异染性脑白质退化症(Meta-chromatic leuko-dystrophy、MLD，亦称:异染性脑白质营养不良症、芳基硫酸酯酶A缺乏症<Arylsulfatase A deficiency/ARSA deficiency>、Greenfield's disease)为一种溶酶体贮积症(lysosomal storage disease)，将MLD的神经纤维切片以结晶紫染色结果不呈紫色而呈黄褐色、故名之"异染性"(meta-chromatic)，一般归于脑白质营养不良(leukodystrophies)的类别、同于鞘脂类代谢障碍(sphingolipidoses)，因其影响神经鞘脂质(sphingolipids)的代谢。而脑白质营养不良影响髓磷脂的"生长"和/或"发展"，而"脂肪"的被覆是作为周围神经纤维的"阻隔体"(insulator)贯穿整个中枢和外周的神经系统。MLD亦涉及脑苷脂的积累。 异染性脑白质退化症、如同大多数的"酶缺陷"(enzyme deficiency)一般，存有「常染色体隐性遗传模式」。

Metachromatic leukodystrophy (MLD, also called Arylsulfatase A deficiency) is a lysosomal storage disease which is commonly listed in the family of leukodystrophies as well as among the sphingolipidoses as it affects the metabolism of sphingolipids. Leukodystrophies affect the growth and/or development of myelin, the fatty covering which acts as an insulator around nerve fibers throughout the central and peripheral nervous systems. MLD involves cerebroside sulfate accumulation. Metachromatic leukodystrophy, like most enzyme deficiencies, has an autosomal recessive inheritance pattern.