**Eisenmenger Complex: A Complex Congenital Heart Disease**

**Core Concept**
Eisenmenger complex, also known as Eisenmenger syndrome, is a congenital heart disease characterized by the reversal of left-to-right shunt to right-to-left shunt due to pulmonary hypertension. This condition often results from uncorrected congenital heart defects, such as ventricular septal defect (VSD), atrial septal defect (ASD), or patent ductus arteriosus (PDA).

**Why the Correct Answer is Right**
The development of Eisenmenger complex in adults is often a result of the delayed diagnosis and treatment of congenital heart defects. As the patient grows, the left-to-right shunt may lead to increased pulmonary blood flow, resulting in pulmonary hypertension. Eventually, the pulmonary vascular resistance increases to the point where the shunt reverses, leading to right-to-left shunting of blood and cyanosis. This condition is also associated with an increased risk of pulmonary embolism, heart failure, and arrhythmias.

**Why Each Wrong Option is Incorrect**
**Option A:** While tetralogy of Fallot is a congenital heart defect, it is not typically associated with Eisenmenger complex in adults.
**Option B:** Atrial septal defect (ASD) can lead to Eisenmenger complex, but it is less common than VSD or PDA.
**Option C:** Ventricular septal defect (VSD) is the most common congenital heart defect associated with Eisenmenger complex, especially if left untreated.

**Clinical Pearl / High-Yield Fact**
Eisenmenger complex often presents with cyanosis, clubbing, and a characteristic "boot-shaped" cardiac silhouette on chest X-ray. It is essential to consider this condition in patients with unexplained cyanosis or pulmonary hypertension.

**Correct Answer:** D. Ventricular septal defect (VSD)