## **Core Concept**
The question describes a patient with pain in calf muscles during exercise and the presence of glycogen in muscle biopsy. This scenario suggests a disorder of glycogen metabolism, specifically one that affects the breakdown of glycogen to provide energy during physical activity.

## **Why the Correct Answer is Right**
The correct answer, **McArdle's disease**, is caused by a deficiency of the enzyme **myophosphorylase**. This enzyme is crucial for the breakdown of glycogen to glucose-1-phosphate in muscle cells. In its absence, muscles cannot properly utilize glycogen as an energy source during exercise, leading to muscle cramps, pain, and fatigue. The presence of glycogen in the muscle biopsy is a hallmark of this condition, as the glycogen cannot be broken down.

## **Why Each Wrong Option is Incorrect**
- **Option A:** A deficiency in **debranching enzyme** leads to Cori's disease (Glycogen Storage Disease Type III), which primarily affects the liver and has neurological manifestations, not just muscle symptoms during exercise.
- **Option B:** A deficiency in **phosphofructokinase** leads to Tarui's disease (Glycogen Storage Disease Type VII), another glycogen storage disease that affects muscles but through a different enzymatic pathway.
- **Option D:** A deficiency in **alpha-glucosidase** leads to Pompe disease (Glycogen Storage Disease Type II), characterized by the accumulation of glycogen in lysosomes of muscle cells, leading to severe muscle weakness.

## **Clinical Pearl / High-Yield Fact**
A key clinical feature of McArdle's disease is the "second wind" phenomenon, where patients experience relief from symptoms after a short rest, due to the switch to fatty acid oxidation for energy. This condition highlights the importance of glycogen metabolism in muscle function during exercise.

## **Correct Answer:** . **Myophosphorylase**