ARSA and lysosomal storage disease: Metachromatic leukodystrophy (MLD) is a type of lysosomal storage disorder that affects sphingolipid metabolism and is classified as both leukodystrophy and sphingolipidosis.1 MLD is characterized by neurodegeneration with progressive demyelination.2 It is most commonly associated with a mutation in the ARSA gene,3 which is mapped to chromosome 22 (22q13.33).