The level of methylmalonic acid in the blood and urine and the level of homocysteine in the plasma increase due to the reduced activity of the methylmalonyl-CoA mutase enzyme and methionine synthase enzymes in vitamin B12 deficiency.12 The levels of plasma homocysteine and methylmalonic acid in the urine and the levels of total, direct, and indirect bilirubin were compared, and once again, there was no significant difference. Here, MMUT is linked to vitamin B12 deficiency.