## **Core Concept**
Tetralogy of Fallot (TOF) is a congenital heart defect characterized by four main features: **ventricular septal defect (VSD)**, **pulmonary stenosis**, **right ventricular hypertrophy**, and an **overriding aorta**. This condition leads to cyanosis due to decreased blood flow to the lungs.

## **Why the Correct Answer is Right**
The correct answer is related to the characteristic features and clinical presentation of Tetralogy of Fallot. TOF is known for its four main components. Understanding these components and their implications on the patient's physiology is crucial.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Typically, TOF involves a VSD, which is a defect in the septum between the heart's two lower chambers. This is a correct association with TOF.
- **Option B:** Pulmonary stenosis, which involves narrowing of the pulmonary valve, is another component. This statement is also true for TOF.
- **Option C:** Right ventricular hypertrophy occurs due to the increased workload on the right ventricle. This too is a characteristic feature.
- **Option D:** The overriding aorta is a hallmark, where the aorta lies on top of the VSD and receives blood from both ventricles.

## **Clinical Pearl / High-Yield Fact**
A key clinical feature of TOF is **cyanotic spells** or "tet spells," episodes of severe cyanosis and distress. These are often precipitated by decreased systemic vascular resistance or increased right-to-left shunting.

## **Correct Answer:** D.