NBME 3 question

Discussion in 'Step I' started by nycmedstudent, May 27, 2008.

  1. nycmedstudent

    Joined:
    Jan 16, 2008
    Messages:
    10
    Likes Received:
    0
    Status:
    Medical Student
    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!
     
  2. Thread continues after this sponsor message. SDN Members do not see this ad.

  3. DwyaneWade

    DwyaneWade Reiging *** Cynic

    Joined:
    May 7, 2006
    Messages:
    669
    Likes Received:
    0
    Status:
    Resident [Any Field]
    ''
     
    #2 DwyaneWade, May 27, 2008
    Last edited: Jun 21, 2008
  4. DocKat

    DocKat Member

    Joined:
    Jun 23, 2005
    Messages:
    109
    Likes Received:
    1
    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.
     
  5. Noroozi

    Noroozi Member

    Joined:
    Feb 11, 2004
    Messages:
    58
    Likes Received:
    0
    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.
     
  6. DwyaneWade

    DwyaneWade Reiging *** Cynic

    Joined:
    May 7, 2006
    Messages:
    669
    Likes Received:
    0
    Status:
    Resident [Any Field]
    ''
     
    #5 DwyaneWade, May 27, 2008
    Last edited: Jun 21, 2008
  7. TonyDaTigrUprct

    Joined:
    Jan 29, 2008
    Messages:
    54
    Likes Received:
    0
    Status:
    Medical Student
    that was EXACTLY my thinking
     

Share This Page