MSFinals-4878

A 36-year-old man arrives at the emergency department after taking an unknown substance during a night out. He has a history of depression. Upon examination, his GCS is 13/15, his pupils are dilated and divergent, and he is tachycardic with a heart rate of 110/min. His blood pressure is 124/70 mmHg, and his ECG shows sinus rhythm with a prolonged QTc duration of 480 msec. He appears dehydrated. What substance is most likely responsible for his symptoms?