Which of the following physiological factor increases the urinary concentrating ability of the kidney?

When a person is dehydrated, glomerular filtration rate (GFR) is somewhat low and levels of ADH are high. The extraction of water in the coical collecting duct removes most of the water from the lumen (and makes it iso-osmotic with the coical interstitium ie, about 300 mOsm/kg). Then, as the remaining but greatly reduced, volume flows through the high osmolality medulla, fuher concentration occurs. The increased urea permeability signaled by ADH greatly assists in generating the medullary osmotic gradient by permitting the recycling of urea. Ref: Eaton D.C., Pooler J.P. (2009). Chapter 6. Basic Renal Processes for Sodium, Chloride, and Water. In D.C. Eaton, J.P. Pooler (Eds), Vander's Renal Physiology, 7e.