Q. A 52-year-old man presents to the emergency department because of chest palpitations. The patient's past medical history is significant for diabetes, hypertension, and ischemic heart disease. The patient's vital signs are normal except for tachycardia. ECG confirms the presence of atrial flutter. The attending physician administers a drug that shifts the black line of the myocardial action potential to the red dashed line (to the right, see image below). Which of the following drugs was likely administered to the patient?