A 32-year-old woman from Delhi presents with a 5-day history of severe vesicular rash on the left side of her face following the distribution of the trigeminal nerve. She has fever (38.5°C), severe pain, and photophobia. Fundoscopy reveals corneal involvement. PCR of vesicular fluid confirms HSV-1. She is on day 3 of intravenous acyclovir 10 mg/kg 8-hourly. Despite adequate dosing, her symptoms are worsening and she develops confusion and tremors. Serum creatinine has risen from 0.9 to 2.1 mg/dL over 48 hours. What is the most likely cause of her clinical deterioration?