Bosentan is
**Core Concept**
Bosentan is an endothelin receptor antagonist, which means it blocks the action of endothelin-1 (ET-1) on its receptors. ET-1 is a potent vasoconstrictor that plays a key role in pulmonary hypertension by constricting pulmonary arteries.
**Why the Correct Answer is Right**
Bosentan works by competitively inhibiting the binding of ET-1 to its receptors, ET(A) and ET(B). This inhibition leads to vasodilation of pulmonary arteries, reducing pulmonary vascular resistance and pressure. Bosentan is specifically used in the treatment of pulmonary arterial hypertension (PAH).
**Why Each Wrong Option is Incorrect**
**Option A:** Bosentan is not a calcium channel blocker. Calcium channel blockers, such as nifedipine, work by inhibiting the influx of calcium ions into vascular smooth muscle cells, leading to vasodilation.
**Option B:** Bosentan is not a phosphodiesterase-5 (PDE-5) inhibitor. PDE-5 inhibitors, such as sildenafil, work by increasing the levels of cyclic guanosine monophosphate (cGMP) in smooth muscle cells, leading to vasodilation.
**Option C:** Bosentan is not an alpha-blocker. Alpha-blockers, such as prazosin, work by blocking alpha-1 adrenergic receptors, leading to vasodilation.
**Clinical Pearl / High-Yield Fact**
Bosentan is a dual endothelin receptor antagonist, meaning it blocks both ET(A) and ET(B) receptors, providing a broader therapeutic effect in PAH.
**Correct Answer:** C. Bosentan is a dual endothelin receptor antagonist used in the treatment of pulmonary arterial hypertension (PAH).