A 52-year-old woman presents with fatigue, bone pain, and recurrent kidney stones. Laboratory studies reveal elevated parathyroid hormone levels and the following electrolytes: serum calcium 11.5 mg/dL (normal 8.5–10.2 mg/dL) and serum phosphate 1.8 mg/dL (normal 2.5–4.5 mg/dL). Imaging confirms a parathyroid adenoma. Increased PTH action reduces phosphate reabsorption in which nephron segment and by which mechanism under normal physiological conditions?