A 7-year-old girl with a history of failure to thrive and recurrent nephrolithiasis is diagnosed with distal (Type I) renal tubular acidosis. She is started on bicarbonate replacement therapy. Despite correction of her metabolic acidosis, she continues to have significant hypercalciuria. Which medication, when added to her regimen, would be most effective at reducing her urinary calcium excretion and risk of future stones?