**Core Concept**
McArdle's disease is a genetic disorder caused by a deficiency of the enzyme myophosphorylase, which is essential for the breakdown of glycogen to glucose in muscle cells. This deficiency leads to an inability to utilize glycogen as a source of energy, resulting in muscle cramps, weakness, and fatigue.

**Why the Correct Answer is Right**
The correct answer is related to the deficiency of myophosphorylase, which is a key enzyme in the glycogenolysis pathway. Myophosphorylase catalyzes the conversion of glycogen to glucose-1-phosphate, which is then converted to glucose-6-phosphate and ultimately enters the glycolytic pathway to produce energy. In McArdle's disease, this enzyme is deficient, leading to an accumulation of glycogen in muscle cells and an inability to utilize it as a source of energy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because McArdle's disease is not caused by a deficiency of the enzyme phosphofructokinase, which is involved in the glycolytic pathway.
* **Option B:** This option is incorrect because McArdle's disease is not caused by a deficiency of the enzyme pyruvate kinase, which is also involved in the glycolytic pathway.
* **Option C:** This option is incorrect because McArdle's disease is not caused by a deficiency of the enzyme creatine kinase, which is involved in energy metabolism in muscle cells.

**Clinical Pearl / High-Yield Fact**
McArdle's disease is an autosomal recessive disorder, meaning that a person must inherit two copies of the mutated gene (one from each parent) to develop the condition. The disease is characterized by muscle cramps, weakness, and fatigue, particularly after exercise or physical activity.

**Correct Answer: D. Myophosphorylase deficiency**