A 38-year-old man with acute gout is started on allopurinol. His serum uric acid drops from 9.5 to 4.2 mg/dL over 4 weeks. A 41-year-old woman with recurrent gout and a history of renal calculi is found to have elevated urinary uric acid (950 mg/day) and is started on febuxostat instead. Which feature best distinguishes the therapeutic approach in overproducers from underexcretors?