This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Methionine and Cysteine details, Subject name: Biochemistry
Which of the following conversion is irreversible 
Select and explain. 
A. Formyl THFA to Methenyl-THFA, B. Methenyl THFA to Methylene-THFA, C. Methylene THFA to Methyl-THFA, D. All of the above
C: Methylene THFA to Methyl-THFA
Answer: C: Methylene THFA to Methyl-THFA 
Explanation: One carbon groups are formed from various sources. Except the formation of methyl-THFA The other one-carbons are interconveible The only way of regeneration of active THFA from methyl-THFA is the methionine synthase reaction.