**Core Concept**
Prostacyclin is a potent vasodilator and inhibitor of platelet aggregation, produced by the endothelium. It plays a crucial role in maintaining vascular homeostasis and preventing thrombosis. A prostacyclin agonist is a medication that mimics the effects of prostacyclin.

**Why the Correct Answer is Right**
Iloprost is a synthetic prostacyclin agonist that is used to treat pulmonary arterial hypertension and systemic sclerosis. It is usually given orally, which is a notable characteristic among prostacyclin agonists. Iloprost works by binding to the IP receptor on smooth muscle cells, leading to vasodilation and inhibition of platelet aggregation. This results in improved blood flow and reduced blood pressure in the pulmonary arteries.

**Why Each Wrong Option is Incorrect**
**Option A:** Beraprost is another prostacyclin agonist, but it is typically administered via inhalation or injection, not orally.
**Option B:** Epoprostenol is a natural prostacyclin that is usually given via continuous intravenous infusion, not orally.
**Option C:** Treprostinil is a prostacyclin analog that can be administered orally, but it is not typically used for systemic sclerosis; iloprost is more commonly used for this indication.

**Clinical Pearl / High-Yield Fact**
Prostacyclin agonists are potent vasodilators, but they can also cause significant side effects, including flushing, diarrhea, and headache. Patients taking these medications should be closely monitored for signs of bleeding or thrombocytopenia.

**Correct Answer:** D. Iloprost.