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