**Core Concept**
Beal's syndrome, also known as late-onset Tay-Sachs disease, is a rare genetic disorder characterized by a deficiency of the enzyme hexosaminidase A (Hex-A). This deficiency leads to the accumulation of GM2 gangliosides within neurons, causing progressive neurodegeneration.

**Why the Correct Answer is Right**
The correct answer is a defect in the **HEXA gene**, which encodes for the alpha-subunit of the hexosaminidase A enzyme. This specific mutation disrupts the normal function of Hex-A, resulting in the inability to break down GM2 gangliosides. Without Hex-A, these gangliosides accumulate within neurons, leading to the characteristic symptoms of Beal's syndrome.

**Why Each Wrong Option is Incorrect**
* **Option A:** The **GLA gene** is associated with Fabry disease, a different lysosomal storage disorder caused by a deficiency of the enzyme alpha-Gal A.
* **Option B:** The **GAA gene** is linked to glycogen storage disease type II (Pompe disease), a condition caused by a deficiency of the enzyme acid alpha-glucosidase.
* **Option D:** The **GBA gene** is associated with Gaucher disease, another lysosomal storage disorder caused by a deficiency of the enzyme glucocerebrosidase.

**Clinical Pearl / High-Yield Fact**
Beal's syndrome is a rare but important consideration in the differential diagnosis of late-onset neurodegenerative disorders, particularly in individuals of Ashkenazi Jewish descent, who have a higher risk of carrying the HEXA gene mutation.

**Correct Answer: C. HEXA gene.**