Which of the following medications is used in treating idiopathic pulmonary arterial hypertension?

Sildenafil is used in treating idiopathic pulmonary arterial hypertension (PAH) due to its mechanism as a phosphodiesterase-5 (PDE-5) inhibitor. By inhibiting PDE-5, sildenafil increases the levels of cyclic guanosine monophosphate (cGMP) within smooth muscle cells, leading to vasodilation of pulmonary and systemic arterial vascular beds. This effect reduces pulmonary vascular resistance and improves blood flow in patients with PAH, thereby alleviating symptoms and improving exercise capacity.

In contrast, other medications listed, such as levosimendan, propranolol, and phenylephrine, do not target the pathological mechanisms specific to PAH. Levosimendan is mainly used as a inotropic agent in heart failure, propranolol is a non-selective beta-blocker often used for hypertension and anxiety, and phenylephrine is a selective alpha-1 agonist used primarily as a decongestant or to raise blood pressure. None of these drugs have proven efficacy in the treatment of idiopathic pulmonary arterial hypertension, thus highlighting the specificity of sildenafil's action in this context.