USMLE (United States Medical Licensing Examination) Practice Exam 14

A 27-year-old woman comes to the clinic for evaluation of recurrent kidney stones. She has had two episodes of flank pain with passage of small stones over the past year. She has no chronic medical conditions and takes no medications. She reports intermittent tingling around her mouth. Physical examination is normal. Laboratory studies show: serum calcium 11.2 mg/dL (8.6–10.2), phosphate 2.1 mg/dL (2.5–4.5), PTH 98 pg/mL (10–65), and 25-hydroxyvitamin D 28 ng/mL (20–50). Which of the following is the most likely mechanism responsible for her hypophosphatemia?