A 40-year-old man is brought to you by his friends. Apparently, he has ingested some unknown medication in a suicide attempt. The patient is disoriented to time. His temperature is 103 F, BP is 120/85 pulse 100/min and irregular, and respirations are 22/min. The skin is flushed and dry. Dilated pupils and muscle twitching are also noted on examination. ECG reveals prolonged QRS complexes. Hepatic transaminases are normal, and ABG shows a normal pH. These findings are most likely due to intoxication by which substance?