A 47-year-old male presents with painless jaundice. He also reports decreased appetite and energy, along with diffuse itching, pale stools, and dark urine. He denies any weight loss or history of liver disease. His physical exam is notable for jaundice and his labs show cholestasis (total bilirubin 6.3, primarily conjugated, and alkaline phosphatase of 347) and elevated transaminases (AST 160 and ALT 547). A MRCP shows intra-hepatic biliary dilation and retroperitoneal fibrosis without a mass or filling defect
What is the most likely underlying diagnosis?