Which of the following is NOT one of the four anatomical defects that constitute Tetralogy of Fallot?

Explanation

## The Four Defects of Tetralogy of Fallot **Key Point:** Tetralogy of Fallot (TOF) is defined by four anatomical abnormalities, which can be remembered by the mnemonic **PROVE**: **Mnemonic:** PROVE — **P**ulmonary stenosis, **R**ight ventricular hypertrophy, **O**verride of aorta, **V**entricular septal defect, **E**ndocardial cushion defect (though the classic four are the first four). Alternatively, the classic four defects are: | Defect | Description | |--------|-------------| | **Ventricular Septal Defect (VSD)** | Hole in the interventricular septum; allows right-to-left shunting | | **Right Ventricular Outflow Tract Obstruction** | Usually pulmonary stenosis; increases RV pressure | | **Overriding Aorta** | Aorta positioned over the VSD rather than solely over the left ventricle | | **Right Ventricular Hypertrophy** | Secondary to chronic pressure overload from RVOT obstruction | **High-Yield:** Atrial septal defect (ASD) is a separate congenital heart lesion and is NOT part of TOF. ASD causes left-to-right shunting at the atrial level, whereas TOF is characterized by right-to-left shunting at the ventricular level. **Clinical Pearl:** The severity of pulmonary stenosis determines the degree of right-to-left shunting and thus the cyanosis. Mild stenosis → acyanotic TOF; severe stenosis → cyanotic TOF. ## Why Atrial Septal Defect Is Wrong ASD is a separate lesion affecting the atrial septum and is not one of the defining features of TOF. TOF is specifically a ventricular-level lesion.