A 6-month-old infant experiencing a "tet spell" is most likely suffering from which congenital heart defect?

The infant experiencing a "tet spell" is most likely suffering from Tetralogy of Fallot. This condition is characterized by four specific heart defects: ventricular septal defect, pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. These anatomical changes interfere with normal blood flow and oxygenation, leading to episodes known as "tet spells," where the infant may exhibit cyanosis, difficulty breathing, and increased irritability.

Tet spells typically occur when the infant experiences a decrease in pulmonary blood flow, which is often triggered by activities such as crying or feeding. The resulting acute oxygen desaturation can lead to the observable symptoms associated with these spells. In contrast, the other congenital heart defects listed do not present with tet spells or similar acute episodes of cyanosis in infants. For example, atrial septal defect and ventricular septal defect may lead to other issues, but they generally do not cause the same acute cyanotic episodes seen in Tetralogy of Fallot. Patent ductus arteriosus may lead to increased pulmonary blood flow and symptoms of heart failure but is also less likely to cause these specific acute cyanotic events.