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NBME 24 Answers

nbme24/Block 1/Question#14 (18.2 difficulty score)
A thin 66-year-old man is brought to the ...
Urine osmolality > plasma osmolality🔍

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 +7  upvote downvote
submitted by andro(109),

SIADH is characterized by :

*Excessive free water retention ( less water in urine means specific gravity increases and not decreases as in option E)

*Euvolemic hyponatremia with continued urinary Na+ excretion ( and so urine Na which actually be greater than and not less than 10 mEq/L option D)

*Urine osmolality > serum osmolality ( option C)

Serum potassium does not actually decrease ( counter-intuitively ) because the excess fluid retention suppresses aldosterone secretion . The two main stimuli for potassium loss/secretion in kidneys are - aldosterone - and high urine flow rates. Both of these are decreased in SIADH ( excluding option B )

Serum urea nitrogen and creatinine are diluted , so their concentration decreases

peridot  Great explanation, thank you so much!! +1  

 +6  upvote downvote
submitted by m-ice(272),
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frijoles  So potassium does not become diluted in SIADH? +1  
ruready4this  I feel like I was overthinking this question so much for some reason!! C definitely makes the most sense but I was also wondering what would happen to potassium. Then I was thinking maybe the excess ADH would suppress aldosterone secretion and serum potassium concentration would actually be higher +1  
peridot  @frijoles Aldosterone can adjust the K+ levels: too much water --> less aldosterone --> no excretion of K+, so this helps retain the K+ to a normal level. However, less aldosterone also means --> more excretion of Na+, so the hyponatremia is not corrected. +