Methylmalonic acidemia is an inborn error of vitamin B12 metabolism characterized by gastrointestinal and neurometabolic manifestations resulting from decreased function of the mitochondrial enzyme methylmalonyl-CoA mutase. The disease is Orphanet rare-disease nomenclature identifier Orphanet_293355 (Methylmalonic acidemia without homocystinuria). Also known as: Methylmalonic aciduria without homocystinuria.