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-   -   Question about function of ADH in medullary osmolarity (https://www.usmle-forums.com/usmle-step-1-forum/298969-question-about-function-adh-medullary-osmolarity.html)

mmor91 08-17-2016 11:26 AM

Question about function of ADH in medullary osmolarity
 
I'm doing some Kaplan q's and then I got confused in something...
A 45-year old man with HTN is participating in a drug trial of an ADH antagonist. The agent is known to only block ADH receptors in the kidney. Which of the following is most likely to occur acutely?
A. A decrease in fluid reabsorption in the loop of Henle. (Hmmm no... there's still going to be fluid absorption in the PCT.)
B. A decrease in the medullary osmotic gradient. (I thought that was this one because ADH increase urea reabsorption in order to increase osmolarity in the medulla... so if there's no ADH...there's no urea reabsorption therefore the osmolarity would decrease i guess)...
C. A decrease in urine osmolarity without a major change in electrolyte excretion. (this was the right one).

Can someone explain to me why ADH absence won't decrease osmolarity? :(
Anyways the explanation that the answer gave me was that if there's no ADH, there's going to be an increase in serum osmolarity, which the kidney is going to try to compensate by reabsorbing more Na (to take water with it), so there's no going to be much fluidity in the vasa recta--> no fluid in vasa recta, no break to take out the cations and stuff in the medulla-> more cations and stuff in medulla->more osmolarity.
But still...urea doesn't play a role here?


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