mucopolysaccharidosis type 3C (MONDO_0009657, a Monarch Disease Ontology entry) is a rare autosomal recessive lysosomal storage disease caused by deficiency of the enzyme acetyl-CoA:alpha-glucosaminide acetyltransferase. It is characterized by behavioral changes, sleep disturbances, and mental developmental delays. Also known as: HGSNAT deficiency, MPS III C, MPS3C, MPSIIIC, Sanfilippo C, Sanfilippo syndrome type C, heparan-alpha-glucosaminide N-acetyltransferase deficiency, mucopolysaccharidosis type IIIC.