A 54-year-old man comes to the emergency room complaining of severe pain in his right toe. The pain is interfering with his ability to walk and he reports no prior trauma to the toe. He has had multiple less severe episodes in the past, which he always treats with pain medications.On physical examination, the toe is red, inflamed, and exquisitely sensitive to movement. An x-ray of the toe is normal and needle aspiration of the joint confirms uric acid crystals. He is treated with oral indomethacin (NSAID) for 7 days, and 1 month later he remains symptom free. Allopurinol is recommended for prevention of this condition. Which of the following is the most likely mechanism of action of allopurinol?