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NBME 23 Answers
A 16-year-old girl with bulimia nervosa is brought ...
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).
Here’s a fun tangent! 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 (which is precisely the “opposite” of vomiting!). So, remember, bulimia ≠ vomiting/purging (although people who are bulimic might also vomit/purge); rather, bulimia = ox hunger!
Chloride is increased though.
I don't think so. I know that K+ levels decrease with laxative use, due to dehydration, which activates the RAAS, which increased aldosterone, which cause Na+ re-absorption and K+ wasting. Aldosterone also causes the alpha intercalated cells to secrete more H+ into the urine, which causes a serum alkalosis. Therefore, in order to correct that, bicarb re-absorption decreases in the kidneys, which brings the pH closer to normal. As far as Chloride, I guess that must be re-absorbed with Na+ due to it being negatively charged (?). That's the one thing I'm not sure about.
I think what they are getting at is that it is Diarrhea--> Non-anion gap metabolic acidosis (HARDASS). This would mean that HCO3- would be low and chloride would be high (in non-anion gap acidosis the chloride increases and that's why you don't have a gap).
Normally, stool's electrolyte content primarily consists of bicarb, potassium, and sodium. Since the colon reclaims sodium in exchange for potassium, the potassium content of stool is usually double that of sodium. Most of our bicarb loss in stool actually occurs through the loss of organic acid anions, i.e. bicarb that's been titrated by the organic acids formed by bacterial fermentation in the colon (e.g. lactic acid).
*Bottom line: our stool is alkaline, with mostly bicarb and potassium.*
Diarrhea is a cause of *NON-anion gap metabolic acidosis* due to bicarb loss in the stool. We aren't adding any acids to the mix -- we're simply losing anions -- which is why our anion gap remains normal. Potassium goes along for the ride and we end up with *hypokalemic* metabolic acidosis. And because we're losing anions, we want to compensate by *increasing retention of Cl-*.
**Anion gap = Na+ - [Cl- + HCO3-]**