Mucopolysaccharidoses (MPSs) are characterized by defective degradation and excessive storage of mucopolysaccharides in various tissues. Hepatosplenomegaly, skeletal deformities, lesions of hea valves, subendothelial aerial deposits, paicularly in the coronary aeries, and lesions in the brain, are features that are seen in all of the MPSs. Of the seven recognized variants, only two well-characterized syndromes are discussed briefly here. MPS type I, also known as Hurler syndrome, is caused by a deficiency of a-L-iduronidase. Accumulation of dermatan sulfate and heparan sulfate is seen in cells of the mononuclear phagocyte system, in fibroblasts, and within endothelium and smooth muscle cells of the vascular wall. The affected cells are swollen and have clear cytoplasm, resulting from the accumulation of material positive for periodic acid-Schiff staining within engorged, vacuolated lysosomes. Lysosomal inclusions also are found in neurons, accounting for the mental retardation. MPS type II or Hunter syndrome differs from Hurler syndrome in its mode of inheritance (X-linked), the absence of corneal clouding, and often its milder clinical course. Despite the difference in enzyme deficiency, an accumulation of identical substrates occurs because breakdown of heparan sulfate and dermatan sulfate requires both a-L-iduronidase and the sulfatase; if either one is missing, fuher degradation is blocked. Diagnosis is made by measuring the level of enzyme in leukocytes. Ref: ROBBINS BASIC PATHOLOGY 10th Ed. pg no: 260