A 30-year-old man with a history of recurrent calcium oxalate kidney stones is diagnosed with gout. Serum uric acid is elevated. Which urate-lowering drug is contraindicated in this patient, and why?
- A Allopurinol; it causes uricosuria and worsens stone formation
- B Probenecid; it increases urinary uric acid excretion and promotes uric acid stone formation ✓
- C Febuxostat; it is nephrotoxic and causes crystalluria
- D Colchicine; it alkalinizes urine and promotes calcium oxalate precipitation
Correct answer: B. Probenecid; it increases urinary uric acid excretion and promotes uric acid stone formation
Explanation
Probenecid is a uricosuric agent that inhibits URAT1 and OAT4 transporters in the proximal tubule, increasing urinary uric acid excretion. In patients with a history of uric acid stones or calcium oxalate stones, uricosuric agents are contraindicated because they increase the risk of new stone formation. Allopurinol or febuxostat (xanthine oxidase inhibitors) are preferred because they lower serum urate without increasing urinary uric acid excretion.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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