A 28-year-old woman presents with muscle cramps, paresthesias, and episodes of fatigue. Laboratory studies reveal hypokalemia, metabolic alkalosis, hypomagnesemia, and a notably low 24-hour urinary calcium excretion. Genetic testing confirms a loss-of-function mutation in the thiazide-sensitive Na+/Cl− cotransporter (NCC) in her distal convoluted tubule cells. Which of the following best explains her decreased urinary calcium excretion?