A 52-year-old man with newly diagnosed acute myeloid leukaemia (AML) presents to the haematology unit. His initial CBC shows WBC 85,000/µL with 70% blasts, Hb 7.2 g/dL, and platelets 35,000/µL. He is started on induction chemotherapy with cytarabine (Ara-C) and daunorubicin. On day 5 of therapy, he develops severe mucositis, diarrhoea, and hand-foot syndrome with erythema and blistering of palms and soles. His renal function remains normal. Which of the following best explains the mechanism of this toxicity?