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NBME 20 Answers

nbme20/Block 3/Question#38

A 23-year-old man with a 3-year history of ...

Psychogenic polydipsia

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In psychogenic polydipsia, serum sodium is low, and after water deprivation test, urine osmolality is increased. Urine osmolality does not increase with vasopressin injection

In nephrogenic diabetes insipidus, serum sodium is high and there is no change/mild increase in urine osmolality after water deprivation

yotsubato  This patient does not undergo a water deprivation test +7  
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. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5579464/ +5  
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 +  

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submitted by dickass(22),

Yeah, so, turns out this is not Nephrogenic DI due to lithium use, you don't give lithium to treat schizophrenia.

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. +