A female infant is born approximately 10 weeks prematurely (at 30 weeks) and weighs 1710 gm. She has respiratory distress syndrome and is treated with endogenous surfactant. She is intubated endotracheally with mechanical ventilation immediately after bih. Over the first 4 days after bih the ventilator pressure and the fraction of inspired oxygen are reduced. Beginning on the fifth day after bih, she has brief desaturations that become more persistent.She needs increased ventilator and oxygen suppo on the seventh day after bih. She becomes cyanotic. Fuher examination, echocardiogram, and x-rays reveal left atrial enlargement, an enlarged pulmonary aery, increased pulmonary vasculature, and a continuous machine-like murmur. Which of the following is the most likely diagnosis?