NBME 22 Diuretics (NBME Answers)

Retired NBME 22 Answers

nbme22/Block 1/Question#45 (reveal difficulty score)
A 16-year-old girl is brought to the ...
Diuretics 🔍 / 📺 / 🌳 / 📖
tags: psych GI repeat

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submitted by ∗cathartic_medstu(37)

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

^^^a beautiful table that shows affects on electrolyte levels in laxatives vs diuretics vs vomiting.

According to the table, its hard to tell diuretic vs laxative abuse by serum electrolytes alone. The urine electrolytes would markedly differ in that diuretic will have increased Na/K/Cl and laxatives would follow the opposite trend (decreased). However, with the fact that BUN is increasing and that we can tell there's a metabolic alkalosis with respiratory acidosis compensation, we can bet on diuretics over laxatives.

EXTRA INFO from the table in the link above:

Vomiting: [K dec] [Cl dec] [HCO3 inc] [pH inc]
Laxatives: [K dec] [Cl inc or dec] [HCO3 dec or inc] [pH dec or inc]
Diuretics: [K dec] [Cl dec] [HCO3 inc] [pH inc]

In urine for vomiting, Na/K/Cl will all be decreased In urine for laxative abuse, Na/K will be decreased. Cl is normal or decreased. In urine for diuretic abuse: Na/K/Cl will all be increased

(Andrew Yang for President)

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drdoom table -> https://imgur.com/vyJZV55 +2

nootnootpenguinn The last sentence is nice touch!!! +2

l0ud_minority What kind of threw me off was the fine hair growth got me thinking hyperthyroidism. But I didn't fall for it. Got it right:) +

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submitted by ∗drmohandes(193)

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Our patient has a metabolic alkalosis with (partial) compensatory respiratory acidosis.

Metabolic alkalosis → H+ loss or HCO3- gain:

vomiting: lose H+ (and lose K+/Cl-)
loop diuretics: lose H+ (and K+)

Metabolic acidosis, possible causes in this context:

diarrhea/laxatives → lose HCO3- (and K+) ; Cl- compensatory increase (normal anion gap)
acetazolamide → lose HCO3- (and K+) ; H+ also decreases but not enough to overcome the alkalosis caused by HCO3- loss
spironolactone

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snripper This makes sense, thanks! +

dysdiadochokinesia I was able to break it down to diuretic or alcohol use and chose alcohol use under the assumption that the patient's serum Cl- levels were low (90; N = 95-105) since Cl- is also lost with vomiting. Im assuming that it was wrong for me to make the association between alcohol use and vomiting. +

avocadotoast @dysdiadochokinesia I think we can rule out alcohol use by looking at our patient's history and demographic. A 16yo girl who is dieting and constantly studying probably isnt getting turnt because 1) alcohol has empty calories (defeats the point of dieting), 2) why would you try to study when you're drunk, 3) where will this 16yo in social isolation get alcohol +1

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submitted by ∗usmleuser007(464)

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This more likely to be diuretics rather than laxatives b/c

the lab study shows a renal dysfunction (BUN & Creatinine are elevated)

Most likely the patient abused loop diuretics; also knows to cause contraction alkaloids, along with renal problems such as interstitial nephritis

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endochondral1 would laxatives also have the low potassium? +2

link981 My question exactly. And what if they were taking Potassium sparing diuretics? Then laxatives would be more likely or am I mistaken? +

link981 Also creatine is normal, it's at the higher limit of normal so we can't say there is renal dysfunction. The BUN is elevated because patient has metabolic alkalosis with respiratory acidosis. +

sweetmed very important to Remember this: Diarrhea causes metabolic acidosis[from bicarb loss in stool], vomiting & loop diuretics cause metabolic alkalosis. +14

hello @usmleuser007 not sure your approach is the best way to think about it. The serum Cr is at the upper limit of normal (1.2). And, even if you calculate the ratio of BUN/Cr, it's 21, which would be a PRE-renal issue. +

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submitted by ∗yotsubato(1208)

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Why cant this be laxatives? Both would cause metabolic alkalosis with hypokalemia... ?

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sup Laxatives would cause an anion gap metabolic acidosis due to loss of bicarbonate in the stool. You would see hypokalemia though as seen in this question. +1

miriamp3 it took me a lot of time choosing between laxatives and diuretics and at the end I choose diuretics. but I didn't realize that the only thing I had to do was check if were a anion gap or not. +

snripper Why would laxatives cause anion gap MA? Isn't it similar to diarrhea? +

castlblack The above comments are incorrect. Diarrhea is a cause of normal-anion-gap metabolic acidosis (D in HARDASS from FA). Laxatives are wrong because they would lower HCO3- but in this scenario it is high. The low K+ and Cl- fits either case though. +4

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submitted by ninja3232(11)

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1) Diuretics cause metabolic alkalosis (contraction alkalosis due to hypovolemia)

2) Diarrhea in general will cause a metabolic acidosis (with NORMAL anion gap) due to loss of K+ and bicarb in the stool (stool has alkaline pH normally).

3) Therefore, laxative abuse normally causes metabolic acidosis BUT chronic abuse can result in metabolic alkalosis due to various mechanisms (response to hypovolemia + more activity of H-K ATP-ase exchanger in the collecting duct due to hypokalemia - you pee out the acid to keep more K+).

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ninja3232 https://www.uptodate.com/contents/acid-base-and-electrolyte-abnormalities-with-diarrhea#H2 +

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submitted by ∗alexb(53)

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Is the part with "constant studying" just supposed to support that she has a psych disorder related to perfectionism, which is why she's going to extremes to control her weight?

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rrasha2 No, the constant studying is to trick you into thinking shes abusing amphetamines.Amphetamines decrease appetite so a lot of people abuse them for weight loss. That combined with increased concentration to study all day errrday.. #onehellofadrug +1

rrasha2 forgot to mention, another side effect of amphetamines would be increased BP due to the increased catecholamines..don't forget to keep an eye out for that +1

dentist would amphetamines influence electrolytes at all? +1

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submitted by presidentdrmonstermd(7)

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Dec. NaCl - general volume loss Dec. K+ - Diuretic (most diuretics, except K+ sparing ones, cause hypoK+) Inc. HCO3-& pH - Volume loss -> RAAS -> aldosterone causes K+ & H+ wasting -> metabolic alkalosis; She may be vomiting as well, which is another possible cause of met. alk. Inc PaCO2 - respiratory compensation for met. alk.

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hello Patient has normal Na. +

hello Lab data indicates serum bicab not ABG bicarb. +

hello oops! just realized bicarb is never given as an ABG haha +

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