An 18-year-old college freshman is transported to the emergency department following a call to EMS by his girlfriend, who found him unresponsive in his room. She reports no history of mental illness, no changes in mood, no suicidal ideation, and no hallucinations or other bizarre behavior. He doesn't use drugs, doesn't smoke, and rarely drinks. She says "I don't know what happened; he was out with his new fraternity brothers." What is the most likely diagnosis?