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NBME 23 Answers

nbme23/Block 3/Question#24 (reveal difficulty score)
A 25-year-old man is brought to the emergency ...
Fractional excretion of sodium greater than 2%๐Ÿ”,๐Ÿ“บ

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submitted by thomasalterman(162),
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eTh teipnta sah TNA royasdenc to aenlr eah.msici uDe ot trlabuu csorn,ies eth ianttep wlli eahv an leveteda a.FeN hTe stetpai'n unrie lwli olsa eb ,idutle tbu stih llwi eb leeecdfrt yb teh low uerni ty,soliolma ont eth NaeF

mousie  Hypotension can also cause pre renal azotemia with a FENa <1%.... How do you know this is ischemic ATN and not hypotension induced Prerenal Azotemia? +13  
sympathetikey  I had the same thought as you @mousie, but I think "azotemia" and low urine output push it more towards ATN (looking back; I got it wrong too). Plus, the initially MVC / muscle damage probably caused some tubule injury by itself. +3  
ajo  This might help clarify why the pt. has ATN rather than pre renal azotemia. The question did mention, though subtly, that the bleeding was controlled. That most likely indicates that his hypovolemia has been corrected. Developing azotemia 24 hrs after correction of hypovolemia is more suggestive of ATN (since he doesn't have hypovolemia anymore). I hope that helps and feel free to correct me, if I am wrong. +45  
ajo  In addition to my earlier comment, I just noticed the question also explicitly mentioned that he was fully volume restored. Which is consistent with my earlier assumption! +17  
gh889  Although initially, hypotension causes prerenal azotemia, the volume correction pushes you away from prerenal azotemia. but they want you to remember that in hypovolemia, the kidneys are also becoming ischemic, and so development of azotemia 24 hours later is more indicative of intrarenal azotemia due to ATN +1  
sugaplum  for anyone who wants to see it: FA 2019 pg591 +2  
divya  i'm confused about one thing. if the tubules aren't working like they should, the bun:cr ratio falls right? doesn't that essentially mean azotemia reduces too? +1  
osler_weber_rendu  Lets all take a moment to admire how shit this question is "Bp 90/60.""Repeated episodes of hypotension in the OR" and still the answer is ATN +5  
donttrustmyanswers  @osler_wever_rendu ATN can be caused by ischemia. +4  

submitted by andro(213),

Step 1 Recognizing its Acute Kidney injury
Step 2 : differentiating between Prerenal (hypotensive) azotemia and Ischemic ATN

From the vignette , Patient is "fully volume restored " ( not hypotensive anymore) but yet he still has a low urine output Note: Failure of AKI to correct after patient is once again euvolemic - would suggest either another prerenal cause such as Heart Failure , Nephrotic syndrome etc or an intrinsic cause like Ischemic ATN

ATN Stages : 1. prodrome
2. Low urine output (Oliguric phase )
3. High urine output (Polyuric phase)

A side note from medscape : Ischemic ATN is a continuum of Prerenal Azotemia . Indeed the cause of the two conditions are the same , ischemic ATN resulting when hypoperfusion overwhelms the kidneys autoregulatory defenses leading to cell death

I think of it as a reversible cause vs an Irreversible one .
Failure of Prerenal Azotemia to correct after fluid resuscitation means it has progressed to the point of cell death ; Acute Tubular Necrosis . (These cells will eventually regenerate with time )

b1ackcoffee  Exactly! +