## Why Leucovorin stabilizes the ternary complex is right The clinical anchor states that leucovorin PARADOXICALLY POTENTIATES 5-FU activity by stabilizing the covalent ternary complex formed between F-dUMP, thymidylate synthase (marked **C**), and N5,N10-methylene-THF. This is mechanistically distinct from leucovorin's rescue role with methotrexate. By providing an abundant pool of N5,N10-methylene-THF, leucovorin shifts equilibrium toward formation and stabilization of the inhibitory complex, trapping the enzyme and preventing dTMP synthesis—the basis of "thymineless death" in dividing cancer cells. This is a well-established principle in 5-FU pharmacology documented in Harper 32e Ch 33 and KD Tripathi 9e Ch 62. ## Why each distractor is wrong - **Leucovorin rescues from 5-FU toxicity by regenerating DHFR**: This describes leucovorin's role with methotrexate (a DHFR inhibitor), NOT its role with 5-FU. The question explicitly states that leucovorin ENHANCES 5-FU efficacy (potentiates, not rescues), making this mechanistically incorrect and contradicts the clinical anchor. - **Leucovorin increases DPYD expression**: Leucovorin does not regulate DPYD (dihydropyrimidine dehydrogenase), the first catabolic enzyme of 5-FU. DPYD deficiency causes severe 5-FU toxicity, but leucovorin does not modulate its expression. This is a distractor that confuses drug metabolism with drug potentiation. - **Leucovorin competitively inhibits thymidylate synthase independently**: Leucovorin is not an inhibitor of thymidylate synthase; it is a cofactor precursor. It does not independently inhibit the enzyme marked **C**. This option misrepresents the role of folate cofactors. **High-Yield:** Leucovorin + 5-FU = POTENTIATION (stabilizes ternary complex); Leucovorin + Methotrexate = RESCUE (regenerates DHFR). This is a classic NEET-PG distinction. [cite: Harper 32e Ch 33; KD Tripathi 9e Ch 62]
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