Review Question - QID 214757

A 10-year-old boy is brought to the emergency department with a 3-hour history of headache, nausea, and slurred speech. His mom says that they were on the first day of a camping trip when he began to complain of these symptoms. He appeared to feel better while hiking during the day but the symptoms returned in the evening and quickly got worse. Of note, the mother states that she and her husband experienced similar symptoms but to a lesser extent. On presentation his temperature is 99°F (37.2°C), blood pressure is 112/71 mmHg, pulse is 112/min, and respirations are 19/min saturating 99% on room air. Intravenous saline is administered, oxygen is started, and labs are drawn. After about 60 minutes he begins to feel significantly better. Which of the following is the most likely underlying etiology of this patient's presentation?