A 42-year-old man presents with several months of pruritus followed by yellowing of the eyes. He has ulcerative colitis, an alkaline phosphatase of 680 U/L (30–110) and an alanine aminotransferase of 96 U/L (5–40); bilirubin is 88 micromol/L (3–20). Magnetic resonance cholangiography shows multiple short narrowings in the intrahepatic and extrahepatic ducts, with normal-appearing segments between them. Which one of the following is the most likely diagnosis?