A 5-year-old boy is brought to the emergency department by his parents due to extreme lethargy, confusion, and difficulty walking. The parents report that he had a febrile upper respiratory tract infection two weeks ago, which resolved without complications. Over the past two days, he became increasingly irritable, complained of a headache, and developed nausea. Today, he was unable to stand on his own and seemed disoriented, failing to recognize his favorite toys. He has no significant past medical history and is up to date on his childhood vaccinations. He takes no medications and has no known allergies. The family history is unremarkable for neurological or autoimmune diseases.

The patient is a somnolent, acutely ill-appearing child who is difficult to arouse but withdraws from painful stimuli. He is febrile with a temperature of 101.3 °F. His blood pressure is 98/60 mmHg with a regular heart rate of 115 bpm and a respiratory rate of 22 breaths per minute.