A 23-year-old man is brought to the emergency department for 2 days of fever, worsening headache, and confusion. His roommate reports that he has been acting “strangely,” speaking incoherently, and had a generalized tonic-clonic seizure at home. On exam, he is febrile and disoriented with brisk reflexes. Lumbar puncture shows: opening pressure normal, protein normal, RBCs elevated, WBCs elevated with 90% lymphocytes. Which of the following is the most appropriate next step to confirm the diagnosis?