Metachromatic leukodystrophy, also known as MLD, is a rare, inherited neurological disorder that affects the nervous system. It is characterized by a deficiency of the enzyme arylsulfatase A, which leads to the accumulation of a fatty substance called sulfatide in the brain and nervous system. This toxic build-up causes damage to the myelin sheath that covers nerve fibers. Synonyms for metachromatic leukodystrophy include arylsulfatase A deficiency, late infantile metachromatic leukodystrophy, juvenile metachromatic leukodystrophy, and adult-onset metachromatic leukodystrophy. While there is no cure for MLD, treatment options aim to manage symptoms and improve quality of life for those affected.