Criggler-Najjar Type 1 is characterized by which UDPGT deficiency?

• A. Type 1/Total (Homozygous) UDPGT deficiency
• B. Type 2/Partial (Heterozygous) UDPGT deficiency
• C. Overactive UDPGT activity
• D. Normal UDPGT activity

Answer: (A)

Crigler-Najjar Type I shows a complete loss of hepatic UDP-glucuronosyltransferase (UGT) activity, usually due to homozygous mutations. Without UGT, bilirubin cannot be conjugated to glucuronic acid, so unconjugated bilirubin builds up from birth, causing severe jaundice and a high risk of kernicterus. This makes a total, homozygous deficiency the best description for Type I. By contrast, Type II has partial enzyme activity and can respond to phenobarbital, which is why that option doesn’t fit. Options suggesting overactivity or normal activity don’t describe the syndrome.