• Diagnosis of MPS IIIA confirmed by the following methods:

‣ No detectable or significantly reduced SGSH enzyme activity by leukocyte assay, and

⁃ Genomic DNA analysis demonstrating homozygous or compound heterozygous mutations in the SGSH gene (based upon review of documented results from a qualified laboratory, and with confirmation with Medical Monitor)

• Age:

‣ For Cohort 1-3: From birth (participating sites in USA and Australia) OR 6 months (participating sites in Spain) to 2 years of age with no BSITD-III Cognitive Development Quotient (DQ) requirement, or older than 2 years with a BSITD-III Cognitive DQ of 60 or above (participating sites globally).

⁃ For Cohort 4 (participating sites in Spain): 3 months to ≤ 2 years of age with no BSITD-III Cognitive DQ requirement, or \> 2 years of age with a BSITD-III Cognitive DQ of 60 or above at Screening Visit 1. Up to 2 additional patients \> 2 years and ≤ 5 years of age with a BSITD-III Cognitive DQ \< 60 at Screening Visit 1 may be enrolled to characterize the optimized prophylactic IM therapy in this more advanced disease patient population.

• Cohort 4 only: Vaccination status based on age according to country-specific guidelines that is up to date 30 days prior to Screening as verified by documentation from the subject's primary care physician, and willing to defer vaccines through 6 months after completion of the subject's IM medication, or longer per Principal Investigator (PI) judgment. Emergency use authorization of coronavirus disease (COVID) vaccines is included unless there is an accepted medical exemption.