Review Question - QID 216382

A 43-year-old woman is brought to the emergency room by her sister after ingesting an unknown medication. Her sister states that she rushed to the patient’s house after the patient called her saying she took "a handful of pills," but found the patient unresponsive when she arrived. She knows the patient is being treated for fibromyalgia and major depressive disorder but is unable to recall her sister's medication regimen. The patient's temperature is 100.9°F (38.3°C), blood pressure is 92/65 mmHg, pulse is 120/min, and respirations are 15/min. She is deeply somnolent but opens her eyes briefly to sternal rub. Her pupils are dilated, equal, and reactive. Her skin is warm and flushed. Bowel sounds are absent. Her ECG is shown in Figure A. Which of the following is the mechanism of action of the most likely ingested substance?