PDE5A and idiopathic pulmonary arterial hypertension: Age 18–75 years with idiopathic PAH (IPAH), familial PAH (FPAH), or PAH associated with connective tissue disease, congenital heart disease, or HIV, Stable background therapy (ERA, PDE-5 inhibitor, or both) for ≥90 days (stable dose for ≥30 days), Baseline 6MWD: 150–425 m, Hemodynamic confirmation of PAH (mPAP >25 mmHg, PCWP ≤15 mmHg, PVR > 3 Wood units).