A 2-year-old boy presents to the emergency department with a 24-hour history of progressive malaise, ataxic gate, and somnolence. He has had bilious vomiting 4 to 5 times a day unrelated to food consumption and constipation for nearly a week. His mother states that he has had no fever, convulsions, diarrhea, jaundice, or symptoms of viral upper respiratory tract infection. There is no history of trauma, exposure to toxic substances, or drug use. He was born from a consanguineous marriage. His medical history is significant for cholestasis that developed during his second month after birth. There is no family history of neonatal cholestasis.On physical examination, the boy has normal vital signs. He has incomprehensible speech, makes purposeless movement in reaction to stimuli, and has hyperactive deep tendon reflexes. His Glasgow Coma Scale score is 9. There are no signs of meningeal irritation and no focal neurologic deficits. The rest of his findings on physical examination are normal.Laboratory studies reveal a normal complete blood cell count, serum electrolyte levels, and blood gas test results. He has mildly elevated aminotransferase levels (alanine aminotransferase [ALT], 85 U/L; aspartate aminotransferase [AST], 131 U/L; and g-glutamyl transferase, 8.6 U/L). The direct bilirubin level is 0.29 mg/dL (5.0 mmol/L), total bilirubin level is 1.05 mg/dL (18.0 mmol/L), serum glucose level is 101 mg/dL (5.6 mmol/L), albumin level is 3.5 g/dL (35 g/L), and international normalized ratio (INR) is 1.73 (reference range, 0.86-1.20). Urine toxicology screening results are negative. Cerebrospinal fluid analysis results are normal. However, his blood ammonia level is elevated at 398 mg/dL (reference range, 15-45 mg/dL). Additional medical history and imaging studies reveal the diagnosis. CASE 1 DISCUSSIONReview of the patient's medical records revealed that he was evaluated at age 9 months and found to have hepatomegaly, jaundice, and a cardiac murmur. He had significantly elevated aminotransferase levels and direct hyperbilirubinemia. Viral serologic test results were negative for acute viral infections. Hepatobiliary THE CASE