A 27-year-old woman comes to the clinic for evaluation of recurrent kidney stones. She reports intermittent flank pain for 2 years and has passed several small stones. She drinks 1–2 L of water daily. She has no history of gout. Medications include a combined oral contraceptive pill. Vital signs are normal. Serum studies show: calcium 9.4 mg/dL, phosphate 3.6 mg/dL, uric acid 4.8 mg/dL. Urinalysis shows pH 6.0 and no protein. A 24-hour urine collection reveals markedly elevated urinary calcium excretion with normal serum calcium. Which of the following is the most likely underlying mechanism of her hypercalciuria?