Review Question - QID 214343

A 10-year-old boy is brought to a new pediatrician for evaluation after his family moved to a new state. On presentation, he is found to be 5th percentile in both height and weight but is otherwise normal in his physical and cognitive development. His parents say that he occasionally complains of muscle weakness and abdominal pain but has never been diagnosed with any diseases. His only complaint currently is that he has to wake up several times during the night in order to urinate. A metabolic panel is obtained with the following results:

Serum:
Cl-: 87 mEq/L
K+: 2.8 mEq/L
HCO3-: 35 mEq/L
Mg2+: 0.9 mEq/L

A 24-hour urine study is also performed showing the following:

Urine Ca2+: 28 mg (normal = 100-300 mg)

Which of the following proteins is most likely mutated in this patient?