**Core Concept**
MCardle's syndrome, also known as McArdle's disease, is a glycogen storage disorder characterized by the inability to break down glycogen to glucose due to a deficiency in a key enzyme. This enzyme is crucial for the final step of glycogenolysis, the process by which glycogen is converted to glucose.

**Why the Correct Answer is Right**
The correct answer is **A. Deficiency of glucose-6-phosphatase**. Glucose-6-phosphatase is the enzyme responsible for the final step of glycogenolysis, catalyzing the conversion of glucose-6-phosphate to glucose. In McArdle's disease, a deficiency of this enzyme leads to an accumulation of glycogen in muscles, resulting in muscle weakness, cramping, and fatigue. The deficiency of glucose-6-phosphatase is inherited in an autosomal recessive manner and is caused by mutations in the G6PC gene.

**Why Each Wrong Option is Incorrect**
* **Option B:** Absence of muscle phosphorylase is associated with another glycogen storage disorder called Hers' disease, not McArdle's disease.
* **Option C:** Deficiency of liver phosphorylase is associated with a glycogen storage disorder called Von Gierke's disease, which primarily affects the liver.
* **Option D:** Deficiency of liver phosphorylase kinase is associated with a glycogen storage disorder called Phosphorylase kinase deficiency, which affects both the liver and muscles.

**Clinical Pearl / High-Yield Fact**
MCardle's syndrome is often referred to as the "exercise-induced" form of glycogen storage disease, as symptoms typically worsen with physical activity. Patients with McArdle's disease may experience muscle cramps, weakness, and fatigue within 5-15 minutes of starting exercise.

**✓ Correct Answer:** A. Deficiency of glucose-6-phosphatase