Metachromatic leukodystrophy is a lysosomal storage disease associated with the synthesis of abnormal my­elin. The term leukodystrophy refers to the abnormality in myelin synthesis. Metachromatic leukodystrophy is an autosomal recessive disease characterized by an arylsulfatase. A deficiency, leading to the accumulation of sulfatides in lysosomes that stain positively with periodic acid-Schiff stain and various metachromatic stains. The clinical findings include various visceral lesions, mental retardation, peripheral neuropathy, and abnormal myelination in the central nervous sys­tem associated with reactive gliosis. It is diagnosed by the absence of arylsulfatase A in the urine. Tay-Sachs disease is an autosomal recessive GM2 gangliosidosis characterized by a hexosaminidase ( ­subunit) deficiency and an accumulation of GM2 gan­glioside in lysosomes. It is commonly found in Ash­kenazi Jews, in which there is a 1 in 30 carrier rate. The patients are normal at birth but develop abnormal­ities by 6 months of age, including severe mental retardation, blindness (cherry red spot in the macula), and muscle flaccidity. The lipid has a whorled configuration in lysosomes when viewed by electron micros­copy. It is a uniformly fatal disease. Niemann-Pick disease is an autosomal recessive ly­sosomal storage disease associated with a sphingomye­linase deficiency and the accumulation of sphingomye­lin in macrophages and neurons. The type A variant is most common. It is characterized by severe mental retardation, hepatosplenomegaly, deterioration of psy­chomotor function, and foamy macrophages. Zebra bodies are noted in the lysosomes when viewed by electron microscopy. The disease is fatal in early life.