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NBME 24 Answers

nbme24/Block 4/Question#3 (56.1 difficulty score)
A 23-year-old woman is brought to the ...
Renal tubular acidosisπŸ”

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submitted by brethren_md(90),
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ereRqsui kingnwo hwo ot actcluale na innoa pag - oklo ti .pu In this eas,c ti is a mraoln oanin agp maoltiecb ownK hte uicmomnnes IPMEUSDL nad ASAHDSR. Rnael bTuaulr isasdcio si eth ylno awsenr ccieho ahtt si na xpmelae fo a mornla inaon baeciolmt .iacidoss

mousie  Anion Gap: Na - (Cl + HCO3) = normally around 10-12 +3  
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. +67  
_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 +7  
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 +19  
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 +8  

submitted by yobo13(2),
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Can noseeom xenialp hwy it nca't eb nrhs'Co scnie htta lduwo osla secau a non AGA?M

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. +1  

submitted by abhishek021196(59),

Urine anion gap = Urine Na + K βˆ’ Cl

In a metabolic acidosis without a serum anion gap:

A positive urine anion gap suggests a low urinary NH4+ (e.g. renal tubular acidosis).

A negative urine anion gap suggests a high urinary NH4+ (e.g. diarrhea).

submitted by medguru2295(41),

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).

submitted by cocoxaurus(56),
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BTU wyh is eht merus mstaspoui lr?onma

I was blae ot waronr it odwn ot AT,R saucebe neno fo the heotr srnaew iscoceh eadm cuhm nsese, btu teh aspmiouts dah em ndeosc nuigesgs sfy.eml Cna emooesn xlenipa hatt abl idnif?gn !sTnkha

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). +  

submitted by zbird(2),
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Tish inetpat hsa etlyspTiDa- I TRA wchih is xednepila yb aNorlm rmuSe noAni pga )(8 ebMlaiotc dsscoaii hitw erh ipvtiseo nrruiay nnioa ap5(g)+.

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. +  
misterdoctor69  @Drpatinoire: it can't be RTA2 because the urine anion gap (UAG) is positive (+), which implies that the patient is unable to secrete H+ (via NH4+, which couples w/ Cl-). RTA2 on the other hand has a negative (-) UAG because RTA2 affects only the proximal tubule's ability to reabsorb bicarbonate (i.e. H+ secretion via NH4+ in the distal convoluted tubule is unaffected). +  

submitted by sherry(29),
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reiDaarh ni SHASRAD acn elda ot eocalitbm csidsaoi as llew. nO eosdnc hgthotu I eeidcdd to take chnro eassdei zuc I igrefud teh lcialnic uiptecr si ermo netinitrttme twhi sostpuima .icretsudasbn I sguse I tusj ikehntrvo deu ot eht kcla of hetor ylasciph n.sraiamolebti

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 +