**Core Concept**
Eisenmenger syndrome is a complex cardiovascular condition resulting from long-standing left-to-right shunting of blood, leading to pulmonary hypertension, right ventricular hypertrophy, and eventual reversal of shunt direction. This syndrome is characterized by increased pulmonary vascular resistance, causing right-to-left shunting of blood.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Eisenmenger syndrome, which involves the development of pulmonary hypertension due to increased resistance in the pulmonary vasculature. This increased resistance is primarily caused by remodeling of the pulmonary arteries, resulting in thickening of the arterial walls and narrowing of the lumens. The reversal of shunt direction is a hallmark of Eisenmenger syndrome, leading to cyanosis and other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might refer to a characteristic of Eisenmenger syndrome, such as cyanosis, clubbing, or right ventricular hypertrophy. However, it is not specified, and Eisenmenger syndrome can indeed present with these features.
**Option B:** This option could be referring to the fact that Eisenmenger syndrome is often associated with congenital heart defects, such as ventricular septal defects or atrial septal defects. However, this is a common cause of Eisenmenger syndrome, so it's not the correct answer.
**Option C:** This option might be related to the fact that Eisenmenger syndrome can be caused by conditions other than congenital heart defects, such as pulmonary vascular disease or chronic lung disease. However, this is not the correct answer as it is not a characteristic of Eisenmenger syndrome.

**Clinical Pearl / High-Yield Fact**
Eisenmenger syndrome is a classic example of a condition where the direction of blood flow through a shunt is reversed due to increased resistance in the pulmonary vasculature. This is a key concept to remember when approaching questions on cardiovascular physiology and pathophysiology.

**Correct Answer: D**