Which condition presents with elevated conjugated bilirubin due to defective canalicular transport?

• A. Elevated unconjugated bilirubin due to increased bilirubin production
• B. Crigler-Najjar syndrome
• C. Elevated conjugated bilirubin due to defective canalicular transport
• D. Hemolytic anemia

Answer: (C)

Elevated conjugated bilirubin from a failure to excrete it into bile points to a problem with canalicular transport. In the liver, bilirubin is conjugated to become water-soluble and then excreted into the bile via canalicular transporters (notably MRP2). If this canalicular excretion is defective, conjugated bilirubin backs up and appears in the bloodstream, producing conjugated hyperbilirubinemia with relatively preserved overall liver function. This pattern matches Dubin-Johnson syndrome, where a defect in canalicular transport causes the rise in conjugated bilirubin.

In contrast, elevated unconjugated bilirubin due to increased bilirubin production reflects prehepatic or hemolytic processes, and Crigler-Najjar syndrome involves deficiency of the conjugating enzyme, leading to unconjugated (not conjugated) hyperbilirubinemia. Hemolytic anemia primarily raises unconjugated bilirubin from increased red cell breakdown.