Extrahepatic bile duct obstruction would most likely result in an increase in which bilirubin fraction?

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Direct (conjugated) bilirubin is the fraction that rises with extrahepatic bile duct obstruction. When the bile duct is blocked, conjugated bilirubin—already processed by the liver and normally secreted into bile—cannot reach the intestine. It backs up into the bloodstream, so the direct bilirubin level increases. Unconjugated (indirect) bilirubin remains unchanged or may even appear relatively lower because the liver’s uptake and conjugation processes are intact upstream. Because bilirubin isn’t delivered to the gut, less bilirubin reaches the intestinal bacteria to be converted into urobilinogen, so urobilinogen decreases rather than increases. Bilirubin diglucuronide is a form of conjugated bilirubin, and the clinically measured direct bilirubin essentially reflects this conjugated pool, which is why the direct fraction is the best answer.