A 57-year-old woman reports increasing symptoms of painful paresthesias in both
legs for the past 18 months. Physical examination reveals impaired position sense
and vibration sense. The serum vitamin B12 level is 205 pg per milliliter (151.2 pmol
per liter), which is above the lower end of the laboratory reference range. The hematocrit
is 42%, with a mean corpuscular volume of 96 fl. The serum methylmalonic acid
level is 3600 nmol per liter (normal level,