ARSA and metachromatic leukodystrophy: Metachromatic leukodystrophy (MLD, OMIM #250100) is a rare, autosomal recessive disease caused by deficient activity of the lysosomal enzyme arylsulfatase A (ARSA), resulting in sulfatide accumulation and subsequent demyelination and neuronal loss within the central and peripheral nervous systems (CNS and PNS).