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?

Gout is a disorder of purine metabolism exacerbated by dehydration, resulting in hyperuricaemia and acute recurrent attacks of synovitis due to urate crystal deposition in the (predominantly) large joints of the body. Aspiration of joint fluid and examination under polarized light shows negatively birefringent crystals, which are characteristic of the condition and suggested in this individual. Hyperuricaemia also predisposes individuals to urate and calcium oxalate stones. Treatment is as for other urinary tract stones, but also includes allopurinol and diet modification in order to lower levels of serum uric acid. Given the age of this man, it is impoant to exclude other impoant differentials (as for any patient), paicularly a ruptured AAA, which can present with similar symptoms and is occasionally missed in such individuals resulting in a catastrophic outcome.