**Core Concept**
Hypoxic pulmonary vasoconstriction (HPV) is a reflexive response to alveolar hypoxia, where pulmonary arteries constrict in response to low oxygen levels, shunting blood away from poorly ventilated areas of the lung to better-ventilated regions.

**Why the Correct Answer is Right**
HPV is primarily mediated by the release of endothelium-derived contracting factors, such as prostaglandin H2 (PGH2), thromboxane A2 (TXA2), and endothelin-1 (ET-1). These factors stimulate the contraction of pulmonary arteries, leading to increased vascular resistance and reduced blood flow to alveoli with low oxygen levels. The mechanism is thought to be an adaptive response to optimize gas exchange in the lungs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as HPV is not primarily mediated by the sympathetic nervous system, although sympathetic activation can contribute to increased pulmonary vascular resistance.
* **Option B:** This option is incorrect as HPV is not a result of decreased pulmonary blood flow, but rather a response to low oxygen levels, which leads to vasoconstriction and reduced blood flow.
* **Option C:** This option is incorrect as HPV is not a result of increased pulmonary blood flow, but rather a response to low oxygen levels, which leads to vasoconstriction and reduced blood flow.

**Clinical Pearl / High-Yield Fact**
HPV is an important mechanism for optimizing gas exchange in the lungs and is impaired in conditions such as pulmonary hypertension and acute respiratory distress syndrome (ARDS).

**Correct Answer: D. Endothelin-1 (ET-1)**