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NBME 23 Answers

nbme23/Block 2/Question#19 (51.2 difficulty score)
A 70-year-old man with severe congestive ...
Serum BUN: 40;
Serum creatinine: 2;
Urine Na+: 5;
Urine specific gravity: 1.025


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submitted by brolycow(27),
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He hsa rahet uerafli chhiw dlesa ot a dceersea in elran lobdo lofw nda rnarelpe .izaamteo In lnarerep ,aaezotmi :rCBNU riaot is t&;=g 20; Aonttvaiic of hte SRAA msytes ued to het naelprre tmeziaao emans atth het spce vgra si gihh at 5.201 nda he si honildg noot doiusm os rinruya udmios liwl be wlo ;0&,2tl( ENaF tl)%&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. +7  
benzjonez  Very helpful video for acute kidney injury: +8  
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). +  
mikay92  Would fully recommend the OnlineMedEd video on AKI. Goes through the differential, lab results, treatment, etc in a very clear and concise manner. +  
drdoom  repost via @benzjonez -> +  
drdoom  @mikay92 is this the OnlineMedEd video you're referring to? -> +1  
drdoom  aha! there is an updated AKI video but you need an OnlineMedEd (free) account to view it: +  
popofo  I understand that BUN:Cr > 20 if renal perfusion is repaired, but in heart failure wouldn't there be increased secretion of ANP/BNP from the atria that pushes up the sodium excretion? +  

submitted by lostdinosaurs(1),
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eyH cna eonmose lxinpea the ralen fn?iignsd osyiPh is neeldytfii ton my gonrst ust.i

Is ti suebcea eh ash dfu+Nla/i onntreiet htta hsi piicfecs aigyrtv and +Na era os w?ol Is terhe a senoar ofr stoeh siicfepc UreBtnaecnNi/i erov teh osethr?

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%). +7  
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  

submitted by haozhier(18),

I chose C because I thought it has been four weeks so it must have been acute tubular necrosis. Can anyone explain? Thanks!

miriamp3  @haozhier if you are deciding to think that he had a ATN because of the 4 weeks.. then he should be by now in the recovery phase(polyuria, Bun/cr fall) But he is with HF and his urine output has progressively decrease. So AKI prerenal HF Bun/cr >20. the only one is D. Don't get confused with the rest of the information. +  
jesusisking  I thought the same thing so chose C as well! +