A 72-year-old man comes to your office complaining of vision change, nausea, vomiting, and diarrhea. His past medical history is significant for gout, hypertension, diabetes, and congestive heart failure. He takes all his prescribed medications regularly. Upon further questioning, he informs you that, he also has a blurry vision and, his vision has a yellow hue to it. An electrocardiogram demonstrates bradycardia, a prolonged PR interval, a shortened QT interval, ST-segment scooping, and T-wave inversion. Of the following, which is the most likely culprit causing distress to this patient?
