**Core Concept**
Reversal of shunt refers to the reversal of the direction of blood flow in a congenital heart defect, particularly in cases where there is a left-to-right shunt. This concept is crucial in understanding the pathophysiology of certain congenital heart defects and their potential complications.

**Why the Correct Answer is Right**
The reversal of shunt is not possible in the natural history of Eisenmenger syndrome. Eisenmenger syndrome is a condition characterized by a congenital heart defect with a left-to-right shunt, which eventually leads to pulmonary hypertension and reversal of the shunt to a right-to-left direction. This reversal of shunt is a result of the increased pulmonary vascular resistance, which makes it impossible for the blood to flow from the left heart to the right heart. The reversal of shunt in Eisenmenger syndrome leads to cyanosis and decreased oxygenation of the blood.

**Why Each Wrong Option is Incorrect**
* **Option A:** Reversal of shunt is not possible in the natural history of Tetralogy of Fallot. This is incorrect because Tetralogy of Fallot is a cyanotic congenital heart defect that involves a right-to-left shunt, not a left-to-right shunt.
* **Option B:** Reversal of shunt is not possible in the natural history of Ventricular Septal Defect (VSD). This is incorrect because while VSD is a common congenital heart defect with a left-to-right shunt, the reversal of shunt is not a characteristic of its natural history.
* **Option C:** Reversal of shunt is not possible in the natural history of Atrial Septal Defect (ASD). This is incorrect because like VSD, ASD is a congenital heart defect with a left-to-right shunt, and the reversal of shunt is not a characteristic of its natural history.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the reversal of shunt is a significant complication in congenital heart defects with left-to-right shunts, leading to decreased oxygenation of the blood and potential cyanosis.

**Correct Answer:** D. Eisenmenger syndrome