Sotatercept in Pulmonary Arterial Hypertension

The goal of this clinical trial is to determine whether sotatercept is effective in improving diffusing capacity in patients with pulmonary arterial hypertension. Participants will be asked to: * Take Sotatercept every 21 days (±3 days) * Each participant will be enrolled in the study for 29 Weeks * Visit the clinic 18 times * Have a physical exam * Perform assessments of lung function and exercise tests * Have an ultrasound of their heart * Have blood draws done at regular intervals The main objectives of the study are: Primary objective: To assess whether sotatercept will improve recruitment of diffusing membrane capacity (DM) with exercise. Secondary objective: To identify components of the diffusing capacity that respond to treatment with sotatercept in pulmonary arterial hypertension.