Usual interstitial pneumonia (UIP) is the histological pattern of idiopathic pulmonary fibrosis (IPF). Which histological feature of UIP is most characteristic and essential for diagnosis, distinguishing it from nonspecific interstitial pneumonia (NSIP)?

• A Temporally uniform interstitial fibrosis and inflammation in all areas of the biopsy
• B Diffuse alveolar damage pattern with hyaline membranes
• C Temporal and spatial heterogeneity with fibroblastic foci alternating with areas of normal lung parenchyma ✓
• D Organising pneumonia pattern with Masson bodies in alveolar spaces

Explanation

The key histological hallmark of UIP pattern (and by extension IPF when idiopathic) is temporal and spatial heterogeneity: areas of dense acellular fibrosis with honeycombing (old injury) alternate sharply with relatively preserved normal lung parenchyma and active fibroblastic foci (new injury) in the same biopsy. This 'patchwork' appearance at low power is pathognomonic of UIP and distinguishes it from NSIP, which shows temporally uniform (all areas the same age) fibrosis without fibroblastic foci, predominantly in a cellular or mixed pattern.

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

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