Q. A 28-year-old male presents to the emergency department with complaints of intermittent heart palpitations for the last 2 days. He states the symptoms begin suddenly and have occurred on three previous occasions. He also reports feeling lightheaded and short of breath. He denies chest pain, nausea, vomiting, or diaphoresis. The patient otherwise has no significant past medical history. He is not taking any medications at home. On physical examination, his blood pressure is 120/80 mm Hg, heart rate 100 beats/min, respiratory rate 18/min, and oxygen saturation is 96 percent on room air. The patient appears uncomfortable but alert. He is tachycardic, though no murmurs, gallops, or rubs are appreciated. His lungs are clear to auscultation. No peripheral edema or cyanosis is detected. An electrocardiogram is performed (see image), which reveals a regular tachycardia with a rate of 100 beats/min. The QRS complex is narrow with a normal axis, and you identify a delta wave. Which of the following medications is most indicated at this time?