Welcome to mikay92’s page.
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I don’t think the NBME ever “intends” to write an ambiguous or poorly worded stem. What they want to do is write questions whose response choices are not “blatantly obvious” but which do have a single, “most correct” choice. That’s actually surprisingly difficult! If the correct choice were “obvious”, the test would not be doing a good job assessing anyone’s ability to make subtle judgment calls (an important skill, one might argue, in the morass that is the real world); this is also the reason they eschew “buzzwords”, generally. If a stem has two or more choices that are “equally correct”, the same lapse has occurred: they would be failing to assess the capacity to make subtle judgment calls.
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?
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.
Very helpful video for acute kidney injury: https://www.youtube.com/watch?v=bMp6IxDKK2Q
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).
Would fully recommend the OnlineMedEd video on AKI. Goes through the differential, lab results, treatment, etc in a very clear and concise manner.
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?