This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins and Minerals, Subject name: Biochemistry
In vitamin B12 deficiency, folate is trapped in the form of 
Select and explain. 
A. Formyl THFA, B. Methenyl THFA, C. Methyl-THFA, D. Methylene-THFA
C: Methyl-THFA
Answer: C: Methyl-THFA 
Explanation: Methylation of homocysteine to methionine is the only way of regeneration of active THFA from methyl THFA. Methionine synthase reaction requires methyl cobalamin. So, deficiency of methyl-B12 leads to trapping of THFA in less utilised methyl-THFA - phenomenon known as folate trap.