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Chapter 11: Carbohydrates: Fructose Metabolism and Feeding Behaviors

Which of the following represents the enzyme deficiency that leads to “essential fructosuria”?

A. fructose-1-phosphate aldolase (aldolase B)

B. fructose-1,6-bisphosphate aldolase (aldolase A)

C. fructokinase

D. hexokinase

E. 6-Phosphofructo-1 kinase, PFK1

Answer C: Essential fructosuria is a benign metabolic disorder caused by the lack of fructokinase which is normally present in the liver, pancreatic islets, and kidney cortex. The fructosuria of this disease depends on the time and amount of fructose and sucrose intake. Since the disorder is asymptomatic and harmless, it may go undiagnosed.

A child born and raised in Chicago planned to spend the summer on a relative's fruit farm and help with the harvest. The summer passed uneventfully, but several days after the harvest began, the child became jaundiced and very sick. On admission to the hospital the following clinical findings were recorded: in addition to the expected hyperbilirubinemia, the patient was hypoglycemic, had a markedly elevated rise in blood fructose concentration, and was hyperlactic acidemic. Further history taking revealed that during the harvest it was customary for the family to indulge in fruit-filled meals and to snack freely on fruit while carrying out the harvest. The elevated blood fructose in this child was most likely due to which of the following?

A. an allergic reaction to constituents in the fruit diet

B. defective hepatic aldolase B

C. defective hepatic fructokinase

D. defective hepatic fructose-1,6-bisphosphatase

E. defective hepatic glucokinase

Answer B: Hereditary fructose intolerance is a potentially lethal disorder resulting from a lack of aldolase B which is normally present in the liver, small intestine, and kidney cortex. The disorder is characterized by severe hypoglycemia and vomiting following fructose intake. Prolonged intake of fructose by infants with this defect leads to vomiting, poor feeding, jaundice, hepatomegaly, hemorrhage, and, eventually, hepatic failure and death. The hypoglycemia that results following fructose uptake is caused by fructose-1-phosphate inhibition of glycogenolysis, by interfering with the phosphorylase reaction, and inhibition of gluconeogenesis at the deficient aldolase step. Patients remain symptom free on a diet devoid of fructose and sucrose.

A 1-year-old girl is being examined by her physician because of a 3-month history of intermittent vomiting, shakiness, and a failure to thrive. The symptoms began when breast-feeding stopped and fruit and vegetables were added to her diet. Her current size is below the fifth percentile for length ...

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