A 48-year-old woman with stage IIIB breast cancer is enrolled in a neoadjuvant chemotherapy protocol. She receives a regimen including 5-fluorouracil (5-FU), cyclophosphamide, and doxorubicin. On day 8 of the first cycle, she develops severe stomatitis with painful ulcers on her buccal mucosa and tongue, along with watery diarrhoea. Her neutrophil count is 800/µL (ANC). She is also noted to have erythema and tenderness of her palms and soles. The oncologist considers administering folinic acid (leucovorin) to mitigate further toxicity. Which of the following best explains the rationale for using folinic acid in this context?