ARSA and metachromatic leukodystrophy: Metachromatic leukodystrophy (MLD) is an autosomal recessive inherited lysosomal storage disorder caused by a deficiency in the lysosomal enzyme arylsulfatase A (ARSA), which catalyzes the degradation of galactosyl-3-sulfate ceramide (sulfatide), a major myelin sphingolipid1.