Which congenital heart defect is characterized by right-to-left shunting and often presents with "blue spells"?

The correct answer is Tetralogy of Fallot, which is characterized by the combination of four specific heart defects that lead to right-to-left shunting of blood. This occurs because of a ventricular septal defect, pulmonary outflow obstruction (stenosis), right ventricular hypertrophy, and an overriding aorta. The right-to-left shunt allows deoxygenated blood to bypass the lungs and enter the systemic circulation, which is why patients may experience "blue spells," or cyanotic episodes, where they appear bluish due to low oxygen levels in the blood. These spells are typically triggered by physical activity or stress, causing increased right-to-left shunting.

In contrast, the other congenital defects listed have different pathophysiological mechanisms. Transposition of the great vessels involves a complete switch in the circulatory pathways, leading to separate systemic and pulmonary circulations, but it commonly presents with severe cyanosis at birth rather than intermittent blue spells. An atrial septal defect primarily causes left-to-right shunting and is not typically associated with cyanotic episodes. A ventricular septal defect, while it can lead to cyanosis in certain cases, usually results in increased pulmonary blood flow and does not typically cause the "blue spells" seen in Tetralogy