Final answer:
Hypoplastic left heart syndrome requires maintaining an open ductus arteriosus to allow adequate blood mixing and circulation in infants, since the left heart is underdeveloped.
Step-by-step explanation:
The heart condition that necessitates keeping the ductus arteriosus open is Hypoplastic left heart syndrome (HLHS). HLHS is a severe congenital defect where the left side of the heart is underdeveloped. The ductus arteriosus, a fetal artery that directs blood away from nonfunctioning lungs by connecting the pulmonary trunk to the aorta, is a crucial bypass for blood flow in infants with HLHS. This condition requires the ductus arteriosus to remain open post-birth to ensure that oxygen-rich and oxygen-poor blood can mix, providing the body with enough oxygenated blood. Without an open ductus arteriosus, infants with HLHS could not survive as the left side of the heart cannot effectively pump blood.