**Core Concept**
Lysosomal storage disorders are a group of genetic conditions caused by deficiencies in enzymes responsible for breaking down and recycling cellular components. These disorders lead to the accumulation of toxic substances within cells, resulting in cellular damage and dysfunction. Mental retardation is a common feature of several lysosomal storage disorders due to the impact of these conditions on brain development and function.

**Why the Correct Answer is Right**
Tay Sach's disease, Fabry's disease, and Metachromatic leucodystrophy are all lysosomal storage disorders associated with mental retardation. Tay Sach's disease is caused by a deficiency in the enzyme hexosaminidase A, leading to the accumulation of GM2 ganglioside within neurons, which results in progressive neurological deterioration, including mental retardation. Fabry's disease is caused by a deficiency in the enzyme alpha-Galactosidase A, resulting in the accumulation of globotriaosylceramide within cells, causing cellular dysfunction and leading to mental retardation. Metachromatic leucodystrophy is caused by a deficiency in the enzyme Arylsulfatase A, leading to the accumulation of sulfatides within neurons, which results in demyelination and neuronal dysfunction, including mental retardation.

**Why Each Wrong Option is Incorrect**
**Option A:** Metachromatic leucodystrophy primarily affects the peripheral nerves and spinal cord, with less prominent involvement of the central nervous system, making it less commonly associated with mental retardation compared to Tay Sach's disease and Fabry's disease.

**Option B:** While Fabry's disease can lead to mental retardation, it is less commonly associated with this condition compared to Tay Sach's disease.

**Clinical Pearl / High-Yield Fact**
Lysosomal storage disorders often present with a wide range of systemic symptoms, including neurodegeneration, and should be considered in the differential diagnosis of patients with unexplained neurological deterioration or systemic symptoms.

**Correct Answer Line**
✓ Correct Answer: D. All of the above