A 27-year-old man presents to a dermatologist because his skin has become chronically itchy. He has also been experiencing chronic, progressive fatigue. On physical examination, no specific skin lesions are seen, but the patient is noted to be mildly jaundiced. Serum chemistry tests demonstrate elevated serum alkaline phosphatase and bilirubin with minimally increased transaminases. He was fuher referred to a surgeon. Endoscopic retrograde cholangiography demonstrates multiple sho strictures and saccular dilatations involving the intrahepatic and extrahepatic bile ducts. Which of the following is the most likely diagnosis?