Cardiovascular Drugs USMLE Step 1 Practice Question

A 26-year-old woman with no significant past medical history presents to the emergency department after developing sudden onset palpitations, lightheadedness, and mild chest tightness while at rest. She denies any prior episodes, illicit drug use, or family history of cardiac disease. Her blood pressure is 118/74 mmHg, heart rate is 178 beats per minute, respiratory rate is 16 breaths per minute, and oxygen saturation is 99% on room air. Physical examination reveals a regular but rapid rhythm without murmurs, and the lungs are clear to auscultation bilaterally. A 12-lead ECG demonstrates a narrow complex tachycardia at 178 bpm without visible P waves, consistent with atrioventricular nodal reentrant tachycardia. A rapid intravenous medication is administered and the rhythm terminates within seconds. The drug acts by increasing potassium efflux and decreasing calcium influx in atrioventricular nodal tissue. Which of the following agents was most likely given?