In usual interstitial pneumonia (UIP), the pathological hallmark is the 'temporal heterogeneity' of fibrosis. Which specific histological finding distinguishes UIP from other interstitial pneumonias?

• A Uniform diffuse alveolar wall thickening with hyaline membrane formation
• B Organizing pneumonia plugs in bronchioles and alveoli without architectural distortion
• C Fibroblastic foci at the edge of dense fibrosis, with alternating areas of normal lung ✓
• D Lymphoid follicles with germinal centers throughout the interstitium

Explanation

Usual interstitial pneumonia is characterized by temporal and spatial heterogeneity: dense, established fibrosis (honeycomb lung) with smooth muscle metaplasia alternates with areas of normal or near-normal lung parenchyma. Fibroblastic foci — clusters of actively proliferating fibroblasts at the advancing edge of fibrosis — are the histological marker of active disease within this heterogeneous pattern. This 'patchwork' pattern distinguishes UIP from non-specific interstitial pneumonia (NSIP), which shows uniform diffuse alveolar inflammation/fibrosis temporally, and from organizing pneumonia (OP) which shows intraluminal plugs (Masson bodies) without architectural distortion.

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

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Written and medically reviewed by the StethoPrep medical team.