Histidine catabolism proceeds through formiminoglutamic acid (FIGLU) before entering the folate one-carbon pool. Urinary FIGLU rises abnormally after a histidine load when which vitamin is deficient?

• A Vitamin B12
• B Riboflavin
• C Thiamine
• D Folic acid ✓

Explanation

Formiminoglutamate transfers its formimino group to tetrahydrofolate, generating glutamate and N5-formiminotetrahydrofolate. In folate deficiency this step stalls, so histidine loading produces excess urinary FIGLU, the basis of the historical FIGLU excretion test. Vitamin D12 deficiency impairs methylmalonyl-CoA mutase and homocysteine remethylation but not the FIGLU step, and thiamine and riboflavin have no role here.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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