A 75-year-old man presents to the surgical unit with a 24-hour history of acuteonset left loin pain, which seems to worsen intermittently and has not settled with regular simple analgesia. He suffers from mild dementia and is unable to recall the details of his past medical history. The foundation year 2 doctor on call suspects that a urinary calculus is the cause of this man's pain and spots an old pathology repo in the patient's notes showing the presence of negatively birefringent crystals in a synol fluid aspirate. Which one of the following substances is likely to make up the majority of this man's calculus?