A 22-year-old woman presents with fever, headache, and confusion. She has not been herself for the past 2 days, including not going to work and forgetting events. On examination, she is moving all her limbs, but not oriented to place or time. The remaining examination is normal. A CT scan shows bilateral, small, low-density temporal lobe lesions. Cerebrospinal fluid (CSF) shows mononuclear cell pleocytosis, increased protein, and normal glucose. The electroencephalogram (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 treatment for her condition?