A 22-year-old male is admitted with fever, headache, photophobia, and neck stiffness. Cerebrospinal fluid (CSF) analysis reveals opening pressure 280 mm H2O, WBC count 2,400 cells/mm^3 with 92% neutrophils, glucose 22 mg/dL (serum glucose 110 mg/dL, ratio 0.20), and protein 210 mg/dL. What is the most likely etiology?

Answer and explanation

Correct answer: B. Acute bacterial meningitis (S. pneumoniae or N. meningitidis)

  1. Step 1: CSF findings showing markedly elevated opening pressure, marked neutrophilic pleocytosis (> 1,000 WBCs), elevated protein (> 100 mg/dL), and low glucose (CSF/serum ratio < 0.4) are classic for acute bacterial meningitis.
  2. Step 2: Streptococcus pneumoniae and Neisseria meningitidis are the leading etiologies in young adults.
  3. Step 3: Viral meningitis typically demonstrates lymphocytic predominance with normal CSF glucose levels.
Answer
Acute bacterial meningitis (S. pneumoniae or N. meningitidis).
Why not the others: 'Enteroviral aseptic meningitis' demonstrates lymphocytic predominance with normal glucose and moderately elevated protein; 'Herpes simplex virus encephalitis' presents with temporal lobe involvement, lymphocytic pleocytosis, and RBCs in CSF; 'Tuberculous meningitis' shows lymphocytic predominance with marked protein elevation and very low glucose.

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