A 45-year-old patient on hemodialysis for one week has noted that his blood pressure is more difficult to control. He reports good compliance with his medications, which include erythropoietin, ferrous sulfate, vancomycin, and vitamin D. His blood pressure is 180/99 mm Hg. Which of the following is the most likely cause for the worsening control of his blood pressure?
B. Ferrous sulfate
D. Vitamin D