Gitelman Syndrome (SLC12A3, AR)

A 19-year-old woman from rural India presents with recurrent muscle cramps, fatigue, and carpopedal spasm. Laboratory investigations reveal serum potassium 2.8 mEq/L, serum magnesium 1.2 mg/dL, HCO3− 31 mEq/L, and notably LOW urinary calcium excretion (FE-Ca <1%). Blood pressure is normal. Genetic testing confirms a homozygous missense mutation in the gene located on the structure marked **A** in the diagram. Which of the following best describes the primary functional consequence of this mutation?