**Core Concept**
Pruning of pulmonary arteries refers to the reduction or obliteration of the smaller branches of the pulmonary arteries, typically seen in conditions that lead to pulmonary hypertension.

**Why the Correct Answer is Right**
Pruning of pulmonary arteries is a hallmark feature of chronic thromboembolic pulmonary hypertension (CTEPH), which is a condition characterized by the presence of chronic pulmonary emboli that lead to increased pressure in the pulmonary arteries. The repeated obstruction of the pulmonary arteries leads to the development of small, irregularly-shaped branches, giving the appearance of pruning.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not specify a condition associated with pruning of pulmonary arteries. While pulmonary embolism is a condition that affects the pulmonary arteries, it does not typically lead to pruning.
**Option B:** This option is incorrect because while pulmonary hypertension is a condition that affects the pulmonary arteries, it does not specifically cause pruning. Pruning is a more specific feature associated with CTEPH.
**Option C:** This option is incorrect because while chronic obstructive pulmonary disease (COPD) is a condition that affects the lungs, it is not typically associated with pruning of pulmonary arteries.

**Clinical Pearl / High-Yield Fact**
CTEPH is often underdiagnosed and can be mistaken for other forms of pulmonary hypertension. A high index of suspicion and imaging studies such as CT angiography or pulmonary angiography are essential for diagnosis.

**Correct Answer: D. CTEPH. Chronic Thromboembolic Pulmonary Hypertension**