Disease-Intrinsic Organising Pneumonia During Biomarker-Confirmed Ulcerative Colitis Remission.

Organising pneumonia is an increasingly recognised pulmonary manifestation of ulcerative colitis, though nearly all reported cases are attributed to drug-induced aetiology. The 2025 ERS/ATS classification of interstitial pneumonias now explicitly lists ulcerative colitis among secondary causes of cicatricial organising pneumonia, formally recognising a disease-intrinsic mechanism for organising pneumonia in ulcerative colitis. We present a 76-year-old woman with 10-year ulcerative colitis pancolitis on stable mesalamine in biochemical remission (faecal calprotectin 62 μg/g, below the 80 μg/g histological activity threshold) who developed progressive dyspnea. Drug-induced aetiology was systematically excluded: mesalamine had been stable for 10 years, discontinuation produced no improvement, and corticosteroids were required for response. Transbronchial cryobiopsy was initially interpreted as fibrotic non-specific interstitial pneumonia; expert pathology consultation revised the diagnosis to organising pneumonia with Masson bodies, changing management from antifibrotics to corticosteroids, illustrating the clinical consequences of cryobiopsy diagnostic discordance. Bronchoalveolar lavage revealed a neutrophil-predominant profile (42% neutrophils, 3% lymphocytes), atypical for cryptogenic organising pneumonia, which typically shows lymphocytic predominance. This pattern may reflect gut-lung axis immune trafficking in secondary organising pneumonia associated with ulcerative colitis. Concurrent seronegative arthritis was reclassified as ulcerative colitis-associated spondyloarthropathy, removing rheumatoid arthritis-associated interstitial lung disease from the differential. This case extends the sole prior report of organising pneumonia during confirmed ulcerative colitis remission by incorporating biomarker-confirmed remission, systematic drug exclusion, cryobiopsy discordance with treatment-altering consequences and contextualisation within the updated 2025 ERS/ATS classification framework.