A 9-month-old child is presented to the emergency room by his parents who report that he has been vomiting and has severe diarrhea. The episodes of vomiting began when the parents started feeding their child cow's milk. The infant exhibits signs of failure to thrive, weight loss, hepatomegaly, and jaundice. Laboratory tests show elevated blood galactose, hypergalactosuria, and metabolic acidosis with coagulation deficiency. These clinical and laboratory findings are most consistent with a deficiency in which of the following enzymes?

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The symptoms and laboratory findings are consistent with classic galactosemia, which is caused by a deficiency in the enzyme UDP-galactose uridyltransferase (GALT). This enzyme deficiency leads to the accumulation of galactose-1-phosphate and galactose, causing the symptoms described.  This patient is suffering from classic (type 1) galactosemia. Type 1 galactosemia occurs as a consequence of mutations in the gene encoding GALT. Classic galactosemia manifests by a failure of neonates to thrive. Vomiting and diarrhea occur following ingestion of milk, hence individuals are termed lactose intolerant. Clinical findings include impaired liver function (which if left untreated leads to severe cirrhosis), elevated blood galactose, hypergalactosemia, hyperchloremic metabolic acidosis, urinary galactitol excretion, and hyperaminoaciduria. Unless controlled by exclusion of galactose from the diet, these patients can go on to develop blindness and fatal liver damage. Blindness is due to the conversion of circulating galactose to the sugar alcohol galacitol, by an NADPH-dependent aldose reductase that is present in neural tissue and in the lens of the eye.

A 9-Month-Old Child Is Presented to the Emergency Room by His