This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
What is involved in the formation of d-TMP from d-UMP? 
Select and explain. 
A. N5, N10-methylene tetrahydrofolate, B. Form imino folate, C. N5 formyl folate, D. Dihydro folate
A: N5, N10-methylene tetrahydrofolate
Answer: A: N5, N10-methylene tetrahydrofolate 
Explanation: The following reaction is catalyzed by thymidylate synthetase:5,10-methylenetetrahydrofolate + dUMP = dihydrofolate + dTMPBy means of reductive methylation, deoxyuridine monophosphate (dUMP) and N5, N10-methylene tetrahydrofolate together used to form dTMP, yielding dihydrofolate as a secondary product.