A 62-year-old woman with a 2-month history of jaundice and right upper quadrant pain undergoes ultrasound which shows a dilated common bile duct (12 mm) with multiple echogenic foci within it and a normal pancreatic head. The gallbladder is contracted with stones. Liver function tests show ALP 380 IU/L and direct bilirubin 7.5 mg/dL. What is the most appropriate next investigation to confirm the diagnosis and guide management?