30th April 2024

A 45-year-old woman presents with a 5-day history of fever, headache, confusion, and lethargy.

A 45-year-old woman presents with a 5-day history of fever, headache, confusion, and lethargy. She has no significant past medical history. On examination, she is febrile, disoriented, and has a stiff neck. There are no focal neurological deficits.

Which investigation is most likely to confirm the diagnosis?