A Natural History Study of Biomarkers and Clinical Outcomes in Mucopolysaccharidosis Type IIIA (MPS IIIA; Sanfilippo Syndrome)

Inclusion Criteria: * Age of enrollment is as follows: 1. All participants must be between 4 months and \< 14 years at time of consent 2. At least 10 participants (ie, approx. 50% of expected total population) must be between 4 months old and \< 4 years old at the time of consent * The participant has a confirmed diagnosis of Mucopolysaccharidosis Type IIIA (MPS IIIA) (i.e., Sanfilippo syndrome) based on at least 2 out of the 3 following criteria: 1. Documented reduced N-sulphoglucosamine sulphohydrolase (SGSH) activity in plasma, white blood cells, and/or skin fibroblasts consistent with MPS IIIA (10% or less of the lower limit of the normal range based on the testing laboratory) 2. Documented likely pathogenic variant of the SGSH gene 3. Documented elevated heparan sulfate levels in urine and/or blood Participants and their families are willing to complete 7 blood draws and 7 Vineland-3 collections Exclusion Criteria: * Known history of HIV, hepatitis, or other infectious diseases * Taken an investigational product in the last 30 days * Experienced excess blood loss, including blood donation, defined as 80 mL in the last month or 160 mL in the previous two months * Undergone an MPS IIIA gene therapy at any time unless prior Sponsor or designee's approval has been received * Documented loss of activity of sulfatases other than N-sulphoglucosamine sulphohydrolase (SGSH), indicating multiple sulfatase deficiency * Known genetic or acquired disorder associated with developmental delay, seizures or other significant CNS dysfunction that would be expected to confound the clinical or biomarker assessments