ARSA and lysosomal storage disease: Metachromatic leukodystrophy (MLD) is an autosomal recessive lysosomal storage disease caused by a deficiency of the enzyme arylsulfatase A.1 Arylsulfatase A deficiency results in the ubiquitous accumulation of sulfatide substrates (3‐O‐sulfogalactosylceramides) in lysosomes.