A 48-year-old man with a history of recurrent biliary colic presents with jaundice, pale stools, and dark urine. Liver function tests show elevated conjugated bilirubin (4.2 mg/dL) and alkaline phosphatase (320 U/L). Ultrasound abdomen shows dilated intrahepatic and extrahepatic bile ducts with a 6 mm common bile duct, but no stone is visualized. What is the most appropriate next investigation to diagnose choledocholithiasis?