Bronchiolitis obliterans (BO), also known as obliterative bronchiolitis or constrictive bronchiolitis, is a form of irreversible obstructive lung disease caused by inflammation and progressive fibrosis of the small airways (bronchioles). Unlike bronchiolitis obliterans organizing pneumonia (BOOP, now termed cryptogenic organizing pneumonia/COP), which primarily involves the alveoli and is typically steroid-responsive, bronchiolitis obliterans specifically targets the bronchiolar epithelium and subepithelial tissues, leading to concentric narrowing and eventual obliteration of the bronchiolar lumen by scar tissue. This distinction is critical for nursing practice because BO is largely irreversible and steroid-resistant, while COP is steroid-responsive -- confusing these entities leads to inappropriate treatment expectations. The pathogenesis of BO involves an initial insult to the bronchiolar epithelium that triggers an exaggerated reparative response. The inciting event causes epithelial cell injury and denudation, exposing the basement membrane and activating an inflammatory cascade. Neutrophils, lymphocytes, and macrophages infiltrate the peribronchiolar tissues, releasing pro-inflammatory cytokines (IL-1beta, IL-6, IL-8, TNF-alpha) and pro-fibrotic mediators (TGF-beta, PDGF). Fibroblasts are recruited to the damaged bronchiolar wall, where they proliferate and differentiate into myofibroblasts -- contractile cells that produce excessive extracellular matrix...