A 58-year-old man from Delhi presents with progressive jaundice for 3 weeks, pale stools, and dark urine. On examination, he has a palpable gallbladder and no hepatomegaly. Serum bilirubin is 8.2 mg/dL (direct 6.8 mg/dL), ALP 280 IU/L, ALT 95 IU/L. Ultrasound abdomen shows dilated intrahepatic bile ducts, dilated common bile duct (12 mm), and a normal gallbladder with no stones. What is the most appropriate next step in management?