Step 1 topics → Neurology and Special Senses → Meningitis (Bacterial, Viral, Fungal)

Meningitis (Bacterial, Viral, Fungal)

USMLE Step 1 trap: Attributes petechial rash in meningitis to S. pneumoniae rather than N. meningitidis. A petechial or purpuric rash in the setting of meningitis is pathognomonic for N. meningitidis, not S. pneumoniae.

Meningitis is one of the highest-yield neurology topics on USMLE Step 1, and the exam tests it from multiple angles simultaneously — you need to know the right organism for the right patient, interpret a CSF profile correctly, and select empiric therapy without missing a coverage gap. The question stem will usually give you a clinical vignette (age, immune status, exposure, rash, travel history) and ask you to identify the organism, explain the pathophysiology, or choose antibiotics. It's not a topic where memorizing a list gets you through — you need to reason through the data the stem gives you.

The trickiest parts are the overlapping CSF profiles and the organism-by-age matrix. Students consistently conflate bacterial and viral findings, especially forgetting that glucose is normal in viral meningitis. TB and fungal meningitis are the silent traps — they look lymphocytic like viral but have very low glucose, which should immediately shift your thinking. USMLE Step 1 loves to put a vignette with a lymphocytic CSF and low glucose expecting you to think 'viral' when the answer is Cryptococcus or TB.

The other major trap is the petechial rash: many students reflexively write 'S. pneumoniae' for any serious-looking meningitis case, but a petechial or purpuric rash is specifically N. meningitidis territory — it reflects endotoxin-driven disseminated intravascular coagulation and meningococcemia. Get the organism-rash-CSF-treatment triad locked down for each major pathogen and you'll handle whatever angle Step 1 comes at you from.

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Common misconceptions

Common mistake
Wrong: S. pneumoniae is the cause of meningitis when a petechial or purpuric rash is present.
Right: A petechial or purpuric rash in the setting of meningitis is pathognomonic for N. meningitidis, not S. pneumoniae.
A petechial or purpuric rash in a meningitis patient is the fingerprint of N. meningitidis, not S. pneumoniae. Meningococcus releases endotoxin (LPS) that triggers a systemic inflammatory cascade leading to DIC and destruction of small vessels — that's the rash. S. pneumoniae is still the most common cause of bacterial meningitis in adults overall, but it does not cause this rash. When the stem shows you a college student or young adult with meningeal signs plus a non-blanching petechial rash, the answer is N. meningitidis every time.
Common mistake
Wrong: Viral meningitis causes low CSF glucose like bacterial meningitis.
Right: Viral meningitis typically shows normal CSF glucose, whereas bacterial meningitis causes low glucose (<45 mg/dL or CSF:serum ratio <0.6).
Normal CSF glucose is one of the most reliable distinguishing features of viral (aseptic) meningitis. Bacteria consume glucose metabolically, and their inflammatory products damage the blood-brain barrier's glucose transport — that's why bacterial meningitis drives glucose low. Viruses do not consume CSF glucose in the same way, so CSF glucose stays normal. If you see a lymphocytic pleocytosis with normal glucose and mildly elevated protein, think viral. Low glucose with lymphocytes should make you think TB or fungal — that's the key shift.
Common mistake
Wrong: Empiric bacterial meningitis treatment in adults requires only a third-generation cephalosporin.
Right: Empiric treatment for bacterial meningitis in adults includes vancomycin plus a third-generation cephalosporin (and dexamethasone); ampicillin is added for patients >50 years or immunocompromised to cover Listeria.
Vancomycin plus a third-generation cephalosporin (ceftriaxone or cefotaxime) covers the major adult pathogens — S. pneumoniae and N. meningitidis — but cephalosporins have no activity against Listeria monocytogenes. Listeria becomes a real threat in adults over 50, pregnant women, and immunocompromised patients, so ampicillin must be added for these groups. Forgetting ampicillin in a question about empiric meningitis therapy in an elderly or immunocompromised patient is a classic Step 1 trap. Also remember dexamethasone given before or with the first antibiotic dose reduces inflammation and improves outcomes, particularly for pneumococcal meningitis.
Common mistake
Gap: Unaware that TB and fungal meningitis produce a lymphocytic CSF profile with very low glucose, mimicking but distinct from viral meningitis
TB and fungal meningitis both show lymphocytic pleocytosis, markedly elevated protein, and very low glucose on CSF analysis, distinguishing them from typical viral meningitis.
TB meningitis and fungal meningitis (especially Cryptococcus neoformans in immunocompromised patients) both produce a lymphocytic pleocytosis with markedly elevated protein and very low CSF glucose — a profile that looks nothing like typical viral meningitis, which has normal glucose. The low glucose in these infections reflects both organism metabolism and blood-brain barrier dysfunction from chronic granulomatous inflammation. Clinically, these present more subacutely than bacterial meningitis. On the exam, look for an HIV patient or someone on immunosuppression with a lymphocytic CSF and low glucose — that's your cue to think Cryptococcus (confirmed by India ink or cryptococcal antigen) or TB, not viral.
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What the exam tests

  1. Given a patient's age and immune status (neonate, infant, college student, adult >50, immunocompromised), identify the most likely causative organism(s) of bacterial meningitis.
  2. Interpret a CSF analysis — cell count and differential, glucose, protein — and correctly classify the meningitis as bacterial, viral, fungal/TB, or another etiology.
  3. Select appropriate empiric antibiotic therapy for bacterial meningitis based on patient age and host factors, including knowing when to add ampicillin for Listeria coverage and when to add dexamethasone.
  4. Identify the clinical and laboratory features that distinguish N. meningitidis from other causes of meningitis, including the significance of a petechial or purpuric rash.
  5. Recognize when to add antifungal therapy (amphotericin B) or TB coverage based on CSF findings and patient risk factors.

Can you avoid these mistakes?

A 68-year-old diabetic man presents with fever, headache, and neck stiffness. CSF shows WBC 1200 cells/µL (90% neutrophils), glucose 28 mg/dL (serum glucose 90 mg/dL), and protein 280 mg/dL. What empiric antibiotic regimen should you start, and why does it differ from empiric therapy for a healthy 25-year-old?
A 19-year-old college student in a dormitory presents with sudden fever, severe headache, photophobia, and a non-blanching purpuric rash on his legs. Which organism is most likely responsible, and what is the mechanism of the rash?
You receive CSF results from two different patients: Patient A has lymphocytes 120 cells/µL, glucose 48 mg/dL (serum 90), protein 65 mg/dL. Patient B has lymphocytes 95 cells/µL, glucose 22 mg/dL (serum 88), protein 190 mg/dL. Both are afebrile at 99°F. How do you differentiate the likely etiology of each, and what additional test would you order for Patient B if they are HIV-positive?
A 2-week-old neonate presents with poor feeding, temperature instability, and bulging fontanelle. What are the three most likely organisms causing meningitis at this age, and what antibiotic regimen covers all three?

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