A 48-year-old man with a 5-year history of recurrent gout attacks is now being evaluated for chronic management. He has had 4 attacks in the past year despite lifestyle modifications. Serum uric acid is 7.8 mg/dL, and 24-hour urinary uric acid excretion is 450 mg/day (normal: 400–800 mg/day). Which investigation would best guide the choice between xanthine oxidase inhibitor versus uricosuric agent?