Q. A 60-year-old man presents to his primary care physician with a three-month history of fatigue and muscle weakness, left flank pain radiates to his penis, and localized pruritus (itching) and pain over areas of his skin. He reports that he has been feeling more tired than usual and has been experiencing muscle weakness, especially in his legs. He also reports a history of end-stage renal disease, which was diagnosed five years ago. He undergoes hemodialysis three days a week. The patient's vital signs on presentation are: blood pressure 140/90 mm Hg, pulse rate 80 beats/minute, and respiratory rate 16 breaths/minute. The physical examination reveals a patient with muscle wasting and joint pain. Urinalysis shows hematuria and wedge-shaped prism crystals. Further studies show hypocalcemia, hyperphosphatemia, with low levels of vitamin D and high levels of PTH. What is the most likely cause of these findings?