A 3-day-old male neonate born to non-consanguineous parents presents with severe cyanosis and respiratory distress. Physical examination reveals 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 pulmonary oligemia. Echocardiography confirms complete transposition of the great arteries (TGA) with an intact ventricular septum. Which embryological defect during weeks 4–6 of cardiac development best explains this finding?