A 35-year-old woman from rural Uttar Pradesh is brought to the hospital in a comatose state. Her family reports that 8 weeks ago she was scratched by a bat while sleeping in her room. She did not seek medical care at that time. Over the past 3 days, she has developed fever, confusion, hallucinations, and involuntary muscle twitching. On neurological examination, she has signs of brainstem dysfunction with irregular respirations (Cheyne-Stokes pattern), dilated pupils, and generalized hyperreflexia. Brain imaging shows no focal lesions. Lumbar puncture reveals CSF with 120 cells/μL (90% lymphocytes), protein 85 mg/dL, and glucose 65 mg/dL (serum glucose 110 mg/dL). What is the most appropriate next diagnostic step?