An 18-year-old man develops fever, headache, confusion, and generalized seizures. On examination, he is moving all his limbs, but incoherent. A CT scan shows bilateral, small, low-density temporal lobe lesions. CSF shows mononuclear cell pleocytosis, increased protein, and normal glucose. The EEG shows bilateral periodic discharges from the temporal leads and slow-wave complexes at regular intervals of 2-3/sec. Which of the following is the most appropriate next step in management?