A 24-year-old man is noted to have mild conjugated hyperbilirubinemia (total bilirubin 3 mg/dL) during a routine checkup. He is asymptomatic, alkaline phosphatase and transaminases are normal, and there is no hemolysis. Liver biopsy shows coarse dark-brown granules of pigment within hepatocytes, giving the liver a black appearance grossly. The defect lies in:
- A UGT1A1 enzyme activity
- B Organic anion transporting polypeptide (OATP) uptake transporter
- C Canalicular multidrug resistance-associated protein 2 (MRP2) ✓
- D ABCB11 bile salt export pump
Explanation
Dubin-Johnson syndrome results from a defect in the canalicular MRP2 transporter (encoded by ABCC2), impairing excretion of conjugated bilirubin into bile. Conjugated hyperbilirubinemia is otherwise benign, and epinephrine metabolites accumulate as dark pigment, producing the characteristic black liver. Gilbert and Crigler-Najjar syndromes involve UGT1A1 defects and cause unconjugated hyperbilirubinemia without pigment deposition. Rotor syndrome also causes conjugated hyperbilirubinemia but lacks pigmentation and involves defective hepatic uptake and storage rather than canalicular excretion.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.