Ethylmalonic acid, also known as alpha-carboxybutyric acid or ethylmalonate, is a member of the class of compounds known as branched fatty acids. Branched fatty acids are fatty acids containing one or more branched chains. Ethylmalonic acid is soluble (in water) and a moderately acidic compound (based on its pKa). Ethylmalonic acid can be synthesized from malonic acid, and can be synthesized into (S)-ethylmalonyl-CoA and (R)-ethylmalonyl-CoA. Ethylmalonic acid can be found in blood, cerebrospinal fluid (CSF), and urine, as well as in human fibroblasts, prostate, and skeletal muscle tissues. Moreover, ethylmalonic acid is found to be associated with anorexia nervosa and malonyl-CoA decarboxylase deficiency. Ethylmalonic acid is a non-carcinogenic (not listed by IARC) potentially toxic compound. Chronically high levels of ethylmalonic acid are associated with at least two inborn errors of metabolism, including short-chain acyl-CoA dehydrogenase deficiency (SCAD deficiency) and ethylmalonic encephalopathy. Ethylmalonic acid is identified in the urine of patients with short-chain acyl-CoA dehydrogenase deficiency, which is a fatty acid metabolism disorder. When present at sufficiently high levels, ethylmalonic 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. Ethylmalonic acid is an organic acid. 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.