'Onion skin' fibrosis of bile duct is seen in

MORPHOLOGY The characteristic features of PSC are different in the extrahepatic and large intrahepatic ducts than in the smaller ducts. The largest ducts have chronic inflammation with superimposed acute inflammation, very similar to the mucosal lesions of ulcerative colitis. These inflamed areas lead to narrowing of the larger ducts either because edema and inflammation narrows the lumen or because of subsequent scarring. The smaller ducts, however, often have little in the way of inflammation and show a striking circumferential fibrosis often referred to as onion skinning around an increasingly atrophic duct lumen Eventually the lumen disappears altogether, leaving just a dense button of scar tissue, the viually diagnostic tombstone scar. Because the likelihood of sampling such smaller duct lesions on a random needle biopsy is minuscule, diagnosis depends not on biopsy but on radiologic imaging of the extrahepatic and largest intrahepatic ducts. TEXT BOOK OF ROBBINS BASIC PATHOLOGY NINTH EDOTION PAGE.628 IMAGE REF: ROBBINS BASIC PATHOLOGY NINTH EDITION FIG 15-24 PAGE 629