A 35-year-old non-smoker woman presents with progressive dyspnea on exertion and dry cough for 8 months. She works as a bird breeder. HRCT of the chest shows bilateral ground-glass opacities predominantly in the upper and middle lobes with mosaic attenuation. BAL fluid shows lymphocytosis with a CD4:CD8 ratio of 0.5. What is the MOST likely diagnosis?
- A Hypersensitivity pneumonitis (extrinsic allergic alveolitis) ✓
- B Idiopathic pulmonary fibrosis
- C Sarcoidosis
- D Pulmonary alveolar proteinosis
Explanation
Hypersensitivity pneumonitis (HP) results from repeated inhalation of antigenic organic dusts; bird breeder's lung (avian antigens) is a classic exposure. HRCT shows diffuse ground-glass opacities with mosaic attenuation (indicating small-airways disease) and upper/mid-lobe predominance. BAL lymphocytosis with a low CD4:CD8 ratio (< 1) is hallmark of HP, in contrast to sarcoidosis where the ratio is elevated (> 3.5). IPF shows basal-predominant honeycombing on HRCT with a usual interstitial pneumonia pattern and does not produce BAL lymphocytosis. Alveolar proteinosis shows a characteristic 'crazy paving' pattern and PAS-positive material on BAL.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.