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nbme24/Block 4/Question#3

A 23-year-old woman is brought to the emergency ...

Renal tubular acidosis

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submitted by yobo13(0),

Can someone explain why it can't be Crohn's since that would also cause a non AGMA?

drpatinoire  If she has Crohn, she has already lost a lot of K, HCO3-, then the compensatory system wouldn't let her keep losing electrolytes in her urine. +  

You didn't really need to calculate Anion gap. The bicarb was EXCEPTIONALLY low. That is generally RTA. Additionally, the Cl was very high in the setting of other normal electrolytes. Additionally,. it was a s low onset (over 2 weeks) knocking out many other choices (Salicylate, Lactic Acidosis, DKA, and Alcoholic Ketoacidosis).

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submitted by zbird(3),

This patient has Distal-Type I RTA which is explained by Normal Serum Anion gap (8) Metabolic acidosis with her positive urinary anion gap(+5).

krewfoo99  Why would the urine Potassium be so high if it is type 1 ? Shouldnt it be type 2? +  
drpatinoire  @krewfoo99 I think it's RTA2 (Fanconi syndrome), he is losing all kinds of Na, K, Cl which should be reabsorbed in PCT. +  

BUT why is the serum potassium normal?

I was able to narrow it down to RTA, because none of the other answer choices made much sense, but the potassium had me second guessing myself. Can someone explain that lab finding? Thanks!

subclaviansteele  My take is that hes not super acidotic and the K is at the low end. +  
nwinkelmann  see the comment by @zbird, which explains that the urine anion gap is important (which I took to interpret as more important than the serum K+ level, lol, because the normal K threw me off too). +  

Requires knowing how to calculate an anion gap - look it up. In this case, it is a normal anion gap metabolic acidosis. Know the mneumonics MUDPILES and HARDASS. Renal Tubular acidosis is the only answer choice that is an example of a normal anion metabolic acidosis.

mousie  Anion Gap: Na - (Cl + HCO3) = normally around 10-12 +1  
seagull  good to know. I keep looking up the urine values but all it said was "varies", then I threw my computer and yelled "does that vary Mother F****ers. I do feel better now. +28  
_yeetmasterflex  glad I wasn't the only one who got very pissed off at the urine values +4  
fulminant_life  Usually the first thing I look at is whether or not the Cl- is high. Generally if the Cl- is high its going to be a normal gap +2  
henoch schonlein  i think they gave you the urine values bc you can calculate the URINE anion gap which is (Na + K - Cl). In this case the Urine Anion Gap is positive (5). Boards and Beyond mentions that a positive UAG is due to Renal Tubular Acidosis Type 1 (inability of alpha intercalated cells to secrete hydrogen ions). just another approach to answer this q +12  
270onstep1  Actually diarrhea is the "D" in "HARDASS"(reason why I was stuck between Chron's and RTA). Ended up getting it right with RTA.. +1  
talha_s  So the reason this is not Crohn's disease is actually what BnB explains in Renal Tubular acidosis video. Anytime there is a Metabolic Acidosis with intact kidney secretion of H+, the URINARY Anion gap (Na+K-Cl) is NEGATIVE. This is because the excess NH4 that is secreted into urine is combined with Cl-. Therefore, in Crohn's disease and Type 2 Renal Tubular Acidosis, the urinary anion gap is NEGATIVE. In this question, the urinary anion gap is POSITIVE so this would be an example of Type 1 RTA because the kidney can not excrete H+. I got it right by chance, definitely did not understand it in this much detail when I was answering it lol +1  

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submitted by sherry(11),

Diarrhea in HARDASS can lead to metabolic acidosis as well. On second thought I decided to take crohn disease cuz I figured the clinical picture is more intermittent with potassium disturbances. I guess I just overthink due to the lack of other physical abnormalities.

daddyyikes  other than the patient not haveing any sxs of crohn's i dont see any reason why its not crohn's. we differ crohn's and rta by urine anion gap. in chronic diarrhoea it is negative while in rta its +ve but in question the anion gap was normal +