The concentration of sodium and chloride ions is lowest in which of the following regions?

The function of the loop of Henle is:

The counter-current multiplier system changes the isotonic glomerular filtrate into

Medullary gradient is mainly developed due to

$A$: The final reabsorption of water from urine into blood occurs through the collecting duct of a mammalian nephron,resulting in the production of hyperosmotic urine.
$R$: The loop of Henle is responsible for the formation of a sodium gradient across the depth of the medullary interstitium of a mammalian kidney.

Explain the mechanism of formation of concentrated urine in mammals.