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NBME 23 Answers

nbme23/Block 2/Question#19

A 70-year-old man with severe congestive heart ...

Serum BUN: 40;
Serum creatinine: 2;
Urine Na+: 5;
Urine specific gravity: 1.025


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submitted by brolycow(12),

He has heart failure which leads to a decrease in renal blood flow and prerenal azotemia. In prerenal azotemia, BUN:Cr ratio is >= 20; Activation of the RAAS system due to the prerenal azotemia means that the spec grav is high at 1.025 and he is holding onto sodium so urinary sodium will be low (<20, FENa <1%).

figprincess  did you figure out the the ratio by actually divding out the numbers since the q didn't give it as a ratio? also what resource tells us what prerenal spec gravity should be? +  
brolycow  I just usually remember from class that spec grav 1.001-1.010 is considered dilute urine, and anything 1.025 and above is concentrated. For this question specifically, I think I remember there only being one option that even had the ratio >=20, all of the others were like 15 or less, so just have to rule them out. +2  
benzjonez  Very helpful video for acute kidney injury: https://www.youtube.com/watch?v=bMp6IxDKK2Q +2  
notadoctor  Another explanation that helped me is that inability to concentrate the urine means something is wrong with the kidneys. If you have dilute urine, or the spec gravity is between 1.001-1.010 in someone with low urine output it suggests something is wrong with the concentration mechanisms of the kidney. Because this person had congestive heart failure we were already looking for something that matched up with prerenal azotemia so we can pretty much get rid of all the answer choices that suggest other azotemias. Then finally to get the precise answer I looked at the BUN/Cr ratio which you would expect to be high(>= 20). +  




Hey can someone explain the renal findings? Physio is definitely not my strong suit.

Is it because he has fluid/Na+ retention that his specific gravity and Na+ are so low? Is there a reason for those specific BUN/creatinine over the others?

brolycow  He has heart failure which leads to a decrease in renal blood flow and prerenal azotemia. In prerenal azotemia BUN/Cr ratio is >= 20; Activation of the RAAS system due to the prerenal azotemia means that the spec grav is high at 1.025 and he is holding onto sodium so urinary sodium will be low (<20, FENa <1%). +5  
mousie  Agree with above, HF and not taking meds would increase or activate RAAS = increase ATII and Aldosterone which leads to body retaining Na and H2O so the urine concentration of Na will be low and the urine will be very concentrated i.e. high SG. I didn't think about the BUN:Cr > 20 but this would have also narrowed it down! +2