A 65-year-old man visits his primary physician because of tingling in his hands and feet, and a sense that he is forgetting things more than usual. A CBC indicates a mild anemia. The patient states that his diet has not changed, other than eating more red meat than before. This patient can be best treated by which one of the following?

The patient has a deficiency of B12 , caused by inadequate intrinsic factor production for the absorption of dietary vitamin B12 , which is required for the conversion of methyl malonyl-CoA to succinyl-CoA and of homocysteine to methionine. A vitamin B12 deficiency results in the excretion of methylmalonic acid in the urine and an increased dietary requirement for methionine. The methyl group transferred from vitamin B12 to homocysteine to form methionine comes from N5-methyl-FH4 , which accumulates in a vitamin B12 deficiency, causing a decrease in free folate levels and symptoms of folate deficiency, including increased levels of FIGLU and decreased purine biosynthesis. Both B12 and folate deficiencies will lead to anemia, but only a B12 deficiency will give rise to the mental status changes and tingling in the hands and feet. As individuals age, a variety of conditions can give rise to reduced intrinsic factor production by the stomach. Because intrinsic factor is low, B12 needs to be delivered by injection, not by the oral route of administration.