ARSA (Arylsulfatase A) is an enzyme that breaks down sulfatides — complex lipids present in many tissues, particularly in the nervous system. A deficiency in ARSA, often caused by genetic mutations, leads to the buildup of sulfatides and can result in Metachromatic Leukodystrophy (MLD), a rare disorder marked by progressive loss of nerve insulation (demyelination) and severe neurological decline.