A 62-year-old woman is evaluated for progressively worsening fatigue and dyspnea on exertion over the past 3 months. She has followed a strict vegan diet for the past 5 years and does not take any nutritional supplements. Physical examination reveals pale conjunctivae, a smooth and red tongue, and decreased vibratory sensation in the lower extremities bilaterally. Laboratory studies show:
| Test | Result | Reference Range |
|---|---|---|
| Hemoglobin | 9.2 g/dL | 12.0-15.5 g/dL |
| Mean corpuscular volume | 114 fL | 80-100 fL |
Which of the following additional laboratory findings is most likely to be present in this patient?
- Elevated methylmalonic acid and elevated homocysteineAnswer
- BNormal methylmalonic acid and elevated homocysteine
- CDecreased serum ferritin and elevated total iron-binding capacity
- DPositive direct antiglobulin test and elevated reticulocyte count
- ENormal methylmalonic acid and normal homocysteine
Answer
Elevated methylmalonic acid and elevated homocysteine
The patient's strict vegan diet, macrocytic anemia (MCV > 100 fL), glossitis, and neurologic symptoms (decreased vibratory sensation) are classic for vitamin B12 (cobalamin) deficiency. Vitamin B12 is a required cofactor for two major reactions: the conversion of methylmalonyl-CoA to succinyl-CoA and the conversion of homocysteine to methionine. Therefore, a deficiency in vitamin B12 leads to the accumulation and elevated serum levels of both methylmalonic acid and homocysteine.
Step-by-Step Solution
Key Concept
Patients with a strict vegan diet are at risk for vitamin B12 deficiency, which presents with macrocytic anemia, glossitis, and neurologic deficits, and is confirmed by elevated levels of both methylmalonic acid and homocysteine.