A 40-year-old man is brought by his friends to you. Apparently, he has ingested some unknown medication. Patient is disoriented. His temperature is 103°F, BP 120/85, pulse 100/min, and respirations 221min. The skin is flushed and dry. Has dilated pupils and muscle twitching. 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 of the following substances?