Pulmonary Arterial Hypertension

A 38-year-old woman with progressive exertional dyspnea (NYHA class III), near-syncope on exertion, and a family history of pulmonary arterial hypertension is evaluated with pulmonary function testing. Spirometry shows FEV1 94% predicted, FVC 96% predicted, and FEV1/FVC 0.82 (normal). Lung volumes are normal (TLC 98%, RV 98% predicted). However, DLCO is markedly reduced at 42% predicted with reduced KCO (56% predicted). Right heart catheterization confirms mean PA pressure 52 mmHg, PAWP 9 mmHg, and PVR 9.2 Wood units, consistent with Group 1 pulmonary arterial hypertension. The pulmonary function pattern marked **A** in the diagram—normal FEV1/FVC with isolated DLCO reduction—is characteristic of which underlying pathophysiology in this patient?