Viral Infections USMLE Step 1 Practice Question

A 19-year-old previously healthy college student from the Midwest presents to the emergency department in early September with a 3-day history of high fever (39.8°C), severe frontal headache, photophobia, and nuchal rigidity. On examination, he is alert but uncomfortable with neck flexion. Kernig and Brudzinski signs are positive. Laboratory studies show WBC 12,500/μL with 85% lymphocytes. Cerebrospinal fluid analysis reveals: WBC 320/μL (92% lymphocytes), protein 95 mg/dL, glucose 28 mg/dL (serum glucose 118 mg/dL), and negative Gram stain. Bacterial culture, enteroviral PCR, and HSV-1/2 PCR are all negative. Which of the following is the most likely viral etiology?