**Core Concept**
Glycogenolysis is the breakdown of glycogen, a complex carbohydrate stored in the liver and muscles, to glucose for energy supply. This process involves a series of enzyme-catalyzed reactions that convert glycogen into glucose-6-phosphate.

**Why the Correct Answer is Right**
The key enzyme in glycogenolysis is glycogen phosphorylase (also known as glycogen phosphorylase enzyme). This enzyme catalyzes the first step in glycogenolysis by converting glycogen into glucose-1-phosphate. Glycogen phosphorylase is a metalloenzyme that requires magnesium ions as a cofactor. It is activated by adrenaline (epinephrine) and cyclic AMP (cAMP), which are signaling molecules released during stress and increased energy demand.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphatase is actually the key enzyme in gluconeogenesis, not glycogenolysis. Gluconeogenesis is the process of generating glucose from non-carbohydrate sources like amino acids and lactate.

**Option B:** Phosphoglucomutase is an enzyme involved in glycogen synthesis (glycogenesis), not glycogenolysis. It catalyzes the conversion of glucose-1-phosphate to glucose-6-phosphate, which is then used to synthesize glycogen.

**Option C:** Hexokinase is an enzyme involved in glycolysis, not glycogenolysis. It catalyzes the conversion of glucose to glucose-6-phosphate, which is then further broken down to produce energy.

**Clinical Pearl / High-Yield Fact**
A key clinical application of glycogenolysis is in the management of hypoglycemia (low blood sugar). Glycogenolysis is inhibited in type 1 diabetes, leading to severe hypoglycemia. Understanding the key enzyme in glycogenolysis is essential for managing glucose levels in diabetic patients.

**Correct Answer: B. Phosphoglucomutase is not correct, the actual answer is A. Glucose-6-phosphatase is the key enzyme in gluconeogenesis, not glycogenolysis, but the key enzyme in glycogenolysis is actually glycogen phosphorylase, the correct answer is not listed.