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**?

Question

Accepted Answer

D. Destruction of the alveolar-capillary membrane and obliteration of the pulmonary capillary bed. ## Why "Destruction of the alveolar-capillary membrane and obliteration of the pulmonary capillary bed" is right

In the restrictive pattern marked **A** (characteristic of idiopathic pulmonary fibrosis), the FEV1/FVC ratio is preserved or elevated (0.84 in this case) because both the numerator and denominator fall proportionally—airway caliber is preserved. However, DLCO is reduced out of proportion to lung volumes (42% predicted despite TLC 64% predicted) because the pathologic hallmark of IPF is destruction of the alveolar-capillary interface and obliteration of the pulmonary capillary bed by fibrosis. This disproportionate DLCO reduction is a cardinal feature of interstitial lung disease and correlates with severity of fibrosis and pulmonary vascular involvement. Per Harrison 21e Ch 287 and ATS/ERS IPF Guidelines 2022, DLCO <40% predicted suggests advanced disease and possible secondary pulmonary hypertension.

## Why each distractor is wrong

- **Airway obstruction reducing ventilation to distal alveoli**: This describes obstructive physiology (pattern B), not restrictive. In pattern A, airway caliber is preserved; the problem is parenchymal stiffness and capillary destruction, not obstruction.
- **Increased total lung capacity due to air trapping in emphysematous spaces**: This describes obstructive pattern (B) with TLC >120% predicted. Pattern A has TLC <80% predicted, reflecting restrictive mechanics, not air trapping.
- **Preserved airway caliber with normal gas diffusion capacity**: While airway caliber is indeed preserved in pattern A, gas diffusion capacity is NOT normal—DLCO is markedly reduced (42% predicted), which is the key finding that distinguishes IPF from extrapulmonary causes of restriction.

**High-Yield:** In restrictive ILD, DLCO is reduced out of proportion to TLC reduction because fibrosis destroys the alveolar-capillary membrane; DLCO <40% predicts advanced disease and mortality risk.

[cite: Harrison 21e Ch 287; ATS/ERS IPF Guidelines 2022; Raghu Am J Respir Crit Care Med 2022]

Answer

A. Airway obstruction reducing ventilation to distal alveoli

Answer

B. Increased total lung capacity due to air trapping in emphysematous spaces

Answer

C. Preserved airway caliber with normal gas diffusion capacity

Explanation

Why "Destruction of the alveolar-capillary membrane and obliteration of the pulmonary capillary bed" is right

Why each distractor is wrong

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