Crigler-Najjar syndrome is best described by which statement?

• A. Inability to transport bilirubin from the sinusoidal membrane to the microsomal region
• B. Deficiency of the enzyme system required for conjugation of bilirubin
• C. Inability to transport bilirubin glucuronides to the bile canaliculi
• D. Severe liver cell damage accompanied by necrosis

Answer: (B)

Crigler-Najjar is defined by a deficiency of the enzyme that conjugates bilirubin in the liver. Bilirubin from the blood is taken up by hepatocytes and normally converted to a water-soluble form by UDP-glucuronosyltransferase, attaching glucuronic acid to produce bilirubin diglucuronide for excretion into bile. When this conjugating enzyme is deficient or absent, unconjugated bilirubin accumulates, causing severe jaundice and, in the most serious form, risk of kernicterus. The option describing a defect in the conjugation step directly captures this mechanism.

The other statements describe problems in different parts of bilirubin handling: one points to a defect in uptake or intrahepatocyte transport, another to transport of conjugated bilirubin into bile, and the last to liver cell damage—none of which define Crigler-Najjar.