Bacteriology USMLE Step 1 Practice Question

A 32-year-old woman presents to the emergency department with a 2-day history of fever, severe headache, and neck stiffness. She also reports photophobia and mild confusion. Temperature is 39.2°C, heart rate 102 bpm, blood pressure 118/76 mmHg, respiratory rate 18/min, and oxygen saturation 98% on room air. Physical examination confirms neck stiffness and a positive Kernig sign. Lumbar puncture is performed and reveals clear cerebrospinal fluid (CSF) with the following results: - Protein: 120 mg/dL - Glucose: 65 mg/dL (CSF glucose-to-serum glucose ratio 0.6) - White blood cell count: 200 cells/μL with 90% lymphocytes, 10% neutrophils - Gram stain: negative - Bacterial culture: pending - Blood cultures: negative after 48 hours of incubation The patient denies recent travel, antibiotic exposure, or immunosuppression. She is pregnant (8 weeks gestation). Which organism is most likely responsible for her meningitis?