McArdle’s disease is caused due to deficiency of
## **Core Concept**
McArdle’s disease, also known as Glycogen storage disease type V, is a genetic disorder that affects the body's ability to metabolize glycogen. It is characterized by the deficiency of an enzyme crucial for glycogen breakdown. This condition leads to an inability to convert glycogen into glucose for energy, particularly affecting muscle tissues.
## **Why the Correct Answer is Right**
The correct answer, **Myophosphorylase**, is the enzyme deficient in McArdle’s disease. Myophosphorylase is crucial for the breakdown of glycogen to glucose-1-phosphate in muscle cells, a key step in glycogenolysis. Without sufficient myophosphorylase, muscles cannot properly utilize glycogen for energy, leading to the symptoms of McArdle’s disease, such as muscle cramps, fatigue, and myoglobinuria after exercise.
## **Why Each Wrong Option is Incorrect**
- **Option A:** Glucose-6-phosphatase deficiency leads to von Gierke’s disease (Glycogen storage disease type I), a different glycogen storage disease that primarily affects the liver and kidneys, causing hypoglycemia, lactic acidosis, and hepatomegaly.
- **Option B:** Debranching enzyme deficiency causes Cori’s disease (Glycogen storage disease type III), affecting the liver, muscles, and sometimes the heart, by impairing the complete breakdown of glycogen.
- **Option D:** Phosphofructokinase deficiency results in Tarui’s disease (Glycogen storage disease type VII), another glycogen storage disease that affects muscles and leads to similar but distinct clinical features compared to McArdle’s disease.
## **Clinical Pearl / High-Yield Fact**
A key clinical feature of McArdle’s disease is the "second wind" phenomenon, where patients experience relief from symptoms after a short rest and can then continue exercising. This occurs because the body begins to utilize alternative energy sources, such as free fatty acids, for muscle activity. Remembering this peculiar symptom can help in distinguishing McArdle’s disease from other myopathies.
## **Correct Answer:** . Myophosphorylase