Question

A 24-year-old man comes to the emergency department due to sudden-onset severe dyspnea, palpitations, and lightheadedness that began while playing basketball. He reports no chest pain but feels as though he might pass out. His medical history is significant for a heart murmur identified during adolescence, though he was lost to follow-up. Vital signs are temperature $37.0^\circ\text{C}$ ($98.6^\circ\text{F}$), blood pressure $92/60\text{ mmHg}$, pulse $132\text{/min}$ and regular, and respirations $24\text{/min}$. Pulse oximetry is $91\%$ on room air. Physical examination reveals a harsh, crescendo-decrescendo systolic murmur at the left lower sternal border; the murmur's intensity increases when the patient transitions from a squatting to a standing position. Pulmonary auscultation reveals fine crackles at both lung bases. An electrocardiogram demonstrates sinus tachycardia with marked left ventricular hypertrophy and deep, narrow Q waves in the lateral leads. Which of the following is the most appropriate initial pharmacologic intervention?