**Core Concept**
Glycogen breakdown, also known as glycogenolysis, is a critical metabolic pathway that occurs in the liver and muscles. This process is essential for maintaining blood glucose levels, especially during periods of fasting or low food intake. The immediate degradation of glycogen yields glucose-1-phosphate.

**Why the Correct Answer is Right**
The enzyme responsible for the initial step of glycogenolysis is glycogen phosphorylase. This enzyme cleaves the alpha-1,4-glycosidic bonds between glucose residues in glycogen, producing glucose-1-phosphate. Glucose-1-phosphate is then converted to glucose-6-phosphate by the enzyme phosphoglucomutase. This glucose-6-phosphate can either be used for energy production or converted back to glucose and released into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphate is an intermediate in glycogen breakdown but is not the direct result of glycogen phosphorylase activity.
**Option B:** Glucose-1-phosphate is indeed a product of glycogen breakdown, but it's not the final product under normal conditions. The enzyme phosphoglucomutase further converts glucose-1-phosphate to glucose-6-phosphate.
**Option D:** Lactic acid is a product of anaerobic glycolysis, not a direct result of glycogen breakdown.

**Clinical Pearl / High-Yield Fact**
Glycogen storage diseases, such as Von Gierke's disease, result from defects in enzymes involved in glycogen metabolism, leading to impaired glycogen breakdown and accumulation of glycogen in the liver.

**Correct Answer: C. Glucose-1-phosphate. Glucose-1-phosphate**