CNS Pharmacology USMLE Step 1 Practice Question

A 22-year-old man with no significant past medical history presents to the emergency department with sudden onset palpitations that began 45 minutes ago while he was at rest. He denies any prior episodes, chest pain, syncope, or use of caffeine or illicit substances. His blood pressure is 118/74 mmHg, heart rate is 172 beats per minute, respiratory rate is 16 breaths per minute, and oxygen saturation is 98% on room air. Physical examination reveals a well-appearing, mildly anxious young man with no murmurs on cardiac auscultation. An ECG demonstrates a narrow complex tachycardia without visible P waves, consistent with supraventricular tachycardia. He is given a very short-acting intravenous drug that transiently causes flushing and chest discomfort but promptly restores normal sinus rhythm by increasing potassium efflux and decreasing calcium influx in AV nodal tissue. Which of the following agents was administered?