Methylmalonic acidemia (MMA) is an autosomal recessive disorder of methylmalonate and cobalamin (cbl; vitamin B12) metabolism. It results commonly from a defect in the gene encoding methylmalonyl CoA mutase apoenzyme MCM (mut MMA or vitamin B12-unresponsive MMA). This work highlights four aspects of methylmalonic aciduria diagnosis: First, it presents an overview of the diagnostic evaluation of patients with organic acidurias and several other IEMs by liquid chromatography- tandem mass spectrometry (LC-MS/MS), gas chromatography- mass spectrometry (GC-MS) and isocratic "high-performance...