A 38-year-old woman from Delhi presents with progressive muscle weakness, fatigue, and recurrent infections over 3 months. She has a history of rheumatoid arthritis treated with methotrexate for 2 years. Laboratory investigations show: Hb 9.2 g/dL, WBC 3.2 × 10⁹/L, platelets 120 × 10⁹/L. Serum folate is low at 2.1 ng/mL (normal >5.4 ng/mL). Her bone marrow shows megaloblastic changes. Which type of enzyme inhibition mechanism best explains methotrexate's toxicity in this patient?