This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cytotoxic Anticancer Drugs, Subject name: Pharmacology
The mechanism of anticancer action of fluorouracil is: 
Select and explain. 
A. Cross linking of double stranded DNA and the resulting inhibition of DNA replication and transcription, B. Cytotoxicity resulting from a metabolite that interferes with the production of dTMP, C. Irreversible inhibition of dihydrofolic acid reductase, D. Inhibiting the formation of mitotic spindle
B: Cytotoxicity resulting from a metabolite that interferes with the production of dTMP
Answer: B: Cytotoxicity resulting from a metabolite that interferes with the production of dTMP 
Explanation: 5-Fluorouracil (5-FU) is an antimetabolite. It is a thymidine analogue and acts by conversion to 5'-dUMP. Latter inhibits the formation of dTMP and results in thymineless death of cells. Option 1 is mechanism of alkylating agents like cyclophosphamide Option 3: methotrexate is example of DHFRase inhibitor Option 4: Vincristine like drugs act by inhibiting mitotic spindle