Maya is 22 years old and attends the GP with a 3 month history of intermittent swollen, painful feet and ankles. She denies any trauma and has no other affected joints. Systems enquiry is normal and there is no fatigue, sweats or weight loss. On examination there is synovitis and pain on squeezing the metatarsophalangeal joints and a normal ankle joint examination.
Which one of the following tests is most likely to aid diagnosis?