An infant appears normal at birth following an uncomplicated pregnancy and begins to nurse. Within 3 days the infant exhibits vomiting and diarrhea. A week later jaundice is evident, and the infant becomes febrile. On physical examination hepatomegaly is present. Laboratory studies show abnormal reducing substances in the urine, and Escherichia coli is cultured from blood. This infant most likely has a deficiency in which of the following enzymes?

This infant has galactosemia, an inborn error of metabolism with an autosomal recessive pattern of inheritance. Lactose in milk is a disaccharide that is converted by lactase to glucose and galactose. An excess of galactose is toxic to liver, brain, and eye. Affected infants begin to develop abnormalities as soon as they begin to ingest milk. Glucocerebrosidase deficiency leads to Gaucher disease. Phenylalanine hydroxylase deficiency leads to the classic form of phenylketonuria. Sphingomyelinase deficiency leads to Niemann-Pick disease. UDP-glucuronosyltransferase deficiency is present in Gilbert syndrome.