**Core Concept**
McArdle's disease, also known as glycogen storage disease type V, is a congenital disorder that affects the body's ability to break down glycogen, a complex carbohydrate stored in muscles. It is caused by a deficiency in a specific enzyme responsible for glycogenolysis, the process of glycogen breakdown.

**Why the Correct Answer is Right**
McArdle's disease is primarily due to the deficiency of myophosphorylase, an enzyme that catalyzes the first step in glycogenolysis. This enzyme is crucial for the breakdown of glycogen to glucose-1-phosphate, which is then converted to glucose-6-phosphate and ultimately enters the glycolytic pathway to produce energy. In the absence of myophosphorylase, glycogen accumulates in muscle cells, leading to muscle cramps, weakness, and fatigue.

**Why Each Wrong Option is Incorrect**
* **Option A:** Debranching enzyme is responsible for breaking down glycogen in the liver and muscles, but its deficiency is associated with Cori's disease, not McArdle's disease.
* **Option B:** Phosphofructokinase is an enzyme involved in the glycolytic pathway, but its deficiency leads to Tarui's disease, a different glycogen storage disorder.
* **Option D:** Glycogen synthase is responsible for glycogenesis, the process of glycogen synthesis, but its deficiency is not associated with McArdle's disease.

**Clinical Pearl / High-Yield Fact**
McArdle's disease is characterized by exercise-induced muscle cramps, weakness, and fatigue due to the inability to break down glycogen for energy. It is often diagnosed through a muscle biopsy or genetic testing.

**Correct Answer:** C.