What exposure history is typical for hypersensitivity pneumonitis and what are its classic HRCT findings?

• A. Exposure to birds, mold, or hot tubs; HRCT shows centrilobular nodules, mosaic attenuation, and air-trapping; BAL lymphocytosis
• B. Exposure to asbestos; HRCT shows pleural plaques
• C. Smoking history with basilar predominant emphysema on HRCT
• D. Bacterial pneumonia pattern on HRCT

Answer: (A)

Hypersensitivity pneumonitis is driven by repeated inhalation of organic antigens, such as birds, mold, or humidifier-related exposures like hot tubs. This immune-triggered reaction primarily affects the small airways and surrounding parenchyma. On high-resolution CT, the classic pattern is small centrilobular nodules accompanied by a mosaic attenuation pattern caused by airway-centered air trapping. Expiratory imaging often makes the air-trapping more evident. BAL typically shows lymphocytosis, supporting an inflammatory, immune-mediated process rather than infection or fibrosis alone. In chronic cases you can see fibrotic changes, but the hallmark features described are most characteristic of the acute/subacute phase. The other exposures and patterns point to different diseases—asbestos with pleural plaques, smoking-associated emphysema, or bacterial pneumonia with focal consolidation.