Glycogen storage disease type III (GSD III)—also known as Cori disease or Forbes disease—is an inherited condition in which the deficiency of an enzyme called glycogen debranching enzyme results in various complications, notably involving the liver and muscles.

The lack of this enzyme means the body cannot properly break down glycogen, a stored form of sugar. As a result, glycogen cannot properly be used to energize the body and glycogen molecules accumulate in the body.




Symptoms of GSD III often appear in infancy or childhood. The liver is enlarged, leading to a noticeably swollen abdomen. This enlargement usually subsides with puberty, although there may be long term liver damage.

Muscle weakness is a problem seen in 85% of people with GSD III. This weakness can at times be severe and may worsen in adulthood. Children with the disease may experience delayed growth, but usually reach normal adult height. A minority of people with the disease also have a mildly enlarged heart, though its function is usually normal.

Low blood sugar and feelings of tiredness may occur with GSD III, although they are less common and less severe than with another form of GSD, type I.










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A female infant appeared normal at birth but developed signs of liver disease and muscular weakness at 3 months. She had periods of hypoglycemia, particularly on awakening. Examination revealed an enlarged liver. Laboratory analyses following fasting revealed ketoacidosis, blood pH 7.25, and elevations in both alanine transaminase (ALT) and aspartate transaminase (AST) levels. Administration of glucagon following a carbohydrate meal elicited a normal rise in blood glucose, but glucose levels did not rise when glucagon was administered following an overnight fast. Liver biopsy revealed an increase in the glycogen content (6% of wet weight).

In which of the following enzymes is a genetic deficiency most likely for this patient?


The correct answer is B.

 Definitive diagnosis would await analysis of the glycogen structure and enzyme activities, but the hepatomegaly, increased liver glycogen content, fasting hypoglycemia, and muscle weakness are consistent with Cori disease type 3 glycogen storage disease. The increase in glycogen content results from an inability to degrade glycogen beyond the limit dextrin of phosphorylase. A deficiency in the debranching enzyme leaves glycogen with short outer branches.

To prevent the frequent episodes of hypoglycemia, which of the following dietary supplements would be most appropriate for this patient?

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The correct answer is E. You answered D.

E Because fasting hypoglycemia results from an inability to break down glycogen past the limit dextrin of phosphorylase, a patient with type 3 glycogen storage disease should be given frequent meals high in carbohydrates. Uncooked cornstarch is an effective supplement because it is slowly digested, and therefore the glucose is released slowly into the bloodstream, helping to maintain blood glucose concentrations.



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