## **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 involves understanding the components of Tetralogy of Fallot. TOF classically includes VSD, which is a key feature allowing for mixing of blood between the ventricles. The other options provided relate to TOF but need evaluation for accuracy in the context of TOF manifestations and treatments.

## **Why Each Wrong Option is Incorrect**
- **Option A: Left axis deviation** - In TOF, the electrical axis is often deviated to the right due to right ventricular hypertrophy, not left.
- **Option B: Left ventricular hypertrophy** - TOF typically presents with right ventricular hypertrophy due to the increased workload on the right ventricle from pulmonary stenosis, not left ventricular hypertrophy.
- **Option E: Morphine is contraindicated in cyanotic spells** - Morphine can actually be used in the management of cyanotic spells (tet spells) to help reduce the infant's stress and anxiety, which can help decrease the severity of the spell.

## **Clinical Pearl / High-Yield Fact**
A critical clinical pearl is that **Blalock-Taussig shunt**, a palliative surgical procedure for TOF, indeed involves creating a connection between the **subclavian artery** and the **pulmonary artery** to increase blood flow to the lungs, thereby reducing cyanosis.

## **Correct Answer: D.**