**Core Concept**
Lysosomal transport defects are a group of rare genetic disorders caused by mutations in genes encoding proteins responsible for transporting molecules in and out of lysosomes. These defects lead to the accumulation of toxic substances within the lysosomes, causing cellular damage and disease.

**Why the Correct Answer is Right**
Cystinosis is a lysosomal storage disorder characterized by the accumulation of cystine within the lysosomes due to a defect in the lysosomal membrane transport protein cystinosin. The mutation leads to the inability of cystine to exit the lysosomes, resulting in its toxic accumulation. This accumulation causes cellular damage and eventually leads to the symptoms of cystinosis, including Fanconi syndrome, renal failure, and growth retardation. The defective transport protein is encoded by the CTNS gene.

**Why Each Wrong Option is Incorrect**
**Option B:** Gaucher's disease is a lysosomal storage disorder caused by a deficiency of the enzyme glucocerebrosidase, which breaks down glucocerebroside. It is not related to a lysosomal transport defect.
**Option C:** Metachromatic leukodystrophy is a lysosomal storage disorder caused by a deficiency of the enzyme arylsulfatase A, which breaks down sulfatides. It is not related to a lysosomal transport defect.
**Option D:** Tay-Sach's disease is a lysosomal storage disorder caused by a deficiency of the enzyme hexosaminidase A, which breaks down GM2 gangliosides. It is not related to a lysosomal transport defect.

**Clinical Pearl / High-Yield Fact**
Lysosomal storage disorders are a group of diseases characterized by the accumulation of toxic substances within the lysosomes due to enzyme deficiencies or transport defects. Understanding the underlying mechanisms of these disorders is crucial for diagnosis and treatment.

✓ Correct Answer: A. Cystinosis