A 58-year-old Indian woman with locally advanced gastric cancer is started on 5-fluorouracil (5-FU) based chemotherapy. The structure marked **C** in the pyrimidine synthesis pathway is the primary target of 5-FU's cytotoxic action. After metabolic activation to F-dUMP, 5-FU irreversibly inhibits this enzyme by forming a covalent ternary complex. Which of the following statements best explains the mechanism by which LEUCOVORIN (folinic acid) enhances 5-FU efficacy in this patient?