Gout

A 55 year-old man comes to the emergency department because of a three-day history of pain, erythema, and swelling in the first metatarsophalangeal joint of his right foot. He is unable to sleep at night because he has severe pain whenever his bed sheet touches the joint. He admits to smoking one pack per day and drinking three to four beers nightly. He denies any previous episodes of pain in this joint and denies trauma to the area. He denies pain in any other joint. His only medication is hydrochlorothiazide for hypertension, and the dose of this medication was recently increased by his primary care provider. Physical examination shows a tender, warm, erythematous right toe. Synovial fluid is removed from the joint, and crystals are seen on microscopy. Which of the following substances is most likely to be seen on analysis?