Abstract: Malonyl-CoA decarboxylase deficiency is an extremely rare autosomal recessive inborn error of fatty acid metabolism. In this study, a Chinese boy was diagnosed with malonyl-CoA decarboxylase deficiency and a novel mutation in the MLYCD gene by the clinical manifestation, Carntine tandem mass spectrometry, analysis of urinary organic acids, imaging examination, genetic features and so on. The patient presented with feeding difficulty, developmental retardation, cardiomyopathy, white matter hyperintensity. Urine organic acids showed marked elevation of malonic acid (901.28mmol/mol creatinine; normal range: <0.10mmol/mol creatinine) and methylmalonic acid (124.86mmol/mol creatinine; normal range: <3.60 mmol/mol creatinine). the levels of malonylcarnitine elevated mildly( 0.86umol/L; normal range: <0.60umol/L), which supported the dignosis of malonyl-CoA decarboxylase deficiency. DNA sequencing

of the MLYCD gene of the boy identified a novel double heterozygous mutation( c.401delT, p.V134fs) and (c.1293G>T, p.W431C), which confirmed the diagnosis of malonyl-CoA decarboxylase deficiency. Treatment with carnitine supplementation together with a low-fat, high-MCT, and high-carbohydrate diet, the neuromotor development progressed mildly and the cardiomyopathy without further exacerbation. The cinical manifestations of MCD are nonspecific. Metabolic detection, imaging examinaion and MLYCD gene analysis play a pivotal role in diagnosis of the disorder.