A 52-year-old man with newly diagnosed multiple myeloma presents to the oncology clinic. He has a serum creatinine of 2.1 mg/dL (baseline 0.9 mg/dL) and haemoglobin of 8.2 g/dL. His physician initiates induction chemotherapy with melphalan, prednisone, and bortezomib. After the first cycle, the patient develops severe mucositis, diarrhoea, and bone marrow suppression. Laboratory tests reveal WBC 1.8 × 10⁹/L (baseline 7.2 × 10⁹/L) and platelets 45 × 10⁹/L (baseline 220 × 10⁹/L). Which of the following best explains the mechanism of melphalan's toxicity in this patient?