MRCP2-4601

A 30-year-old Caucasian farmer presents to the outpatient department with a complaint of generalised aches and pains that have been bothering him for the past two months. He used to be an active runner but had to give up competitive running a year ago due to a hip injury. Two years ago, he had a painful red eye for which he received treatment with eye drops in the Emergency department. He is a heavy smoker, consuming 45 cigarettes a day, and drinks five pints of beer every weekend. He denies any skin rashes or mucosal ulceration. His mother had rheumatoid arthritis, and his father had severe gout. On examination, the left first metatarsophalangeal joint was swollen and tense, but all the other joints were unremarkable. Rotation of the lumbar spine was restricted. What test would be most useful in establishing the diagnosis?