Скачать или смотреть Crigler-Najjar Syndrome Type II 🧬 Biochemistry & Pediatrics | USMLE Step 1 & Step 2 CK

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Crigler-Najjar Syndrome Type II 🧬 Biochemistry & Pediatrics | USMLE Step 1 & Step 2 CK

Crigler-Najjar Syndrome Type II is a rare autosomal recessive disorder of bilirubin conjugation, caused by a partial deficiency of the enzyme UDP-glucuronosyltransferase (UGT1A1). Unlike Type I, where UGT activity is completely absent, patients with Type II retain some residual enzyme activity, resulting in milder, non-life-threatening unconjugated hyperbilirubinemia. This distinction is a frequently tested concept on USMLE Step 1 (Biochemistry) and Step 2 CK (Pediatrics/Internal Medicine).

Clinically, Crigler-Najjar Type II presents in childhood or adolescence with persistent or fluctuating jaundice, but unlike Type I, bilirubin levels are usually less than 20 mg/dL and neurologic complications such as kernicterus are rare. Patients are often otherwise asymptomatic, with normal growth and development. The unconjugated (indirect) bilirubin fraction remains elevated, but the condition has a benign course.

A key diagnostic and therapeutic distinction is that Crigler-Najjar Type II responds to phenobarbital, a medication that induces UGT1A1 activity, leading to a reduction in serum bilirubin levels. This response is diagnostic and helps differentiate Type II from Type I, which does not respond to phenobarbital. Liver function tests (AST, ALT, ALP) are normal, and there is no evidence of hemolysis.

Diagnosis is confirmed by:
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Persistent unconjugated hyperbilirubinemia with normal liver enzymes
A positive response to phenobarbital (↓ bilirubin)
Genetic testing of UGT1A1 mutations if needed
No evidence of hemolysis or conjugated hyperbilirubinemia

Management is usually conservative, with phenobarbital used intermittently or continuously in patients with higher bilirubin levels. In most cases, no aggressive intervention is needed, and liver transplantation is not required, unlike in Type I.

On USMLE Step 1/Step 2 CK, expect questions comparing Crigler-Najjar Type I vs Type II, focusing on enzyme activity, response to phenobarbital, and risk of kernicterus. Also know how to distinguish it from Gilbert syndrome (milder, stress-induced jaundice) and Dubin-Johnson syndrome (conjugated hyperbilirubinemia with black liver).


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