**Core Concept:** Glycogen is a complex carbohydrate stored in the liver and muscles for energy and gluconeogenesis. It is broken down into glucose, which can be released into the bloodstream when glucose levels are low. Glycogen degradation is mediated by a series of enzymes, but the main one is glycogen phosphorylase (GP).

**Why the Correct Answer is Right:** Glycogen phosphorylase (GP) is a key enzyme responsible for the breakdown of glycogen into glucose-1-phosphate, which can be further converted to glucose. GP has two isoforms: GPa and GPb, where GPa is primarily found in the liver and GPb in skeletal muscles. The correct answer, GPb, is responsible for the breakdown of glycogen in skeletal muscles during periods of high energy demand, such as during exercise or hypoglycemia.

**Why Each Wrong Option is Incorrect:**

A. Phosphorylase kinase (PK) is a regulatory enzyme that phosphorylates glycogen phosphorylase (GP), activating it for glycogenolysis (glycogen breakdown). However, it is not the main enzyme involved in glycogen degradation.

B. Glucose-6-phosphatase (G6Pase) is an enzyme responsible for the final step in glycogen synthesis, converting glucose-6-phosphate to glucose. It is not the main enzyme involved in glycogen breakdown.

C. Phosphorylase kinase (PK) and glycogen synthase (GS) are involved in glycogen synthesis, not breakdown.

D. Glucose-6-phosphatase (G6Pase) is involved in glycogen synthesis, not breakdown.

**Clinical Pearl:** A thorough understanding of glycogenolysis (glycogen breakdown) and glycogenesis (glycogen synthesis) is crucial for understanding glucose homeostasis and the regulation of blood glucose levels, particularly in conditions such as diabetes mellitus and hypoglycemia.