Methylmalonic acid is a dicarboxylic acid and byproduct of propionate catabolism.¹ It impairs mitochondrial respiration, increases glucose consumption, and downregulates the transcription of the mature neuronal markers neuron-specific enolase (ENO2) and synaptophysin (SYP) in neuronally differentiated SH-SY5Y cells. Intrastriatal administration of methylmalonic acid (100 mg/ml) induces neuronal loss in rats.² Methylmalonic acid accumulates in the tissues and body fluids of patients with methylmalonic acidemia, an inborn error of metabolism characterized by a deficiency in the activity of L-methylmalonyl-CoA mutase or its cofactor 5-deoxyadenosylcobalamin and leads to seizures, intellectual disabilities, psychomotor abnormalities, and coma.¹