(methylmalonic acidaemia, aminoacidopathy) a defect of amino acid metabolism causing an excess of methylmalonic acid in the urine and blood. There are two types: one is an *inborn error of metabolism due to a deficiency of the enzyme methylmalonyl-CoA mutase; the other is an acquired type due to deficiency of vitamin B12, which results in defective synthesis of adenosylcobalamin (a cofactor in this metabolic process).
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