## **Core Concept**
Pompe's disease, also known as Glycogen storage disease type II (GSD II), is a genetic disorder caused by the deficiency of a specific lysosomal enzyme. This condition leads to the accumulation of glycogen within lysosomes, primarily affecting muscles. The underlying principle here involves the breakdown and storage of glycogen, a complex carbohydrate used by the body for energy.

## **Why the Correct Answer is Right**
The correct answer, **Acid alpha-glucosidase**, is the enzyme deficient in Pompe's disease. Acid alpha-glucosidase (GAA) is crucial for breaking down glycogen into glucose within lysosomes. Without sufficient GAA activity, glycogen accumulates within lysosomes, particularly in muscle cells, leading to cellular dysfunction and the clinical manifestations of Pompe's disease, such as muscle weakness and cardiomegaly.

## **Why Each Wrong Option is Incorrect**
- **Option A: Glucokinase** - This enzyme is involved in glucose metabolism in the liver, facilitating the conversion of glucose to glucose-6-phosphate. Its deficiency leads to maturity-onset diabetes of the young (MODY 2), not Pompe's disease.
- **Option B: Debranching enzyme** - Deficiency of the debranching enzyme leads to Cori's disease or Glycogen storage disease type III, characterized by the accumulation of limit dextrins. This is not related to Pompe's disease.
- **Option D: Phosphorylase** - This enzyme is involved in glycogenolysis, and its deficiency results in McArdle's disease or Glycogen storage disease type V, primarily affecting muscle energy metabolism but distinct from Pompe's disease.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for Pompe's disease is that it presents with **cardiomegaly** and **muscle weakness**, particularly in infants, but can also have a late-onset form presenting in adulthood with muscle weakness. Early diagnosis and treatment, including enzyme replacement therapy (ERT) with recombinant human GAA, can significantly improve outcomes.

## **Correct Answer: C. Acid alpha-glucosidase**