A 35-year-old woman presents with jaundice, fever, and right upper quadrant pain for 3 days. Serum bilirubin is 6.8 mg/dL (conjugated 5.2 mg/dL), ALT 680 U/L, AST 520 U/L, ALP 145 U/L. Abdominal ultrasound shows dilated intrahepatic and extrahepatic bile ducts with a 12 mm stone in the common bile duct. Which investigation is most appropriate to confirm the diagnosis and plan definitive management?