**Core Concept**
Pulmonary vasodilation refers to the widening of blood vessels in the lungs, which reduces the resistance to blood flow and improves oxygenation. This process is crucial in conditions such as pulmonary hypertension, where the blood vessels become constricted, leading to increased pressure and strain on the right heart.

**Why the Correct Answer is Right**
Pulmonary vasodilation is primarily mediated by the release of nitric oxide (NO) and prostacyclin (PGI2). Nitric oxide binds to guanylyl cyclase, increasing cyclic guanosine monophosphate (cGMP) levels, which causes smooth muscle relaxation and vasodilation. Prostacyclin also stimulates adenylate cyclase, leading to an increase in cyclic adenosine monophosphate (cAMP), which further relaxes vascular smooth muscle. **Option C**, prostacyclin, is the correct answer because it directly leads to pulmonary vasodilation through the increase of cAMP.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because endothelin-1 is a potent vasoconstrictor that causes pulmonary vasoconstriction, not vasodilation. Endothelin-1 binds to endothelin receptors on vascular smooth muscle cells, leading to an increase in intracellular calcium and contraction.

**Option B:** This option is incorrect because thromboxane A2 is a potent vasoconstrictor that causes platelet aggregation and pulmonary vasoconstriction. Thromboxane A2 binds to thromboxane receptors on platelets and vascular smooth muscle cells, leading to an increase in intracellular calcium and contraction.

**Clinical Pearl / High-Yield Fact**
In conditions like pulmonary hypertension, prostacyclin analogs, such as epoprostenol, are used as a treatment to cause pulmonary vasodilation and reduce pulmonary vascular resistance.

**Correct Answer: C. Prostacyclin**