Viral Infections USMLE Step 1 Practice Question

A 41-year-old woman with multiple sclerosis on natalizumab presents with progressive visual deficits, clumsiness, and aphasia over 4 weeks. Vital signs: BP 128/82, HR 88, RR 16, Temp 37.2°C, SpO2 98%. MRI reveals multiple nonenhancing white matter lesions without mass effect. CSF analysis shows lymphocytic pleocytosis (45 cells/μL, 90% lymphocytes), negative bacterial cultures, and undetectable glucose. No fever or meningismus noted. Which organism is most likely responsible?