A 32-year-old woman presents to the emergency department with a 3-day history of severe headache, high fever (39.8°C), and neck stiffness. She has a vesicular rash on the genitals and a history of recurrent genital herpes. CSF analysis shows lymphocytic pleocytosis (180 cells/μL, 85% lymphocytes), protein 65 mg/dL, and glucose 42 mg/dL (serum glucose 98 mg/dL). PCR for HSV-2 in CSF is positive. She is started on intravenous acyclovir 10 mg/kg every 8 hours. After 48 hours of therapy, her fever persists, and she develops confusion and tremors. Serum creatinine rises from 0.9 to 2.1 mg/dL. What is the most likely cause of her neurological deterioration?