A 42-year-old woman has been diagnosed with liver cancer and is being treated with 5-FU. 5-FU is successful in destroying the tumor cells because it blocks the production of which one of the following?

5-FU inhibits the thymidylate synthase reaction, which produces dTMP from dUMP. It does not inhibit FH2 reductase, which would block the production of FH4 . Methotrexate inhibits FH2 reductase. UMP is made via the de novo pyrimidine biosynthetic pathway, which is not inhibited by 5-FU. Methyl cobalamin is produced from N5-methyl-FH4 and vitamin B12 , and 5-FU does not block this activation of B12 . CTP is produced from UTP, using glutamine as the nitrogen donor, and that reaction is also not affected by the presence of 5-FU.