A Study to Assess the Safety, Pharmacokinetics, and Pharmacodynamics of mRNA-3705 in Participants With Isolated Methylmalonic Acidemia

Key Inclusion Criteria: * (Part 1 only) Participant has a body weight of ≥11.0 kilograms at the screening visit. * Participant has a diagnosis of isolated MMA due to MUT deficiency confirmed by molecular genetic testing. * Participant has a blood vitamin B12 level equal to or above the lower limit of normal (based on laboratory reference range) confirmed in the screening period. * Participant or their legally authorized representative is willing and able to provide informed consent and/or assent as mandated by local regulations and is willing and able to comply with study-related assessments. * Sexually active participants of childbearing or reproductive potential agree to use a highly effective method of contraception, consistent with local regulations, during the study and for 3 months after the last administration of study drug. * (Part 2 only) Participants with 2 screening MMA levels ≥400 micromolar. * (Parts 2 and 3 only) Participant is ≥5 years of age at the time of informed consent/assent. Key Exclusion Criteria: * Participant has a diagnosis of isolated MMA cofactor adenosyl-cobalamin (cb1A, cb1B, or cb1D) enzymatic subtypes or methylmalonyl-CoA epimerase deficiency or combined MMA with homocystinuria. * Participant has previously received gene therapy for the treatment of MMA. * Participant has a history of organ transplantation or planned organ transplantation during the period of study participation. * Participant has an active, unstable, or clinically significant medical condition not related to MMA or history of noncompliance that, in the investigator's opinion, could potentiate the risk while participating in this study, interfere with the interpretation of study results, or limit the participant's participation in the study. This may include, but is not limited to, history of relevant food or drug allergies; history of cardiovascular, central nervous, gastrointestinal, or infectious disease; history of clinically significant pathology; and/or history of cancer. * (Part 2 only) Participant has the partial MUT deficiency disease phenotype, as assessed by genotyping, clinical phenotype/presentation, or vitamin B12-responsive MMA. Note: Additional inclusion/exclusion criteria may apply, per protocol.