Which laboratory finding is most characteristic of megaloblastic anemia due to vitamin B12 or folate deficiency?

• A. Low RBC count
• B. Low B12/folate
• C. Normal MCHC
• D. Low Platelet count

Answer: (B)

The key idea is that megaloblastic anemia from vitamin B12 or folate deficiency is driven by a shortage of the specific nutrient needed for DNA synthesis. Because these vitamins are essential cofactors for making thymidine and, consequently, for proper DNA replication in rapidly dividing cells, their deficiency directly shows up as low levels of the vitamin in the blood. So the most characteristic laboratory finding is a low serum vitamin level—either low B12 or low folate—depending on which deficiency is causing the anemia.

You’ll still often see macrocytosis with an elevated MCV, and peripheral smears may show macro-ovalocytes with hypersegmented neutrophils, reflecting the defective maturation. Other findings like a low RBC count or low platelets can occur but aren’t as specific to this condition, and MCHC is typically normal as a general rule.