Mucopolysaccharidosis type IIIA (MPS3A), also known as Sanfilippo syndrome type A, is an inherited lysosomal storage disorder that mainly affects the brain and spinal cord. It is due to a deficiency of the N-sulfonoglucosamine sulfohydrolase enzyme. Children with MPS3A appear normal at birth, but develop symptoms during early childhood. Early symptoms of MPS3A include delayed speech and behavior problems. Children with MPS3A will have worsening intellectual disability and will lose skills they had previously learned, such as crawling and walking. They will eventually develop seizures, movement problems, and hearing and vision loss. People with MPS3A may live into early adulthood. There is no cure for MPS3A; however medical surveillance and care may help to improve some symptoms and overall condition of life. MPS3A is caused by pathogenic variants in the SGSH gene.