Which of the following factors is responsible for the formation of concentrated urine?

Counter current flow in loop of Henle and vasa recta helps maintain a concentration gradient in the medullary interstitium. There is increasing osmolarity towards the inner medullary interstitium (from $300 \mathrm{mOsml}/\mathrm{L}$ in the cortex to about $1200 \mathrm{mOsml}/\mathrm{L}$ in the medulla). Such a gradient helps in concentration of urine.