A 52-year-old man with chronic congestive heart failure has been treated with the loop diuretic bumetanide for six weeks. He now presents with muscle cramps and paresthesias in his hands and feet. Laboratory studies reveal a serum magnesium concentration of 1.2 mg/dL (normal 1.7–2.2 mg/dL) and increased urinary magnesium excretion. Which segment of the nephron is the primary site of bumetanide-induced magnesium wasting?