Why Does the Peritubular Fluid of the Renal Medulla Contain

Why does the peritubular fluid of the renal medulla contain an unusually high solute concentration?

A) The cells of the juxtaglomerular complex secrete the hormone erythropoietin and the enzyme renin.
B) The collecting system reabsorbs water and reabsorbs or secretes sodium, potassium, hydrogen, and bicarbonate ions.
C) The cells of the proximal convoluted tubule reabsorb organic nutrients, plasma proteins, and ions from the tubular fluid.
D) The ascending limb of the nephron loop actively transports sodium and chloride ions out of the tubular fluid.
E) The filtration slits between the slender processes of the podocytes prevent the passage of blood cells and most plasma proteins.