I think you are misunderstanding the question. In the absence of ADH (i.e. no free water reabsorption from the collecting duct) the highest osmolarity of tubular fluid will be seen in the descending limb of loop of Henle. If you remember descending limb is selectively permeable to water leaving all the solute behind and making the fluid highly osmolar. From that point on, there is no more free water reabsorption without ADH. Free water reabsorption means absorbing just water, without the solute, as seen with ADH. Contrast that to proximal tubule where all the water is reabsorbed along with sodium. In the presence of ADH collecting duct will reabsorb free water also and there is no way to say whether the collecting duct or the descending limb would be the section with the highest osmolarity. I hope that clears it up.UworldQ: In the absence of ADH highest osmolarity seen in?
A: end of descending limb of loop of henle
How does ADH effect the osmolarity of loop of Henle while its action on collecting duct?