MEDizzy - A 3-month-old girl with developing cataracts is shown to

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Glycolysis and Gluconeogenesis

A 3-month-old girl with developing cataracts is shown to contain a reducing sugar in her urine, but the glucose oxidase test was negative. She has had no problems eating, and her growth curve is at the 60th percentile. Fasting blood glucose tests show normal levels of circulating glucose. A likely enzyme deficiency is which of the following?

ExplanationThe answer is C: Galactokinase. The child has nonclassical galactosemia, a defect in galactokinase. With this disorder, galactose cannot be accumulated within cells, and so it accumulates in the blood, spilling over to the urine. Because of its high level, the galactose can enter the eye and be reduced to galactitol by aldose reductase, trapping the galactitol within the eye. As galactitol accumulates, an osmotic imbalance is created, leading to cataract formation. However, since galactose1-phosphate is not accumulating (as occurs in classical galactosemia, a defect in galactose-1-phosphate uridylyl transferase), the other effects seen with classical galactosemia (hypoglycemia and neurological deficit) do not occur. The sugar that is accumulating in the urine is galactose, which contains an aldehyde, which generates a positive response in a reducing test. A defect in fructokinase leads to fructosuria, a benign condition (fructose is not a substrate for aldose reductase, as it is a ketose and not an aldose). A defect in hexokinase would lead to elevated glucose levels, and can lead to sorbitol production in the lens of the eye, but the urine reducing sugar test was negative for glucose