**Core Concept**
Hexosaminidase A (Hex-A) is a lysosomal enzyme responsible for breaking down GM2 gangliosides in neurons. Deficiency or dysfunction of this enzyme leads to the accumulation of GM2 gangliosides, resulting in a severe neurodegenerative disorder.

**Why the Correct Answer is Right**
Hex-A deficiency is the primary cause of Tay-Sachs disease, an autosomal recessive disorder characterized by the accumulation of GM2 gangliosides within neurons. This accumulation leads to neuronal damage and death, resulting in progressive neurodegeneration. The deficiency is due to mutations in the HEXA gene encoding the alpha-subunit of hexosaminidase A. The enzyme is crucial for the breakdown of GM2 gangliosides, and its deficiency disrupts the normal degradation pathway, causing the toxic accumulation of these lipids.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Sandhoff disease is caused by a deficiency of both hexosaminidase A and B, not just hexosaminidase A.

**Option B:** This option is incorrect as Fabry disease is caused by a deficiency of alpha-Galactosidase A, not hexosaminidase A.

**Option C:** This option is incorrect as Niemann-Pick disease is caused by a deficiency of sphingomyelinase, not hexosaminidase A.

**Clinical Pearl / High-Yield Fact**
Tay-Sachs disease is an autosomal recessive disorder, meaning that both parents must be carriers of the mutated HEXA gene to pass the disease to their offspring.

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