Q. A 3500 g (7.7 Ib) baby is delivered to a 36-year-old woman. The delivery was uneventful. The patient has a history of bipolar disorder; however, she has been stable for years on lithium. On delivery, the infant has lip cyanosis with a respiratory rate 72/min. Cardiac auscultation reveals fixed splitting of the second heart sound. An additional early diastolic heart sound is heard. Echocardiography shows a persistent opening between the right and left atrium, and the four pulmonary veins are draining into the superior vena cava. Which of the following is the most probable diagnosis?