Which histologic feature is typically present in usual interstitial pneumonia but may be absent in nonspecific interstitial pneumonia?

• A. Granulomas
• B. Hyaline membranes
• C. Pleural plaques
• D. Fibroblastic foci

Answer: (D)

Fibroblastic foci are a hallmark of usual interstitial pneumonia because they represent sites of ongoing fibrogenesis at the interface between already fibrotic and relatively preserved lung. In UIP, you see patchy, temporally heterogeneous fibrosis with architectural distortion, and these active foci at the borders help distinguish it from other patterns. Nonspecific interstitial pneumonia, on the other hand, tends to show more uniform interstitial inflammation or fibrosis without these distinct foci, reflecting a more uniform, less active fibrotic process.

Granulomas would point toward granulomatous diseases (like sarcoidosis or hypersensitivity pneumonitis), hyaline membranes indicate acute lung injury or ARDS, and pleural plaques are associated with asbestos exposure—none of these define UIP or NSIP.