A 68-year-old woman presents to the clinic with progressive fatigue and a tingling sensation in her hands and feet for the past month. She has a history of Crohn disease and underwent an ileal resection five years ago. Laboratory studies show a hemoglobin of 9.2 g/dL and an MCV of 112 fL. A peripheral blood smear reveals hypersegmented neutrophils.
Which of the following laboratory profiles would most definitively distinguish vitamin B12 deficiency from folate deficiency in this patient?
The correct answer is:
A) Elevated homocysteine and elevated methylmalonic acid
Both vitamin B12 and folate are required for the conversion of homocysteine to methionine. Therefore, a deficiency in either will lead to elevated serum homocysteine. However, only vitamin B12 is a cofactor for the enzyme methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA. In vitamin B12 deficiency, both homocysteine and methylmalonic acid (MMA) are elevated. In folate deficiency, homocysteine is elevated, but MMA is normal. In this clinical vignette, the patient's history of ileal resection is a major clue, as the terminal ileum is the primary site of vitamin B12 absorption. Furthermore, her paresthesias suggest neurological involvement (subacute combined degeneration), which is unique to B12 deficiency.
Answer choice B: Elevated homocysteine and normal methylmalonic acid, is incorrect. This profile is diagnostic of folate deficiency. While both cause megaloblastic anemia, the presence of neurological symptoms and the patient's surgical history make B12 deficiency the more likely diagnosis.
Answer choice C: Low reticulocyte count and elevated indirect bilirubin, is incorrect. These findings are common to all megaloblastic anemias. The low reticulocyte count reflects ineffective erythropoiesis (underproduction), and the elevated indirect bilirubin reflects intramedullary hemolysis (destruction of fragile megaloblasts before they leave the marrow).
Answer choice D: Presence of Howell-Jolly bodies on peripheral smear, is incorrect. Howell-Jolly bodies (nuclear remnants in RBCs) are typically seen in patients with functional or surgical asplenia. They are not a specific finding for megaloblastic anemia.
Answer choice E: Normal homocysteine and elevated methylmalonic acid, is incorrect. Homocysteine is virtually always elevated in B12 deficiency. A normal homocysteine level would significantly decrease the likelihood of either deficiency.
Key Learning Point
The primary biochemical differentiator between vitamin B12 deficiency and folate deficiency is methylmalonic acid (MMA).
High MMA + high homocysteine = Vitamin B12 deficiency.
Normal MMA + high Hhmocysteine = Folate deficiency.
Additionally, remember that B12 deficiency presents with neurological deficits (dorsal column and lateral corticospinal tract involvement), whereas folate deficiency does not.
