A 38-year-old woman with a history of recurrent biliary colic presents with jaundice, pale stools, and dark urine. Laboratory tests show elevated direct bilirubin and alkaline phosphatase. Abdominal ultrasound confirms dilated intrahepatic bile ducts and a dilated common bile duct (8 mm) but no gallstones are visualized. Which investigation is most appropriate to identify the cause of biliary obstruction?