The correct answer is option (3) as mammals have the ability to produce a concentrated urine. The Henle's loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle's loop as well as flow of blood through the two limbs of vasa recta are in a counter current pattern. The proximity between the Henle's loop and vasa recta, as well as the counter current in them help in maintaining the increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmol L−1 in the cortex to about 1200mOsmolL−1 in the inner medulla. So, counter current mechanism of loop of Henle of juxtamedullary nephron participates in concentration of urine.