**Core Concept**
Forbes disease, also known as glycogen storage disease type III, is a rare genetic disorder caused by a deficiency in the enzyme responsible for breaking down glycogen in the liver and muscles. This enzyme plays a crucial role in glycogen metabolism, allowing the body to convert glycogen into glucose for energy.

**Why the Correct Answer is Right**
The correct answer is **B. Debranching enzyme**, also known as amylo-1,6-glucosidase. This enzyme is essential for the breakdown of glycogen, particularly at branch points where alpha-1,6-glycosidic bonds are present. In the absence of this enzyme, glycogen accumulates in the liver and muscles, leading to the characteristic symptoms of Forbes disease, including hypoglycemia, hepatomegaly, and progressive muscle weakness.

**Why Each Wrong Option is Incorrect**
**Option A:** Branching enzyme is involved in glycogen synthesis, not breakdown. It creates branch points in glycogen molecules, which are then targeted by the debranching enzyme.

**Option C:** Myophosphorylase is a different enzyme involved in glycogen metabolism, specifically in the breakdown of glycogen in muscles. Its deficiency leads to McArdle disease, a distinct glycogen storage disorder.

**Option D:** Hepatic phosphorylase is another enzyme involved in glycogen metabolism, specifically in the breakdown of glycogen in the liver. Its deficiency leads to Hers disease, a different glycogen storage disorder.

**Clinical Pearl / High-Yield Fact**
Forbes disease is characterized by a unique "chewed-up" appearance of glycogen on electron microscopy, reflecting the incomplete breakdown of glycogen due to the debranching enzyme deficiency. This distinctive feature helps in the diagnosis of this rare disorder.

**Correct Answer Line**
✓ Correct Answer: B. Debranching enzyme