A 40-year-old man experiences weakness and easy fatigability of 2 months' duration. Physical examination yields no remarkable findings. Laboratory studies show a serum calcium of 11.5 mg/dL, inorganic phosphorus of 2.1 mg/dL, and serum parathyroid hormone of 58 pg/mL, which is near the top of the reference range. A radionuclide bone scan fails to show any areas of increased uptake. What is the most likely cause of these findings?