A 26-year-old pregnant woman at 20 weeks gestation has a hemoglobin of 9 g/dL, MCV 104 fL, and hypersegmented neutrophils. Serum folate is low and homocysteine is elevated, but methylmalonic acid is normal. Why is her MMA normal despite overt megaloblastic anemia?
- A The fetus supplies methylmalonic acid scavenging enzymes across the placenta
- B Pregnancy suppresses renal reabsorption of methylmalonic acid
- C Folate deficiency does not affect the methylmalonyl-CoA mutase pathway, which requires adenosylcobalamin ✓
- D Homocysteine elevation competitively inhibits MMA accumulation
Correct answer: C. Folate deficiency does not affect the methylmalonyl-CoA mutase pathway, which requires adenosylcobalamin
Explanation
Methylmalonic acid accumulates only when B12 (adenosylcobalamin) is deficient because methylmalonyl-CoA mutase depends on it. Folate deficiency impairs thymidylate synthesis via reduced 5,10-methylene THF, causing megaloblastosis and raised homocysteine through the methionine synthase pathway, but MMA stays normal. This distinction separates folate from B12 deficiency and predicts absence of neurological signs here.
Reference: Williams Obstetrics, 25th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.