Inclusion Criteria: * At the time of dosing, participants must be 6 months to 12 years of age * Males and females with diagnosis of severe MMA meeting all the following; 1. Isolated MMA with genetically confirmed, pathogenic mutations in the MMUT gene 2. Screening serum/plasma methylmalonic acid level of \>100 µmol/L 3. One or more of the following considered by the PI to be MMA-related: (i) An unscheduled ER visit, hospitalization or requirement for sick day diet in the year prior to screening visit (ii) Developmental delay, movement disorder, optic neuropathy or feeding disorder with tube feeding requirement 4. Medically stable for the 2 months prior to the start of screening Exclusion Criteria: * Participants with organic acidemias other than isolated MMA, or with any other causes of hyperammonemia * Having received MMA-targeted gene therapy or nucleic acid therapy * Participants on insulin or high dose hydroxocobalamin (\> 1 mg/day OHB12 parenteral) * Kidney or liver transplant, including hepatocyte cell therapy * Estimated glomerular filtration rate (eGFR) of \< 60 mL/min/1.73 m2 based on age appropriate equations, or ongoing dialysis for renal disease * Participant tests positive for anti-rAAV-LK03-neutralizing antibodies