**Core Concept**
The question is testing knowledge of **lysosomal storage diseases**, specifically the enzyme **hexosaminidase** and its role in breaking down **glycosaminoglycans**. A defect in this enzyme leads to accumulation of toxic substances within cells.

**Why the Correct Answer is Right**
A defect in **hexosaminidase** leads to **Tay-Sachs disease**, a genetic disorder causing progressive destruction of nerve cells in the brain and spinal cord. This enzyme is crucial for breaking down **GM2 gangliosides**, and its deficiency results in their accumulation, leading to neurodegeneration.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the disease associated with hexosaminidase deficiency is not related to the breakdown of **glycogen**.
**Option B:** This option is incorrect as it is associated with a different enzyme deficiency.
**Option C:** This option is also incorrect as the disease caused by hexosaminidase deficiency is not primarily related to **mucopolysaccharide** metabolism.

**Clinical Pearl / High-Yield Fact**
Remember, **Tay-Sachs disease** is a classic example of a **lysosomal storage disease**, and its diagnosis is often suspected in infants with progressive neurological deterioration and characteristic **cherry-red macular spots**.

**Correct Answer:** D. Tay-Sachs disease