NBME 23 Potassium: decreased;<br />Chloride: increased;<br />Bicarbonate: decreased (NBME Answers)

Retired NBME 23 Answers

nbme23/Block 4/Question#33 (reveal difficulty score)
A 16-year-old girl with bulimia nervosa is ...
Potassium: decreased;
Chloride: increased;
Bicarbonate: decreased 🔍 / 📺 / 🌳 / 📖
tags:

+13 upvote downvote
submitted by ∗nwinkelmann(366)

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I literally just realized why this question confused me so much (and I've tried to figure it out a couple of times, lol). I let the colloquial definition of bulimia (i.e. vomiting) stick in my mind, that I forgot the actual medical definition = normal BMI (>18) + binge eating and purging (where purging could be induced vomiting or diuretic use or laxative use or/and excessive exercise). So really, what this question was asking is simply what is the electrolyte balance of excessive diarrhea? GEEZ! I made it so much harder in my head when trying to answer it originally.

Diarrhea causes non-anion gap (i.e. hyperchloremic) metabolic acidosis. Stool predominantly contains HCO3- and K+, so excessive diarrhea = excessive loss of HCO3- and K+. Chloride levels in the serum will be increased due to the normal HCO3-/Cl- equilibrium, so as negative charge dissipates due to loss of HCO3-, Cl- will increase correcting the anion-gap.

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drdoom Bulimia comes from Greek "ravenous hunger"; the term is a literal concatenation of the words for ox (bous) + hunger (limos). So, in Greek, bou-limia is literally "ox hunger", which is a nod to how the word is used in medicine = perpetual and insatiable appetite for food (the very "opposite" of vomiting). +5

abhishek021196 I agree with your reasoning but the classic case description of Bulima lists electrolyte disturbance of HypOkalemia, HypOchloremia, and Metabolic Alkalosis, along with other things like parotid hypertrophy and dorsal hand calluses due to the induced vomiting. I tripped up there. :/ FA 20 Pg 567 +

llamastep1 Take home lesson: reasoning > memorizing +3

lulumomovicky I thought more of it by the following: the patient loses HCO3 in the stools and this diarrhea causes the Metabolic acidosis. Because normal HCO3/Cl equilibrium should be maintained, chloride levels in serum go up to compensate the loss of Bicarb. As the patient is also HYPOVOLEMIC due to BP of 89/61, then RAAS activates. Aldosterone will increase Na and water reabsorption to keep volemia and K will be excreted (hypokalemia). Also H-ATPase in the intercalated cells of the collecting duct will be activated by aldosterone, further propagating the metabolic acidosis. +1

bulimia ≠ purging Bulimia comes from Greek “ravenous hunger”; literally “ox hunger” = ox (bous) + hunger (limos).

+5/- drdoom(1206)

N.B., there are many ways to “purge”; the method of purge determines the observed electrolyte disturbance. A nice description of bulimia nervosa from MeSH:

BULIMIA NERVOSA. An eating disorder that is characterized by a cycle of binge eating (BULIMIA or bingeing) followed by inappropriate acts (purging) to avert weight gain. Purging methods often include self-induced VOMITING, use of LAXATIVES or DIURETICS, excessive exercise, and FASTING.

+1/- drdoom(1206)

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submitted by brookly_(1)

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I thought bulimia give rise to metabolic alkalosis ...can someone elaborate ?

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drdoom Remember, bulimia itself does not mean “purging”; it means “ox-hunger”. It is purging (e.g., intentional vomiting, laxative abuse, diuretic abuse, excessive exercise, or extreme fasting) which creates metabolic disturbance. The type of disturbance depends on your preferred route of “exit”. +2

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submitted by ∗dr.xx(176)

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Abuse of laxatives => usually decreased serum potassium. As to chloride and bicarb, that depends. They could be increased or decreased.

https://www.aafp.org/afp/2004/0315/p1530.html

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-4 upvote downvote
submitted by ∗usmleuser007(464)

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Treat this like a VIPoma (Watery diarrhea, achlorhydria = reduced HCl in the lumen, & hypokalemia)

this will lead to metabolic acidosis d/t loss of bicarb in stool

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btl_nyc Chloride is increased though. +

maxillarythirdmolar This comment is gold. @btl_nyc, this is actually accute. you would expect hyperchloremia https://www.ncbi.nlm.nih.gov/books/NBK507698/ +1

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submitted by ∗maxillarythirdmolar(45)

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Best comment i read was to treat this like it's a VIPoma. You would get all three electrolyte/metabolic disturbances.

https://www.ncbi.nlm.nih.gov/books/NBK507698/

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