**Core Concept**
Glycogen, a complex carbohydrate stored in the liver and muscles, is broken down into glucose through a process called glycogenolysis. This process involves the sequential action of enzymes that cleave the glycosidic bonds between glucose molecules in glycogen.

**Why the Correct Answer is Right**
In the liver, glycogen is broken down into glucose-1-phosphate by the enzyme glycogen phosphorylase. However, in the muscle, glycogen is broken down into glucose-1-phosphate by the enzyme glycogen phosphorylase, but this glucose-1-phosphate is then converted to glucose-6-phosphate by the enzyme phosphoglucomutase. The glucose-6-phosphate is then dephosphorylated to form glucose by the enzyme glucose-6-phosphatase. In contrast, glycogen is not broken down in the gut, as it is not a major source of glucose in the gut.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glycogen is indeed broken down in the liver, which is a major site of glycogenolysis.
* **Option C:** Muscle cells do break down glycogen, albeit differently than liver cells.
* **Option D:** The gut does not break down glycogen, but it does break down other complex carbohydrates like starch and cellulose.

**Clinical Pearl / High-Yield Fact**
The enzyme glucose-6-phosphatase is a key regulator of glucose production in the liver and is deficient in the genetic disorder glycogen storage disease type I (GSD-I), also known as von Gierke's disease.

**Correct Answer: D. Glycogen is not broken down in the gut.**