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