**Core Concept**
Glycogenolysis is the breakdown of glycogen into glucose, which is then released into the bloodstream. This process is crucial for providing energy to muscle cells. In muscle cells, glycogenolysis occurs through two mechanisms: the phosphorolysis pathway, which is the primary pathway, and the debranching enzyme pathway, which is involved in the release of glucose from the branch points of glycogen.

**Why the Correct Answer is Right**
The debranching enzyme (also known as amylo-alpha-1,6-glucosidase) is a key enzyme involved in glycogenolysis in muscle cells. This enzyme catalyzes the release of free glucose from the branch points of glycogen, allowing the breakdown of glycogen to continue. The debranching enzyme is essential for the complete breakdown of glycogen, as it removes the alpha-1,6-glycosidic bonds that connect the branch points of glycogen. This enzyme is unique to muscle cells and is not found in liver cells, where the primary pathway of glycogenolysis is phosphorolysis.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycogen phosphorylase is the enzyme responsible for the phosphorolysis pathway of glycogenolysis, which is the primary pathway in muscle cells. It catalyzes the conversion of glycogen to glucose-1-phosphate, but it is not responsible for the release of free glucose from glycogen.
**Option B:** Glucose-1-phosphatase is not a recognized enzyme involved in glycogenolysis. Glucose-1-phosphate is a product of glycogen phosphorylase activity, but it is not further converted to free glucose by this enzyme.
**Option C:** Glucose-6-phosphatase is an enzyme involved in gluconeogenesis and glycogenolysis in the liver, but it is not responsible for the release of free glucose from glycogen in muscle cells.

**Clinical Pearl / High-Yield Fact**
The debranching enzyme is unique to muscle cells and is essential for the complete breakdown of glycogen during glycogenolysis. Inhibiting this enzyme can lead to glycogen accumulation in muscle cells, resulting in muscle weakness and cramps.

**Correct Answer:**
✓ Correct Answer: D. Debranching enzyme