Question

A child with exercise fatigue presented to OPD. The features were suggestive of McArdle disease. Which of the following enzymes will be deficient in this?

• A. Myo-Phosphorylase
• B. Glucose 6 Phosphatase
• C. Lysosomal Glucosidase
• D. Phosphofructokinase

Hint:

McArdle disease results in muscle fatigue and exercise intolerance due to the inability to break down glycogen in muscle cells.

Answer 1

The Correct Option is A

The question is regarding a clinical condition known as McArdle disease, associated with exercise fatigue. McArdle disease is a type of glycogen storage disease (Type V) that is characterized by a deficiency in the enzyme Myo-Phosphorylase. Let's understand why Myo-Phosphorylase deficiency leads to these symptoms and why other options are incorrect.

1. Understanding McArdle Disease:
   • McArdle disease is a genetic disorder that affects the muscle cells' ability to break down glycogen into glucose during physical activity.
   • The enzyme Myo-Phosphorylase, also known as glycogen phosphorylase, is crucial for glycogenolysis in muscle tissue.
   • A deficiency in Myo-Phosphorylase leads to an inability to use muscle glycogen for energy, causing exercise intolerance and fatigue.
2. Evaluating the Options:
   • Myo-Phosphorylase: This enzyme is necessary for glycogen breakdown in muscles. Its deficiency results in McArdle disease, leading to exercise-induced muscular issues.
   • Glucose 6 Phosphatase: This enzyme is involved in gluconeogenesis and glycogenolysis, particularly in the liver, and its deficiency is related to Von Gierke disease (Glycogen storage disease Type I), not McArdle disease.
   • Lysosomal Glucosidase: This enzyme is associated with Pompe disease (Glycogen storage disease Type II), which involves lysosomal glycogen breakdown deficiency. It's unrelated to McArdle disease.
   • Phosphofructokinase: This is a key regulatory enzyme in glycolysis. Its deficiency causes Tarui's disease (Glycogen storage disease Type VII), not McArdle disease.
3. Conclusion: The correct answer is Myo-Phosphorylase, as its deficiency directly corresponds with the symptoms observed in McArdle disease, i.e., exercise fatigue due to the lack of glycogen breakdown in muscles.