**Core Concept**
McArdle's syndrome, also known as glycogen storage disease type V, is a genetic disorder caused by a deficiency of the enzyme myophosphorylase. This enzyme plays a crucial role in the breakdown of glycogen to glucose in muscle cells, particularly during intense physical activity.

**Why the Correct Answer is Right**
Myophosphorylase is responsible for catalyzing the conversion of glycogen to glucose-1-phosphate, which is then further metabolized to provide energy for muscle contractions. In McArdle's syndrome, the deficiency of myophosphorylase leads to an accumulation of glycogen in muscle cells, resulting in muscle cramps, weakness, and fatigue after exercise. This condition highlights the importance of proper glycogen metabolism in muscle function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because McArdle's syndrome is specifically related to myophosphorylase deficiency, not hexokinase, which is involved in glucose phosphorylation.
**Option B:** This option is incorrect because debranching enzyme deficiency is associated with glycogen storage disease type III, not McArdle's syndrome.
**Option C:** This option is incorrect because phosphofructokinase deficiency is associated with glycogen storage disease type VII, not McArdle's syndrome.
**Option D:** This option is incorrect because phosphoglucomutase deficiency is not associated with McArdle's syndrome.

**Clinical Pearl / High-Yield Fact**
McArdle's syndrome is characterized by muscle cramps, weakness, and fatigue after exercise, which can be relieved by rest. A classic clinical finding is the inability to perform a prolonged bout of exercise, such as a 10-minute run, due to the accumulation of glycogen in muscle cells.

**Correct Answer:** C. Phosphorylase deficiency.