## Vitamin B12 Deficiency — Biochemical Markers **Key Point:** Vitamin B12 (cobalamin) is an essential cofactor for two critical enzymes: methionine synthase and methylmalonyl-CoA mutase. ### Metabolic Consequences of B12 Deficiency When B12 is deficient, both enzyme pathways are impaired: 1. **Methylmalonyl-CoA mutase block** → accumulation of methylmalonic acid (MMA) 2. **Methionine synthase block** → accumulation of homocysteine Both MMA and homocysteine are excreted in elevated quantities in urine and serum, making them **pathognomonic biochemical markers** of B12 deficiency. ### Diagnostic Value | Parameter | B12 Deficiency | Folate Deficiency | B6 Deficiency | | --- | --- | --- | --- | | **Methylmalonic acid** | ↑↑ | Normal | Normal | | **Homocysteine** | ↑↑ | ↑ | Normal | | **Serum B12** | Low | Normal | Normal | **High-Yield:** The **simultaneous elevation of both MMA and homocysteine** is the biochemical hallmark that distinguishes B12 deficiency from folate deficiency (which raises only homocysteine). **Clinical Pearl:** In India, B12 deficiency is common due to vegetarian diets and pernicious anemia; checking serum MMA and homocysteine is more specific than serum B12 alone, which can be falsely normal in early deficiency.
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