A 48-year-old man with no significant past medical history presents with painless jaundice noticed by his wife 1 week ago. He also reports dark urine, pale stools, and generalised itching. He denies abdominal pain, fever, or weight loss. Examination confirms jaundice and scratch marks, but no abdominal tenderness or palpable masses. Liver function tests show: Bilirubin 150 µmol/L (conjugated), ALT 80 U/L, AST 70 U/L, ALP 450 U/L, GGT 550 U/L. What is the most appropriate initial imaging investigation?