Methylmalonic acidemia is an autosomal recessive disorder, caused by complete or partial deficiency of the enzyme methylmalonyl-CoA mutase, a defect in the transport or synthesis of its cofactor, adenosyl-cobalamin, or deficiency of the enzyme methylmalonyl-CoA epimerase. Onset of the manifestations is usually in early infancy, and varies from mild to life threatening. These manifestations include: lethargy, failure to thrive, recurrent vomiting, dehydration, respiratory distress, muscle hypotonia, hepatomegaly and coma. Patients may also show signs of anemia (not megaloblastic), have potentially life-threatening ketoacidosis and/or hyperammonemia, and developmental delay and intellectual deficit, with metabolic stroke affecting the brain stem.