Usual interstitial pneumonia (UIP) is the histological pattern underlying idiopathic pulmonary fibrosis (IPF). The hallmark histological feature of UIP that distinguishes it from nonspecific interstitial pneumonia (NSIP) is:

• A Diffuse, temporally uniform fibrosis throughout the biopsy with preserved lung architecture
• B Organizing pneumonia pattern with Masson bodies filling alveolar spaces
• C Spatial and temporal heterogeneity — patchy fibrosis with fibroblast foci adjacent to normal-appearing alveoli, and honeycombing predominantly in subpleural/paraseptal distribution ✓
• D Diffuse alveolar damage with hyaline membranes indicating acute phase UIP

Explanation

UIP pattern is defined by spatial and temporal heterogeneity — the key distinguishing features are: (1) patchy subpleural and basal-predominant fibrosis with alternating areas of dense fibrosis, fibroblast foci (areas of active myofibroblast proliferation and young collagen), and adjacent normal-appearing parenchyma (temporal heterogeneity — old fibrosis and new foci coexist); (2) honeycombing (cystic air spaces lined by bronchiolar-type epithelium); and (3) absence of features suggesting alternative diagnoses (granulomas, prominent inflammation, organizing pneumonia). NSIP shows temporally uniform fibrosis (all fibrosis at the same stage) without honeycombing. Masson bodies (organizing pneumonia) are absent in UIP.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.