistic infection risk and when to start antiretroviral therapy.
Traps
Examiners test the CSF pattern: bacterial (high neutrophils, low glucose, high protein) vs viral (lymphocytes, normal glucose) vs TB/fungal (lymphocytes, very low glucose, very high protein). Remember that antibiotics should not be delayed for lumbar puncture in suspected bacterial meningitis - give dexamethasone and antibiotics first. Watch for the "culture-negative endocarditis" causes (HACEK, Coxiella, Bartonella) and for Clostridioides difficile after antibiotic use.
Worked rule of thumb
Suspected bacterial meningitis: draw blood cultures, give empiric antibiotics plus dexamethasone immediately, then lumbar puncture if safe. Do not delay treatment for imaging unless there are focal deficits, seizures, or immunocompromise.
Key points
- Sepsis: cultures then antibiotics within one hour.
- CSF glucose ratio <0.4 and neutrophilia favor bacterial meningitis.
- Add ampicillin for Listeria in neonates, elderly, and immunocompromised.
- Latent TB treatment prevents reactivation.
- HIV opportunistic infection risk is CD4-driven.