## **Core Concept**
Glycogen storage diseases (GSDs) are a group of inherited disorders that result from defects in enzymes involved in glycogen synthesis or breakdown. These disorders affect various tissues, including the liver, muscles, and heart. Muscle involvement in GSDs often presents with symptoms like muscle cramps, weakness, and exercise intolerance.

## **Why the Correct Answer is Right**
The correct answer, **McArdle's disease (Glycogen Storage Disease Type V)**, is characterized by a deficiency of the enzyme **myophosphorylase**, which is crucial for glycogen breakdown in muscle cells. This deficiency leads to an inability to mobilize glycogen stores during exercise, resulting in muscle cramps, fatigue, and myoglobinuria after intense physical activity. McArdle's disease predominantly affects muscles, making it a classic example of a GSD with muscle involvement.

## **Why Each Wrong Option is Incorrect**
- **Option A: Pompe disease (Glycogen Storage Disease Type II)** primarily affects the heart and skeletal muscles due to a deficiency of **acid alpha-glucosidase**. While it does involve muscles, it's not limited to muscle involvement and has significant cardiac implications.
- **Option B: von Gierke's disease (Glycogen Storage Disease Type I)** mainly affects the liver, leading to severe hypoglycemia, hepatomegaly, and lactic acidosis. It does not predominantly involve muscles.
- **Option D: Cori's disease (Glycogen Storage Disease Type III)** involves a deficiency of the **debranching enzyme** and affects the liver and muscles but is not characterized by predominant muscle involvement.

## **Clinical Pearl / High-Yield Fact**
A key clinical feature of McArdle's disease is the **"second wind" phenomenon**, where patients experience relief of symptoms after a brief rest, due to the switch to fatty acid oxidation for energy. This phenomenon is a classic clue for the diagnosis of McArdle's disease.

## **Correct Answer:** . **McArdle's disease**