NBME 3 question

[nycmedstudent]

To those of you who took this exam...

there was a question about figuring out the osmolarity of a solution that caused a RBC to swell...the osmolarity of the original solution when RBC was its original size was not given. did you just use normal serum osmolarity of 290, or the sum of serum Na and Cl osmolarity? I couldn't figure this one out, I've been told the answer was 75mM, but with a RBC in normal serum of somewhere between 250 and 300, it would have to swell 4x if the new solution was 75mM...what am i doing wrong??? or was I misinformed about the answer. In the graph it appeared as though the RBC only swelled to max 1.5X it's original size...

thanks!

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[DwyaneWade]

To those of you who took this exam...

there was a question about figuring out the osmolarity of a solution that caused a RBC to swell...the osmolarity of the original solution when RBC was its original size was not given. did you just use normal serum osmolarity of 290, or the sum of serum Na and Cl osmolarity? I couldn't figure this one out, I've been told the answer was 75mM, but with a RBC in normal serum of somewhere between 250 and 300, it would have to swell 4x if the new solution was 75mM...what am i doing wrong??? or was I misinformed about the answer. In the graph it appeared as though the RBC only swelled to max 1.5X it's original size...

thanks!

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[DocKat]

I guess 75 makes sense if you are thinking about concentration, not osmolality, and the serum NaCl conc is around 150. I don't know if this is right though, since I thought that it was 150 as well.

 

[Noroozi]

Correct me if i'm wrong but I believe the answer is 75. Remember NaCl is two ions therefore osmolarity becomes 150, making it the only option on the list that is hypotonic and thus causes RBC swelling. 150 would be isotonic (150 x 2) and the other options were hypertonic.

 

[DwyaneWade]

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[TonyDaTigrUprct]

Was it describing the sodium concentration or osmolarity? I thought it said concentration (osmolarity) so normal plasma osmolarity is 300, if the red cell doubles you get 150.

that was EXACTLY my thinking