Which of the following conditions does not contribute to the creation and/or maintenance of the medullary osmotic gradient?

a. The countercurrent exchanger of the vasa recta

b. The countercurrent multiplier of the nephron loops of cortical nephrons

c. The countercurrent multiplier of the nephron loops of juxtamedullary nephrons

d. The permeability of the medullary collecting system to urea and other ions