A 58-year-old farmer presents with a 6-month history of progressive dyspnea, dry cough, and weight loss. He works daily in a hay barn with known water damage and mold growth. High-resolution CT chest shows centrilobular ground-glass nodules with mosaic attenuation and traction bronchiectasis. Pulmonary function testing reveals the pattern marked **B** in the diagram. Which of the following best explains the physiological abnormality shown in this pattern?
A. Reduced FEV1 and FVC with FEV1/FVC ratio <0.7, indicating concurrent obstructive airway disease from smoking
B. Normal spirometry with reduced DLCO, suggesting early disease without parenchymal involvement
C. Reduced FEV1 with normal FVC and elevated FEV1/FVC ratio (>0.8), indicating small airway obstruction from peribronchiolar inflammation
D. Reduced FVC and TLC with preserved FEV1/FVC ratio (>0.7), accompanied by disproportionately reduced DLCO reflecting alveolar-capillary membrane damage from interstitial fibrosis
Explanation
Why option 1 is correct
The pattern marked B (restrictive pattern with reduced DLCO) is the hallmark of chronic/fibrotic hypersensitivity pneumonitis (CHP/FHP). Per the ATS/JRS/ALAT 2020 guidelines, CHP presents with a restrictive spirometric pattern—reduced FVC and reduced TLC with a preserved or elevated FEV1/FVC ratio (>0.7 or >LLN)—reflecting pulmonary fibrosis from repeated immune-mediated injury. The DLCO is characteristically reduced out of proportion to spirometric changes because the interstitial fibrosis and granulomatous inflammation damage the alveolar-capillary membrane, impairing gas diffusion. This farmer's exposure to thermophilic actinomycetes in moldy hay, combined with his HRCT findings (centrilobular nodules, mosaic attenuation, traction bronchiectasis) and clinical presentation, confirms fibrotic HP with the expected restrictive-plus-reduced-DLCO physiology.
Why each distractor is wrong
Option 2: This describes a pattern of reduced FEV1 with normal FVC and elevated FEV1/FVC ratio, which is inconsistent with a restrictive pattern. While peribronchiolar inflammation in HP can contribute to a mixed pattern, the primary spirometric finding in CHP is restrictive (reduced FVC/TLC), not isolated small airway obstruction.
Option 3: Normal spirometry with reduced DLCO may occur in early or mild interstitial lung disease, but this patient has established fibrotic HP with HRCT evidence of centrilobular nodules, mosaic attenuation, and traction bronchiectasis—findings that mandate a restrictive pattern on spirometry, not normal values.
Option 4: An FEV1/FVC ratio <0.7 indicates an obstructive pattern (COPD, asthma), not the restrictive pattern seen in CHP. Although this farmer may have smoking history, the clinical scenario, imaging, and exposure history point to fibrotic HP, which presents with a restrictive, not obstructive, pattern.
High-YieldNEET PG
In chronic hypersensitivity pneumonitis, the combination of restrictive spirometry (reduced FVC/TLC with preserved FEV1/FVC ratio) + disproportionately reduced DLCO is pathognomonic for interstitial fibrosis; DLCO reduction reflects alveolar-capillary damage and is often the most sensitive early marker.
ATS/JRS/ALAT HP Guideline 2020; INBUILD NEJM 2019
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