A 3-month-old infant is brought to the clinic for poor feeding and poor weight gain. The parents report that the baby becomes sweaty and tachypneic during feeds and has had several “colds” since birth. On examination, temperature is 36.8°C (98.2°F), pulse is 158/min, respirations are 52/min, and oxygen saturation is 99% on room air. Cardiac auscultation reveals a grade II/VI holosystolic murmur best heard at the left lower sternal border and a low-pitched diastolic rumble at the cardiac apex. The pulmonic component of S2 is loud. There is mild hepatomegaly.

Which of the following best explains the auscultatory findings in this patient?