ARSA and metachromatic leukodystrophy: Metachromatic leukodystrophy (MLD) is a rare inherited lysosomal disorder caused by recessive gene mutations in ARSA (OMIM: 607574) and PSAP (Kihara, 1982; Kihara, Fluharty, O'Brien, & Fish, 1982; van Rappard, Boelens, & Wolf, 2015; Schielen, Kemper, & Gelb, 2017).