The patient in the question likely has hereditary fructose intolerance, manifest only when fructose in fruit juice is provided in the diet. Galactosemia, fructosemia, tyrosinosis, and glucose-6-phosphatase deficiency represent diseases in which a congenital deficiency of enzyme causes an interruption of a normal metabolic pathway and an accumulation of metabolic precursors that damage vital organs. Galactose (found in milk) and fructose (found in fruit juices) produce urinary-reducing substances in their respective disorders. The mode of inheritance of galactosemia, fructosemia, and most forms of glucose-6-phosphatase deficiency is autosomal recessive. In galactosemia and fructosemia, errors in carbohydrate metabolism cause the accumulation of toxic metabolites when specific dietary sugars are introduced (lactose in galactosemia; fructose and sucrose in fructosemia). Exclusion of the offending carbohydrate from the diet will prevent liver damage. In tyrosinemia type I, or tyrosinosis, the accumulation of tyrosine and its metabolites is associated with severe involvement of the liver, kidney, and CNS. Manifestations of acute liver failure can appear in infancy. A chronic form of the disorder presents as progressive cirrhosis and leads to liver failure or hepatoma. Dietary management does not prevent liver disease. Glucose-6-phosphatase deficiency often presents at 3 to 4 months of age with failure to thrive, hypoglycemia, hepatomegaly, and acidosis. a1-Antitrypsin deficiency causes liver disease through accumulation of an abnormal protein, caused by a single amino acid substitution on chromosome 14. It has a variable presentation, but the following are common in infancy: cholestasis; bleeding into the CNS, gastrointestinal (GI) tract, or at the umbilical stump; and elevation of transaminase concentrations. Affected children may have chronic hepatitis with cirrhosis and portal hypertension.