A 54-year-old man comes to the emergency room with severe pain in his right toe. He has had less severe episodes in the past, which he always treated with pain medications. The toe is red, inflamed, and exquisitely sensitive to movement. Needle aspiration of the toe confirms uric acid crystals, and he is treated with oral indomethacin (NSAID). One month later, he remains symptom free, and allopurinol is recommended for prevention of this condition. Which of the following is the most likely mechanism of action of allopurinol?