Infants exhibiting profound metabolic ketoacidosis, muscular hypotonia, developmental retardation, and who have very large accumulations of methylmalonic acid in their blood and urine suffer from a disorder known as methylmalonic acidemia. This disorder results from a defect in which of the following enzymes?

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Methylmalonic acidemia is caused by a defect in the enzyme methylmalonyl-CoA mutase, which leads to the accumulation of methylmalonic acid.  Defects in methylmalonyl-CoA mutase activity comprise 4 distinct genotypes whose clinical symptoms are remarkably similar. Characteristic findings in methylmalonyl-CoA mutase deficiency include failure to thrive leading to developmental abnormalities, recurrent vomiting, respiratory distress, hepatomegaly, and muscular hypotonia. In addition, patients have severely elevated levels of methylmalonic acid in the blood and urine. Unaffected individuals have nearundetectable levels of methylmalonate in their plasma, whereas, affected individuals have been found to have levels ranging from 3 to 40 mg/dL in their blood.

Infants Exhibiting Profound Metabolic Ketoacidosis, Muscular Hypotonia, Developmental Retardation, and Who