ARSA and lysosomal storage disease: Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder (LSD) caused by mutations in the gene coding for the lysosomal enzyme arylsulfatase A (ARSA) (EC 3.1.6.8), a key enzyme in the catabolism of myelin‐enriched sphingolipids 27, 28.