Hepatobiliary Disease USMLE Step 1 Practice Question

A 34-year-old man with 8-year history of ulcerative colitis presents with progressive fatigue and jaundice. Vital signs: BP 118/76, HR 88, RR 16, Temp 37.2°C, SpO2 98% on room air. Laboratory studies reveal elevated alkaline phosphatase (287 U/L), total bilirubin 3.2 mg/dL, and normal transaminases. Abdominal ultrasound shows intrahepatic bile duct dilation. ERCP demonstrates multifocal stricturing of intrahepatic and extrahepatic bile ducts. No fever or pruritus noted. Which antibody is most likely positive?