CLINICAL CASES
187
[20.4]
A 23-year-old male has been vigorously working on the yard and
begins to feel slightly light-headed from hypoglycemia. He drinks a
can of soda and is aware of the competition for the glucose to be stored
in his liver as glycogen versus used as energy in his muscles. What is
the best explanation regarding the fate of the glucose in the soda?
A. The lower Km of hexokinase versus the Km of glucokinase will tilt
the glucose toward glycolysis.
B. The bolus of glucose via the soda will lead to a higher glucose
level, inducing storage of the glucose into glycogen in the liver.
C. The muscle is using high levels of glucose, leading to an increased
level of glucose 6-phosphate thus inhibiting glucokinase.
D. The glucose will be equally used by muscle for metabolism and
liver for glycogen storage.
A nsw ers
[20.1]
B. Definitive diagnosis would await analysis of the glycogen struc-
ture and enzyme activities, but the hepatomegaly, increased liver
glycogen content, fasting hypoglycemia, and muscle weakness are
consistent with Cori disease, glycogen storage disease type III. 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.
[20.2]
E. Because fasting hypoglycemia results from an inability to break
down glycogen past the limit dextrin of phosphorylase, a patient with
type III 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.
[20.3]
C. Although a deficiency in a number of enzymes can result in exer-
cise intolerance, the lack of an increase in serum lactate following
ischemic exercise points to an inability to a defect in the breakdown
of glycogen in the muscle. The muscle depends on glycogenolysis for
intense exercise, and fatigue rapidly ensues when glycogen is
depleted. Patients with a deficiency in the muscle isoform of glyco-
gen phosphorylase (McArdle disease) can tolerate mild to moderate
exercise, but get muscle cramps with strenuous exercise as a conse-
quence of the lack of glycogenolysis in the muscle cell.
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