Review Question - QID 213850

A 72-year-old man presents to the emergency department after he was found by his daughter to be unarousable at home. His daughter says that when she returned from work she found him slumped over at the table and became concerned when he didn't respond to her questions. His past medical history is significant for schizophrenia, depression, and early Alzheimer disease. She does not recall what medications he is currently taking because he manages his own medications. An electrocardiogram is obtained in the emergency department showing the pattern demonstrated in Figure A. The most likely cause of this patient's symptoms has which of the following mechanisms of action?