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Glycogen Storage Disease 4

Glycogen storage disease
biochemistry

Question

Vignette: A 28-year-old man presents to his physician with complaints of muscle weakness and fatigue. He reports that these symptoms seem to increase after he exercises. He also reports occasional episodes of dark urine. His medical history is unremarkable. His physical examination reveals mild bilateral muscle weakness but is otherwise normal. Laboratory tests reveal increased levels of creatine kinase. A muscle biopsy is performed, and upon microscopic examination, vacuoles containing glycogen are found within the muscle cells. These findings are indicative of a deficiency in which of the following enzymes?

Choices

A. Glucokinase

B. Glucose-6-phosphatase

C. Phosphofructokinase

D. Myophosphorylase

E. Hexokinase

Answer

D. Myophosphorylase

Explanation

The patient's symptoms of weakness and fatigue that worsen with exercise, combined with the presence of glycogen within vacuoles in muscle cells, are indicative of McArdle disease (glycogen storage disease type V). This is an autosomal recessive disorder caused by a deficiency in the enzyme myophosphorylase, which is essential for glycogenolysis in muscle cells. Glycogenolysis is the process by which glycogen stored in the liver and muscle tissue is converted into usable glucose. Without myophosphorylase, muscle cells are unable to break down glycogen into glucose-1-phosphate for ATP production during exercise, leading to symptoms such as muscle weakness, fatigue, and myoglobinuria (dark urine). Increased levels of creatine kinase in the blood are indicative of muscle damage. The other options listed are also involved in glucose metabolism but are not associated with the symptoms and findings presented in this vignette.

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