A 38-year-old woman from rural India presents with fatigue, dyspnea on exertion, and glossitis. Laboratory studies show hemoglobin 7.2 g/dL, MCV 72 fL, serum B12 level 150 pg/mL (normal >200), and serum folate 2.8 ng/mL (normal >5.4). Peripheral blood smear shows hypersegmented neutrophils and target cells. All of the following would be expected findings in this patient EXCEPT:

Explanation

## Combined B12 and Folate Deficiency: Clinical and Laboratory Profile **Key Point:** This patient has combined B12 and folate deficiency (both low), presenting with macrocytic features (hypersegmented neutrophils) and microcytic features (MCV 72 fL, target cells). The question tests understanding of B12 deficiency pathophysiology and distinguishing it from iron metabolism abnormalities. ### Pathophysiology of B12 Deficiency **High-Yield:** B12 is essential for: 1. DNA synthesis (via methylation) → megaloblastic anemia 2. Myelin formation → neurological complications 3. Homocysteine metabolism → elevated homocysteine and methylmalonic acid ### Expected Findings in B12 Deficiency | Finding | Mechanism | Present in This Case? | | --- | --- | --- | | Elevated methylmalonic acid | Impaired methylmalonyl-CoA mutase activity | Yes ✓ | | Elevated homocysteine | Impaired methionine synthase | Yes ✓ | | Intrinsic factor antibodies | Autoimmune pernicious anemia | Possible (if PA is cause) ✓ | | Elevated serum iron/ferritin | B12 deficiency does NOT affect iron metabolism | No ✗ | | Subacute combined degeneration | Demyelination of dorsal/lateral spinal cord tracts | Yes ✓ | **Clinical Pearl:** B12 deficiency does NOT cause iron overload or abnormal iron metabolism. Iron stores are independent of B12 status. Elevated serum iron and ferritin would suggest secondary iron overload (from transfusions or hemochromatosis), not B12 deficiency. ### Why Option 3 Is Wrong Elevated serum iron and ferritin with low transferrin saturation is NOT an expected finding in B12 deficiency. This pattern is inconsistent with B12 pathophysiology: - B12 does not regulate iron absorption or storage - Elevated ferritin + low transferrin saturation suggests iron sequestration (as in anemia of chronic disease), not B12 deficiency - This patient's anemia is due to impaired DNA synthesis, not iron metabolism **Warning:** Do not confuse the microcytic features in this patient (MCV 72, target cells) with iron deficiency. The microcytosis here is due to **concurrent folate deficiency** or chronic disease, not iron deficiency. Iron studies would be normal or elevated, not depleted. **Mnemonic:** **B12 Metabolic Effects** — **DNA** synthesis (megaloblastosis), **Myelin** formation (neurological), **Homocysteine** metabolism (elevated levels).