Which of the following has the greatest impact on the physiology of tetralogy of Fallot?

C. The degree of pulmonary stenosis.

DISCUSSION: The VSD in tetralogy of Fallot is nonrestrictive, and therefore its size does not affect the degree of shunting that can occur. Likewise, an ASD, which may or may not be a component of tetralogy of Fallot, can provide right-to-left shunting at the atrial level but is not the main contributor to the cyanosis of this disease. The degree of right-to-left shunt across the VSD is dictated by the variable compliance of the downstream chambers, and the increased resistance imposed by severe pulmonary stenosis creates greater amounts of right-to-left shunting and, therefore, more intense cyanosis. The position of the aorta in relation to the VSD is not important as long as no subaortic obstruction exists.