A 27-year-old man is brought to the emergency department for fever, headache, and progressive confusion over 24 hours. His partner reports he has been increasingly irritable and had a focal seizure en route to the hospital. Temperature is 39.2°C (102.6°F). Neurologic exam shows disorientation and expressive aphasia. A lumbar puncture is performed.

CSF analysis shows:

• WBC: 180/µL (predominantly lymphocytes)
• RBC: 90/µL
• Glucose: 65 mg/dL (serum glucose 100 mg/dL)
• Protein: 110 mg/dL

MRI of the brain demonstrates hyperintense lesions in the medial temporal lobe on T2-weighted imaging. Which of the following is the most appropriate next step to confirm the diagnosis and initiate targeted therapy?