A 35-year-old woman presents with 2 weeks of jaundice, pruritus, and pale stools. Serum bilirubin is 6.8 mg/dL (predominantly conjugated), ALT 580 U/L, AST 420 U/L, ALP 280 U/L, and GGT 320 U/L. Abdominal ultrasound shows dilated intrahepatic and extrahepatic bile ducts with a 1.5 cm stone in the common bile duct. What is the most appropriate next step in management?