A 24-year-old asymptomatic man is found to have total bilirubin 4 mg/dL with a direct fraction of 2.8 mg/dL on routine testing. Liver function tests are otherwise normal. At diagnostic laparoscopy for an unrelated issue the liver appears uniformly black. Fractionation of urinary coproporphyrins shows more than 80% as coproporphyrin I. The underlying defect is in:

• A UGT1A1, bilirubin UDP-glucuronosyltransferase
• B SLCO1B1, the basolateral organic anion transporting polypeptide 1B1
• C ABCC2, the canalicular multidrug resistance-associated protein 2 ✓
• D ABCB11, the bile salt export pump

Explanation

Dubin-Johnson syndrome arises from ABCC2 (MRP2) loss at the canalicular membrane, causing defective excretion of conjugated bilirubin and epinephrine metabolites that produce melanin-like pigment and a grossly black liver. Coproporphyrin I predominance with a normal total level is characteristic. This pattern kills the strongest distractor, Rotor syndrome, which shows elevated total urinary coproporphyrin with a normal fraction and no hepatic pigmentation.

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.