**Core Concept**
Glycogen storage disease type 5 (GSD-5) is a rare genetic disorder caused by a deficiency of the enzyme glycogen-branching enzyme (GBE). This enzyme plays a crucial role in glycogen synthesis and branching, which is essential for the proper storage and breakdown of glycogen in the liver and muscles.

**Why the Correct Answer is Right**
The glycogen-branching enzyme is responsible for creating branch points in glycogen molecules, allowing for efficient storage and breakdown. A deficiency in this enzyme leads to the accumulation of abnormal, linear glycogen molecules in the liver and muscles, resulting in the symptoms of GSD-5. The liver and muscles are unable to properly break down glycogen, leading to hypoglycemia and other complications.

**Why Each Wrong Option is Incorrect**
* **Option A:** Debranching enzyme deficiency is associated with glycogen storage disease type 3 (Cori's disease), not GSD-5. Debranching enzyme is responsible for breaking down glycogen molecules at branch points.
* **Option B:** Phosphorylase deficiency is associated with glycogen storage disease type 0. Phosphorylase is responsible for breaking down glycogen into glucose-1-phosphate.
* **Option D:** Glycogen synthase deficiency is associated with glycogen storage disease type 0. Glycogen synthase is responsible for adding glucose molecules to the growing glycogen chain.

**Clinical Pearl / High-Yield Fact**
Glycogen storage diseases are a group of rare genetic disorders that can be caused by deficiencies in various enzymes involved in glycogen synthesis and breakdown. Accurate diagnosis and management of these disorders require a thorough understanding of the underlying biochemical and genetic mechanisms.

**Correct Answer: C. Glycogen-branching enzyme.**