A 72-year-old woman is admitted to the hospital with acute onset confusion, disorientation, and visual hallucinations over the past 12 hours. She was previously independent and cognitively intact. On examination, she is agitated, her attention is poor, and she has fluctuating consciousness. Temperature is 38.5°C, heart rate 110/min. Urine dipstick shows leukocyte esterase and nitrites. Which investigation is most appropriate to establish the diagnosis?