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?

Explanation

## Why "Parenchymal restriction due to interstitial lung disease affecting the alveolar-capillary membrane" is right The disproportionate reduction in DLCO (45% predicted) compared to the mild reduction in FVC (65% predicted) is the hallmark of **parenchymal restriction** — specifically interstitial lung disease (ILD). In IPF and other parenchymal ILDs, the alveolar-capillary membrane is damaged (fibrosis, inflammation, thickening), which impairs CO diffusion far more than it reduces lung volumes. This dissociation — reduced DLCO out of proportion to FVC — is the key diagnostic clue that distinguishes parenchymal from extraparenchymal restriction. The clinical presentation (progressive dyspnea, dry cough, age > 60) and restrictive spirometry with preserved FEV1/FVC (0.78) further support ILD as the cause of restriction. ## Why each distractor is wrong - **Extraparenchymal restriction from severe kyphoscoliosis**: In chest wall disease, lung volumes are reduced mechanically, but the alveolar-capillary membrane is structurally normal. DLCO remains **normal or near-normal** because gas exchange is unaffected. The marked reduction in DLCO (45%) rules out this mechanism. - **Neuromuscular weakness causing reduced respiratory muscle strength**: Neuromuscular disorders (myasthenia gravis, ALS, Guillain-Barré) reduce ventilation and lung volumes but do not damage the parenchyma. DLCO is **preserved** in neuromuscular disease. The disproportionate DLCO reduction here is incompatible with pure neuromuscular restriction. - **Pleural effusion restricting lung expansion**: Pleural disease reduces lung volumes by external compression but does not affect the alveolar-capillary interface. DLCO remains **normal** in uncomplicated pleural effusion. The marked DLCO reduction (45%) is not explained by pleural restriction alone. **High-Yield:** In restrictive ventilatory defects, DLCO is the discriminator — **reduced DLCO out of proportion = parenchymal ILD**; **normal DLCO = extraparenchymal** (chest wall, pleural, neuromuscular). [cite: Harrison 21e Ch 290; Pulmonary Function Tests in Restrictive Lung Disease]