The 7-year-old boy now in your office was last seen 2 weeks ago with a mild viral upper respiratory tract infection. Today, however, he presents with fever, ataxia, weakness, headache, and emesis. In the office, he has a 3-minute left-sided tonic-clonic seizure. You send him to the hospital and order a magnetic resonance imaging (MRI) with contrast of the brain, the results of which are shown. This boy's likely diagnosis is which of the following?