A 91-year-old woman in a nursing home has been prescribed Trimethoprim for a urinary tract infection. Her regular medications include Methotrexate 7.5 mg weekly for rheumatoid arthritis, Folic acid 5 mg weekly, Amlodipine 5 mg, and Paracetamol 1 g QDS. Why is Trimethoprim the MOST concerning antibiotic in this patient?