MUC5AC and neoplasm: Other contributors include neoplastic transformation of the cavity wall, protease- or mucin-mediated destruction of the alveolar septa, and a bronchiolar “check-valve” effect, where tumor infiltration narrows terminal bronchioles, permitting air entry on inspiration but impeding its exit on expiration, leading to progressive air trapping and cystic degeneration of the mass [8–10].