## Why option 1 is correct The enzyme marked **B** (methylmalonyl-CoA mutase) catalyzes the conversion of methylmalonyl-CoA → succinyl-CoA and requires B12 as a cofactor but is **independent of folate**. In contrast, methionine synthase (the enzyme responsible for converting homocysteine → methionine) requires **both B12 and methyl-THF (derived from folate)**. Therefore, elevated MMA is **specific for B12 deficiency** because it reflects impaired function of a B12-only enzyme. Homocysteine rises in **both B12 and folate deficiency** because methionine synthase is blocked by either deficiency, making it non-discriminatory. This biochemical distinction is the clinical basis for using MMA (and methylmalonic aciduria) to confirm B12 deficiency when homocysteine alone is ambiguous. [Harper 32e Ch 27; Harrison 21e Ch 95] ## Why each distractor is wrong - **Option 2**: This is factually backwards. Homocysteine is elevated in **both** B12 and folate deficiency (not just B12), whereas MMA is elevated **only** in B12 deficiency. This reversal makes it incorrect. - **Option 3**: While MMA accumulation may contribute to neurologic injury through effects on myelin fatty acid composition, the question asks why MMA is a **better diagnostic marker**, not why it causes symptoms. The diagnostic specificity hinges on the biochemical pathway, not the clinical consequence. - **Option 4**: While methylmalonyl-CoA mutase is indeed mitochondrial and methionine synthase is cytoplasmic, subcellular localization does not explain why one enzyme depends on folate and the other does not. This is a structural detail that does not address the cofactor specificity that drives the clinical differentiation. **High-Yield:** MMA is specific for B12 deficiency; homocysteine is non-specific (rises in both B12 and folate deficiency). Always measure MMA to confirm B12 deficiency before giving folate alone. [cite: Harper 32e Ch 27; Harrison 21e Ch 95]
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