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NBME 20 Answers

nbme20/Block 3/Question#38 (44.2 difficulty score)
A 23-year-old man with a 3-year history of ...
Psychogenic polydipsiaπŸ”

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submitted by drzed(206),

Patient has low serum sodium = hyponatremia.Given that the patient has a LOW URINE OSMOLARITY, it suggests that ADH is NOT active. The only way for someone to have hyponatremia AND a low ADH (in this case) is through psychogenic polydipsia (e.g. if it was SIADH, the urine would be MAXIMALLY concentrated and it is NOT in this case)

(A) would cause central DI -- no ADH means one develops hypernatremia as free water is lost in the urine, thus concentrating the serum.

(B) osmotic diuresis could cause hypernatremia due to loss of free water in the urine

(C) degradation of ADH leads to DI which means one develops hypernatremia

(E) resistance to ADH (nephrogenic DI), again, hypernatremia.

lovebug  thanks for kindful labeling! :) +1  

submitted by masonkingcobra(314),
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nI gsincceyohp iyispopal,d mesur mdousi is wol, nad tfera awrte ntiiavrodep t,tse ruine lyoaotsilm is rndi.naceeirUes omitlsoayl esod not cenerias twhi seoisrvpsna jeointicn

In gpoeicrnhne bteaisde iispdu,ins smure idmuos is high dna erhte is no decg/amlinh ncieresa ni rueni olylmtoisa faret trewa vrioptdanei

yotsubato  This patient does not undergo a water deprivation test +12  
niboonsh  Compulsive water drinking or psychogenic polydipsia is now increasingly seen in psychiatric populations. Effects of increased water intake can lead to hyponatremia causing symptoms of nausea, vomiting, seizures, delirium and can even be life threatening if not recognized and managed early. +7  
missi19998  Just wondering why it in not resistance to ADH action of vasopressin +  
amarousis  because he would be hypernatremic with no ADH. can't resorb any water +1  
minhphuongpnt07  low osm/urine, low os/plasma => psychogenic polydipsia +  
benitezmena  In this question the pt had a normal urine osm (80) a low urine osm would be <50mosmol/kg. +  
euchromatin69  u world 212 +  

submitted by dickass(84),
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Yh,ae s,o nurts otu htis si ton nNcprgeoeih ID eud ot imhtiul es,u you d'not eigv tlhuiim to eartt

mbourne  If it was Nephrogenic DI, you would have essentially have the effects of too LITTLE ADH. This patient shows severe hyponatremia, essentially the effects of too much water in the serum. This could be from SIADH or polydipsia, and the question stem and answer choices leave us with Psychogenic Polydipsia as the correct response. +1  
rockodude  I was thinking about carbamazepine causing SIADH but that is an anti-epileptic not antipsychotic and also as someone said above, the urine would have high osmolarity due to water reuptake at the collecting duct. just fyi +  

Anecdotally, some of my clinical faculty have mentioned that typical antipsychotics have a very high predilection to cause psychogenic polydipsia.

+3/- ibestalkinyo(39),