A 6-month-old infant with Down syndrome is found to have moderate atrioventricular septal defect (AVSD) on echocardiography. He is asymptomatic at rest but has poor weight gain and mild tachypnea on exertion. His parents wish to defer surgery for 2–3 months. Which drug is the first-line choice for preoperative medical management of his left-to-right shunt?