A 28-year-old woman with newly diagnosed acute lymphoblastic leukaemia (ALL) is started on high-dose methotrexate (MTX) chemotherapy. The structure marked **A** in the diagram is the primary target of methotrexate. Which of the following best explains the mechanism by which methotrexate toxicity is managed in this patient, and why folinic acid (leucovorin) is administered as rescue therapy?