**Core Concept**
Eisenmenger syndrome is a severe, irreversible condition resulting from long-standing left-to-right shunt lesions, which eventually leads to pulmonary hypertension and reversal of the shunt direction to right-to-left. This condition is characterized by increased pulmonary vascular resistance, leading to elevated pulmonary artery pressure and subsequent cyanosis.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Eisenmenger syndrome. In this condition, the increased pulmonary vascular resistance is due to remodeling and proliferation of pulmonary arterial smooth muscle cells, leading to obliteration of the small pulmonary arteries. This, in turn, causes a significant increase in pulmonary artery pressure, resulting in the reversal of the shunt direction.

**Why Each Wrong Option is Incorrect**
**Option A:** Cyanosis is a hallmark of Eisenmenger syndrome, resulting from the right-to-left shunt. Therefore, this option is incorrect as it is a characteristic of the syndrome.
**Option B:** Pulmonary hypertension is a key feature of Eisenmenger syndrome, leading to increased pulmonary artery pressure. This option is incorrect as it is a fundamental aspect of the condition.
**Option C:** Clubbing is a common finding in patients with Eisenmenger syndrome, resulting from chronic hypoxia and polycythemia. This option is incorrect as it is a characteristic of the syndrome.

**Clinical Pearl / High-Yield Fact**
Eisenmenger syndrome is often a complication of untreated congenital heart disease, particularly ventricular septal defects (VSD) or atrial septal defects (ASD). Early diagnosis and treatment of these conditions can prevent the development of Eisenmenger syndrome.

**Correct Answer:** D.