Why does tetralogy of Fallot cause blue baby syndrome?

Tetralogy of Fallot causes "blue baby" syndrome, or cyanosis, because it forces deoxygenated blood into the body's arterial circulation.12

This condition involves four specific congenital heart defects:
* Ventricular septal defect: A hole in the partition between the lower heart chambers.13
* Pulmonary stenosis: A narrowing of the opening to the pulmonary artery, which restricts blood flow to the lungs.12
* Overriding aorta: The aorta is displaced and positioned over the septal defect, allowing it to receive blood from both the right and left ventricles.32
* Right ventricular hypertrophy: Thickening of the right ventricle muscle due to increased workload.13

Because of the pulmonary stenosis, the right ventricle cannot effectively pump blood to the lungs for oxygenation. Instead, deoxygenated venous blood is shunted through the septal defect into the aorta and the systemic circulation. The presence of this deoxygenated blood in the arteries results in the characteristic blue-gray discoloration of the skin.123