**Core Concept**
Primary pulmonary hypertension (PPH) is a rare and severe disorder characterized by elevated pulmonary artery pressure in the absence of any known cause. The underlying pathophysiological mechanism involves structural and functional changes in the pulmonary vasculature, leading to increased vascular resistance and right ventricular failure.

**Why the Correct Answer is Right**
The basic abnormality in PPH lies in the proliferation of endothelial and smooth muscle cells in the small pulmonary arteries, resulting in thickening of the vessel walls and narrowing of the lumens. This process is often associated with the activation of various growth factors, including transforming growth factor-beta (TGF-β) and platelet-derived growth factor (PDGF), which promote cellular proliferation and fibrosis. The increased vascular resistance and pressure in the pulmonary arteries ultimately lead to right ventricular hypertrophy and failure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specifically address the underlying pathophysiological mechanism of PPH.
**Option B:** This option is incorrect as it refers to a secondary cause of pulmonary hypertension, which is not relevant to primary PPH.
**Option C:** This option is incorrect as it refers to a different cardiovascular disorder, not related to PPH.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that PPH is often associated with a poor prognosis and high mortality rate, making early diagnosis and treatment crucial. The use of vasodilators and pulmonary vasodilators, such as calcium channel blockers and endothelin receptor antagonists, can help manage symptoms and improve quality of life.

**Correct Answer: D. Abnormal proliferation of endothelial and smooth muscle cells in pulmonary arteries.**