Review Question - QID 214509

A 67-year-old woman presents to her primary care physician with a 1-month history of constipation and flank pain. She says that she has been feeling down for the last few months and initially thought that these symptoms were simply because she was getting old. She became concerned when she started noticing intermittent flank pain that radiates to the groin as well as weakness throughout the day. Her past medical history is significant for hypertension and diabetes for which she takes metformin and lisinopril. Her only other medication is a vitamin supplement that she recently started taking. She drinks 2-3 drinks on the weekends and has never smoked. Laboratory tests are obtained, and the results are as follows:

Serum:
Na+: 138 mEq/L
Cl-: 104 mEq/L
K+: 4.0 mEq/L
HCO3-: 24 mEq/L
BUN: 16 mg/dL
Glucose: 138 mg/dL
Creatinine: 1.2 mg/dL
Ca2+: 11.8 mg/dL
PO4-: 2.4 mg/dL

Which of the following is the most likely cause of this patient's symptoms?