Endocardial cushion defect
Aortic arch constriction
Abnormal neural crest cell migration
Ans. C. Abnormal neural crest cell migration
This patient is presenting with occasional cyanosis that has been present since birth. Tetralogy of Fallot is the most common cause of cyanotic congenital heart disease, although it does not always present with cyanosis in the immediate newborn period, especially when the pulmonary stenosis component is only mild. Mild pulmonary stenosis allows the ventricular septal defect (VSD) to act mainly as a left-to-right shunt early in childhood, resulting in relatively infrequent cyanotic episodes. A progressive increase in pulmonary hypertension (due to increased right-sided heart flow) eventually leads to shunting reversal with a significant increase in cyanotic symptoms. Squatting improves cyanosis because the resulting increase in systemic pressure reduces the amount of right-to-left shunting through the VSD (decreasing the amount of blood bypassing the lungs). The heart and its inflow and outflow tracts form from a primitive tube-like structure in the developing embryo. This tube undergoes folding and septation under the influence of a variety of factors. One step involves the division of the primitive truncus arteriosus and bulbus cordis into the aorta and pulmonary artery, which is accomplished by the process of aorticopulmonary septation. Here, neural crest cells migrate into the truncal and bulbar ridges of the truncus arteriosus and bulbus cordis, respectively, and grow in a spiraling fashion to separate the aorta and pulmonary artery. Failure of this process is responsible for three of the five cyanotic heart diseases: tetralogy of Fallot, truncus arteriosus, and transposition of the great vessels.