HRCT of a 48-year-old female bird fancier with recurrent episodes of dyspnoea shows patchy ground-glass opacity, normal lung and areas of decreased attenuation in the same axial slice, with air trapping confirmed on expiratory images. There is relative sparing of the lung bases. This combination indicates:

• A Subacute hypersensitivity pneumonitis ✓
• B Idiopathic pulmonary fibrosis
• C Desquamative interstitial pneumonia
• D Lymphangitic carcinomatosis

Explanation

The three-density sign, coexistence of ground-glass, normal and hypoattenuating lobules on one slice, with expiratory air trapping and mid-zone predominance sparing the bases, is characteristic of subacute hypersensitivity pneumonitis. IPF shows basal subpleural honeycombing without air trapping, DIP occurs in smokers with uniform ground-glass and basal subpleural cysts, and lymphangitic carcinomatosis shows septal thickening with nodules rather than mosaic attenuation.

Reference: Webb, Muller and Naidich: High-Resolution CT of the Lung, 5th ed.

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Written and medically reviewed by the StethoPrep medical team.