Methylmalonic acid is a malonic acid derivative, which is a vital intermediate in the metabolism of fat and protein. In particular, the coenzyme A-linked form of methylmalonic acid, methylmalonyl-CoA, is converted into succinyl-CoA by methylmalonyl-CoA mutase in a reaction that requires vitamin B12 as a cofactor. In this way, methylmalonic acid enters the Krebs cycle and is thus part of one of the anaplerotic reactions. Abnormalities in methylmalonic acid metabolism lead to methylmalonic aciduria. This inborn error of metabolism is attributed to a block in the enzymatic conversion of methylmalonyl CoA to succinyl CoA. Methylmalonic acid is also found to be associated with other inborn errors of metabolism, including cobalamin deficiency, cobalamin malabsorption, malonyl-CoA decarboxylase deficiency, and transcobalamin II deficiency. When present in sufficiently high levels, methylmalonic acid can act as an acidogen and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of methylmalonic acid are associated with at least 5 inborn errors of metabolism, including Malonyl CoA decarboxylase deficiency, Malonic Aciduria, Methylmalonate Semialdehyde Dehydrogenase Deficiency, Methylmalonic Aciduria and Methylmalonic Aciduria Due to Cobalamin-Related Disorders. Methylmalonic acid is an organic acid and abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart abnormalities, kidney abnormalities, liver damage, seizures, coma, and possibly death. These are also the characteristic symptoms of the untreated IEMs mentioned above. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures.