A 28-year-old woman in Mumbai presents with fever, headache, myalgia, and neck stiffness for 3 days. CSF shows lymphocytic pleocytosis (120 cells/μL, 90% lymphocytes), normal glucose (55 mg/dL, serum 95 mg/dL), and elevated protein (65 mg/dL). Gram stain and bacterial culture are negative. PCR for enterovirus is pending. What is the most appropriate initial antiviral therapy?