A 2-week-old baby is irritable and feeding poorly. On physical examination, the infant is irritable, diaphoretic, tachypneic and tachycardic. There is circumoral cyanosis, which is not alleted by nasal oxygen. A systolic thrill and holosystolic murmur are heard along the left sternal border. An echocardiogram reveals a hea defect in which the aoa and pulmonary aery form a single vessel that overrides a ventricular septal defect. What is the appropriate diagnosis?