**Core Concept**
Glycogenolysis is the breakdown of glycogen, a complex carbohydrate stored in the liver and muscles, into glucose. This process is crucial for maintaining blood glucose levels, particularly during fasting or exercise. The key enzyme responsible for glycogenolysis is glycogen phosphorylase.

**Why the Correct Answer is Right**
Glycogen phosphorylase catalyzes the conversion of glycogen to glucose-1-phosphate, which is then converted to glucose-6-phosphate by phosphoglucomutase. Glucose-6-phosphate can then be released into the bloodstream or converted to glucose-6-phosphate dehydrogenase for energy production. The activity of glycogen phosphorylase is regulated by several factors, including epinephrine, cortisol, and glucagon, which stimulate glycogen breakdown during stress or fasting.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as glycogen synthase is the enzyme responsible for glycogen synthesis, not breakdown.
* **Option B:** This option is incorrect as phosphoglucomutase is involved in the conversion of glucose-1-phosphate to glucose-6-phosphate, but it is not the primary enzyme responsible for glycogenolysis.
* **Option D:** This option is incorrect as hexokinase is an enzyme involved in glycolysis, not glycogenolysis.

**Clinical Pearl / High-Yield Fact**
Glycogenolysis is an important mechanism for maintaining blood glucose levels during fasting. However, excessive glycogen breakdown can lead to hypoglycemia, particularly in individuals with diabetes or those taking certain medications that stimulate glycogen breakdown.

**Correct Answer: C. Glycogen phosphorylase.