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NBME 22 Answers

nbme22/Block 2/Question#2

A 65-year-old man starts using topical fluorouracil ...

Thymidylate synthase

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submitted by usmleuser007(87),

Two major mechanisms of action have been elucidated:

1) Flucytosine is intrafungally converted into the cytostatic fluorouracil which undergoes further steps of activation and finally interacts as 5-fluorouridinetriphosphate with RNA biosynthesis thus disturbing the building of certain essential proteins.

2) Flucytosine also undergoes conversion into 5-fluorodeoxyuridinemonophosphate which inhibits fungal DNA synthesis.

3) Thymidylate synthetase is an enzyme that catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP).

Thymidine is one of the nucleotides in DNA.

With inhibition of TS, an imbalance of deoxynucleotides and increased levels of dUMP arise. Both cause DNA damage.

(WIKI)

link981  Just look at page 36 of FA 2018 and memorize that shitty diagram o De novo pyrimidine and purine synthesis they ask so much about. No need for scientific explanations for this one unless you like to waste time. +2