Spirometry — Interstitial Lung Disease with Reduced DLCO

A 68-year-old retired construction worker presents with 18 months of progressive dyspnea on exertion and non-productive cough. Examination reveals fine bibasilar velcro crackles and clubbing. HRCT shows subpleural, basal-predominant reticulation with honeycombing and traction bronchiectasis consistent with a definite UIP pattern. Spirometry demonstrates the pattern marked **A** in the diagram, with FVC 58% predicted, FEV1 62% predicted, FEV1/FVC 0.84, TLC 64% predicted, and DLCO 42% predicted. A 6-minute walk test shows exercise-induced desaturation from 96% to 84%. Which of the following best explains why the DLCO is reduced out of proportion to the reduction in lung volumes in the pattern marked **A**?