Review Question - QID 215055

A 2-day-old boy presents in the neonatal intensive care unit (NICU) with cyanosis and tachypnea. He was born at 38 weeks gestation via spontaneous vaginal delivery to a G1P1 woman who did not receive prenatal care. There were no complications during pregnancy or delivery. He does not have any family history of congenital heart disease. His temperature is 98.6°F (37.0°C), blood pressure is 55/20 mmHg, pulse is 155/min, and respirations are 45/min. Arterial blood gas reveals a partial pressure of oxygen (PaO2) of 45 mmHg on 100% oxygen. Physical examination reveals an ill-appearing neonate with a single second heart sound (S2) and a 3/6 holosystolic murmur best heard in the left lower sternal border. Chest radiograph is notable for decreased pulmonary vasculature markings. ECG is notable for tall, peaked P waves and small R waves in the precordial leads, with left axis deviation. Which of the following is the most likely diagnosis?