A 58-year-old woman presented with 9 months of pruritus and was hospitalized 1 month ago after others noticed jaundice. No abdominal pain, normal appetite and bowel movements. Physical exam showed marked jaundice, excoriations, liver 3 cm below the costal margin (firm consistency), splenomegaly (6 cm below costal margin). Lab results: TBil 85.5 μmol/L, CB 46.8 μmol/L, albumin 30g/L, globulin 42g/L, ALT 56U, ALP 820U/L, GGT 94.5U/L, IgG 19.6g/L, IgA 16.9g/L, IgM 8.6g/L. Ultrasound showed hepatosplenomegaly, common bile duct 6mm without intrahepatic duct dilation. The most critical investigation is: