McArdle’s disease causes muscle cramps and muscle fatigue with increased muscle glycogen. Which of the following enzymes is deficient?
**Core Concept**
McArdle's disease, also known as Glycogen Storage Disease Type V, is a metabolic disorder that affects the breakdown of glycogen in muscle cells, leading to impaired energy production and exercise intolerance. This condition is characterized by muscle cramps, muscle fatigue, and increased muscle glycogen levels due to the accumulation of glycogen.
**Why the Correct Answer is Right**
McArdle's disease is caused by a deficiency of the enzyme myophosphorylase, which is responsible for breaking down glycogen into glucose-1-phosphate in muscle cells. This enzyme plays a crucial role in the glycogenolysis pathway, allowing muscles to generate energy from stored glycogen. The deficiency of myophosphorylase leads to the accumulation of glycogen in muscle cells, resulting in the characteristic symptoms of McArdle's disease.
**Why Each Wrong Option is Incorrect**
* **Option A:** Debranching enzyme deficiency is associated with Cori's disease (Glycogen Storage Disease Type III), which affects the breakdown of glycogen in both liver and muscles.
* **Option B:** Phosphofructokinase deficiency is associated with Tarui's disease (Glycogen Storage Disease Type VII), which affects the breakdown of glycogen in muscles.
* **Option D:** Glycogen synthase deficiency is associated with Hers' disease (Glycogen Storage Disease Type VI), which affects glycogen synthesis in the liver.
**Clinical Pearl / High-Yield Fact**
McArdle's disease is an autosomal recessive disorder, and affected individuals often experience muscle cramps, fatigue, and pain after short periods of exercise, which can be relieved by rest.
**Correct Answer:** C. Myophosphorylase deficiency.