A 65-year-old male taking warfarin for a history of deep vein thrombosis develops a urinary tract infection. The physician prescribes trimethoprim-sulfamethoxazole. What is the most important monitoring parameter due to this new medication?

✓ Correct Answer: Option B

Trimethoprim-sulfamethoxazole can potentiate the anticoagulant effect of warfarin, significantly increasing the INR and risk of bleeding. Therefore, close monitoring of the INR is crucial when these medications are co-administered to prevent hemorrhagic complications.