Spirometry — Restrictive Pattern (IPF)

A 68-year-old man with a 2-year history of progressive exertional dyspnea and dry cough presents for pulmonary function testing. Spirometry shows FVC 65% predicted, FEV1 70% predicted, and FEV1/FVC ratio 0.78 (normal). Body plethysmography confirms TLC 72% predicted. The diffusing capacity for carbon monoxide (DLCO) is 45% predicted. The parameter marked **D** in the diagram — reduced DLCO out of proportion to the reduction in FVC — is most consistent with which underlying pathophysiology?