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Chapter 12: Carbohydrates: Galactose Metabolism

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 failing to thrive. Laboratory tests show elevated blood galactose, hypergalactosuria, metabolic acidosis, albuminuria, and hyperaminoaciduria. These clinical and laboratory findings are most consistent with which of the following disorders?

A. alkaptonuria

B. essential fructosuria

C. hereditary galactosemia

D. Menkes disease

E. von Gierke disease

Answer C: 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 galactitol, by an NADPH-dependent aldose reductase that is present in neural tissue and in the lens of the eye.

You are examining a patient who complains of gastric discomfort following the consumption of milk. Additional signs and symptoms include liver and kidney impairment as well as neural involvement. Blood work indicates an increased concentration of galactose-1-phosphate. This patient likely has a defect in which of the following enzymes?

A. fructose-1,6-bisphosphatase

B. galactokinase

C. galactose-1-phosphate uridyltransferase

D. glucokinase

E. ketohexokinase (fructokinase)

Answer C: This patient is suffering from classic (type 1) galactosemia. Type 1 galactosemia occurs as a consequence of mutations in the gene encoding galactose-1-phosphate uridyltransferase (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.

A defect in which of the following enzymes of galactose metabolism would most likely be associated with hypoglycemia, hepatomegaly, and hyperaminoaciduria?

A. galactokinase

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