This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which of the following physiological factor increases the urinary concentrating ability of the kidney? 
Select and explain. 
A. ECF volume contraction, B. Increase in RBF, C. Reduction of medullary hyperosmolarity, D. Increase in GFR
A: ECF volume contraction
Answer: A: ECF volume contraction 
Explanation: 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.