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NBME 21 Answers

nbme21/Block 2/Question#35 (71.2 difficulty score)
A 4-month-old female infant is brought to the ...

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submitted by champagnesupernova3(75),
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rrDehaai casseu sols of ,waetr aN dna .rbcBai rRemmebe ttha rareaihd is a escau fo nno naoni gap bcmolteai siadocsi ecbause het lsos of cerabobiatn is dmesnoeaptc by nsigarceni iherlodC paesno.toirrb oS she nca evha aplyihcrhoeemr ro hyoiaemntarp edu to enktai of ynol efre wrtea ofr 24 huros. utB seh sha SIESERZU os hyampnaoreit si mots lkeyil

drzed  I think the initial hyperchloremia would be quickly diluted out by the large consumption of water, so those two competing processes would likely neutralize the chlorine level, or even make the child hypochloremic. +1  

submitted by haliburton(215),
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iThs si rawte icx.tootainin hsihciev8p.wmp:7/nn.9/.75mtn.wbgo/7du1wbl/t

thisisfine   Agreed! It's along the lines of those marathon runners who collapse questions. Nothing but water for 24 hours = getting rid of too much sodium. +1  
temmy  are we just going to ignore the diarrhea for 3 days? what is its significance +4  
kard  Temmy, We aint Ignoring the Diarrhea, Actually the most likely electrolytes to get lost with it is sodium> chloride> potassium> bicarbonate... Plus the Water intoxication -> HYPONATREMIA +1  
bronchophony  why not hypoglycemia? +1  
saulgoodman  Because glucose is not an electrolyte, it does not conduct electricity in solution. The question is asking "Which of the following electrolyte abnormalities". +4  

submitted by haozhier(19),

Why is it not hypoglycemia?? Hypoglycemia can also lead to seizure

cuthbertallg0od  Same thoughts here, and I think hypoglycemia occurs earlier in kids/infants than in adults (like 8 hours?) -- maybe just more likely to be hyponatremia since Na+ lost in the diarrhea... +2  
fbehzadi  I think mostly the fact that 24 is not gonna reduce his glucose to the point of causing a seizure. +  
fbehzadi  24 hours* +  

submitted by readit(14),
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ehT kye ot tish snouetiq si that hte ntiptae si 4 ntohms ldo nda gitegnt

rewbnsoN lhsodu NTO be iteggnt iapln rtaew unlit erfat 6 nsohtm fo aeg caeseub ti can ecusa aaoptmyherin -&-t;g ruzeesis

submitted by didelphus(59),
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nAy eadi ywh lecharyhirmpeo ts'ni na ?nesarw hTe aairderh udowl uceas an olmran nioan agp cirle()rhhopycem otmabicel i.caidsso

charcot_bouchard  this is the problem bet uw and nbme. in uw it would be for sure a gotcha ques. but in nbme they are usually looking for most obvious. also look what they are asking "most likely". baby would dev low Na before acidosis. Thats my 2 cents +26  
temmy  hyperchloremia will not account for the seizure that brought the patient to the hospital. seizures according to first aid is caused by hypocalcemia and hyponatremia +1  
cry2mucheveryday  Children with diarrhoea who drink large amounts of water or other hypotonic fluids containing very low concentrations of salt and other solutes, or who receive intravenous infusions of 50% glucose in water, may develop hyponatraemia. This occurs because water is absorbed from the gut while the loss of salt (NaCl) continues, causing net losses of sodium in excess of water. The principal features of hyponatraemic dehydration are: there is a deficit of water and sodium, but the deficit of sodium is greater; serum sodium concentration is low (<130 mmol/l); serum osmolality is low (<275 mOsmol/l); the child is lethargic; infrequently, there are seizures. +  
cry2mucheveryday  Also, why is this being given formula...? May be lactase deficiency...which leads to osmotic diarrhea...leads to hyponatremia(goljan) Aren't newborns supposed to be kept on exclusive breast milk till 6 months?? +  
hello  @cry2mucheveryday Don't read too much into it. The fact that the baby is receiving formula isn't relevant to answering the Q. Btw, not everyone breast feeds. Additionally, the Q wouldn't make much sense if it said "they ran out of breastmilk"... +1  
hello  @cry2mucheveryday Being on formula then the parents running out of formula is more of a clue for water intoxication. This is typically the scenario that water intoxication presents. However, I suppose if for some reason the baby was being breastfed and the parents switched to exclusively waterfeeding (and no other foods), then water intoxication would also result. +  

submitted by drzed(221),

Perhaps I under-thought this questions, but it is highly unlikely to have HYPER- of anything when consuming large amounts of water, because whatever ion is present is going to get diluted. So in the case of normal gap acidosis from diarrhea, yes there may be an initial hyperchloremia, but the water is going to dilute it out.

Between hypoglycemia and hyponatremia, it is more likely to be hyponatremia because the child had seizures

submitted by champagnesupernova3(75),
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eDaihrra aucsse lsos fo wrat,e aN nad bcr.aBi mRbereme thta rhareida is a ceuas of onn ninoa apg cmloiatbe codsaisi ebsueac the slos of anobtecbria si nmcasepdote yb easncgirni Cidrohle ptsbro.earoni oS she nac heva pehlhmraercioy or tonharmeiyap ued to aektni fo olyn refe rtewa ofr 24 ou.shr But seh has ESRZSEIU os iatpnoreayhm is smto leklyi