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:
A. Elevated serum iron and ferritin with low transferrin saturation
B. Positive intrinsic factor antibodies
C. Elevated methylmalonic acid and homocysteine levels
D. Neurological manifestations such as subacute combined degeneration
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-YieldNEET PG
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
Table
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.
