The most common cause of Blue baby
Tetralogy of Fallot—a Right-to-Left Shunt:
Tetralogy of Fallot is shown in Figure 23-5;
It is the most common cause of “blue baby”. Most of the blood bypasses the lungs, so the aortic blood is mainly unoxygenated venous blood. In this condition, four abnormalities of the heart occur simultaneously:

The aorta originates from the right ventricle rather than the left, or it overrides a hole in the septum, as shown in Figure 23-5, receiving blood from both ventricles.
The pulmonary artery is stenosed, so much lower than normal amounts of blood pass from the right ventricle into the lungs; instead, most of the blood passes directly into the aorta, thus bypassing the lungs.
Blood from the left ventricle flows either through a ventricular septal hole into the right ventricle and then into the aorta or directly into the aorta that overrides this hole.
Because the right side of the heart must pump large quantities of blood against the high pressure in the aorta, its musculature is highly developed, causing an enlarged right ventricle.
Tetralogy of fallot
Cardiac Ischemia
VSD
None of the above