**Core Concept**
Pruning of pulmonary arteries refers to the narrowing or obliteration of small pulmonary arteries, typically seen in conditions with high pulmonary vascular resistance. This phenomenon is often observed in patients with chronic hypoxia, leading to pulmonary hypertension and subsequent vascular remodeling.

**Why the Correct Answer is Right**
The correct answer is associated with a condition characterized by chronic hypoxia and pulmonary vascular remodeling. In this condition, the high pulmonary vascular resistance leads to vascular remodeling, including the narrowing or obliteration of small pulmonary arteries, known as pruning. This process is mediated by the release of vasoconstrictive substances, such as endothelin-1, and the proliferation of vascular smooth muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because pruning of pulmonary arteries is not typically seen in conditions with low pulmonary vascular resistance, such as pulmonary veno-occlusive disease.
**Option B:** This option is incorrect because pruning of pulmonary arteries is not a characteristic feature of pulmonary embolism, which primarily involves the obstruction of larger pulmonary arteries.
**Option C:** This option is incorrect because pruning of pulmonary arteries is not typically seen in conditions with normal pulmonary vascular resistance, such as primary pulmonary hypertension without chronic hypoxia.

**Clinical Pearl / High-Yield Fact**
The pruning of pulmonary arteries is a hallmark of chronic hypoxia-induced pulmonary vascular remodeling, which can lead to right ventricular failure and increased morbidity and mortality in affected patients.

**Correct Answer:** D. Chronic Hypoxia