Glycogen storage disorder due to muscle phosphorylase deficiency
**Core Concept**
Muscle phosphorylase deficiency leads to a glycogen storage disorder characterized by the accumulation of abnormal glycogen in muscle cells, resulting in muscle weakness and cramps. This condition is a type of glycogen storage disease (GSD), specifically type V, also known as McArdle's disease.
**Why the Correct Answer is Right**
Muscle phosphorylase is an enzyme responsible for breaking down glycogen into glucose-1-phosphate in muscle cells. In McArdle's disease, the deficiency of muscle phosphorylase prevents the breakdown of glycogen, resulting in its accumulation and leading to muscle damage. This condition is inherited in an autosomal recessive manner and is characterized by exercise intolerance, muscle cramps, and weakness.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because GSD type III is caused by a deficiency of the debranching enzyme, not muscle phosphorylase.
* **Option B:** This option is incorrect because GSD type VI is caused by a deficiency of liver phosphorylase, not muscle phosphorylase.
* **Option C:** This option is incorrect because GSD type VII is caused by a deficiency of phosphofructokinase, not muscle phosphorylase.
**Clinical Pearl / High-Yield Fact**
McArdle's disease is often referred to as the "exercise-induced cramp" due to its characteristic symptom of muscle cramps and weakness triggered by physical activity.
**Correct Answer:** D. McArdle's disease is caused by a deficiency of muscle phosphorylase.