A 3-day-old male neonate born to a 28-year-old primigravida presents with severe cyanosis and respiratory distress. On examination, the infant is deeply cyanosed, with a single loud S2 and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows an 'egg-on-string' appearance with decreased pulmonary vascular markings. Echocardiography confirms anteroposterior transposition of the great arteries with an intact ventricular septum. Which embryological defect in the truncus arteriosus division is responsible for this condition?