A 48-year-old man with a history of recurrent biliary colic presents with jaundice (total bilirubin 4.2 mg/dL, direct 3.8 mg/dL), pale stools, and dark urine. Ultrasound shows multiple gallstones, a dilated common bile duct (8 mm), but no stone is visualized in the duct. What is the most appropriate next investigation to confirm choledocholithiasis?