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Above. Post-delivery anatomy. The aorta (AO) and pulmonary artery (PA) are transposed, or switched in transposition of the great arteries (D-TGA). The aorta arises from the right ventricle instead of the left ventricle and the pulmonary artery arises from the left ventricle instead of the right ventricle. The coronary arteries arise from the transposed aorta. The AO is to the right of the PA, and the 2 great vessels (PA and AO) are in parallel orientation.
After birth, the systemic blood to the body does not receive oxygen from the lungs and the blood which is oxygenated is pumped from the left side of the body to the lungs. The vital organs including the heart does not receive adequate amounts of oxygen. Therefore, the blood circulates in 2 circuits, and without neonatal intervention. a large atrial septal defect (ASD) or ventricular septal defect (VSD) is necessary for survival.

Image courtesy: The Childrens’ Heart Clinic, PA, and Childrens’ Hospitals and Clinics of Minnesota.