NBME3 spoiler physio question

[Phloston]

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Why is the answer A?

Thanks,

 

[Boardz]

Why is the answer A?

Thanks,

the fluid is hypotonic. the cell is introduced to the hypotonic solution, and swells.

 

[Cwing99]

It's a hypotonic solution... the cell will swell.

 

[Phloston]

Yeah, but isn't B also hypotonic?

Plasma osmolarity should be 290 mOsm.

Why is it specifically A?

 

[HelpPleaseMD]

B is hypertonic bro.
150x2

 

[Phloston]

Wait...if I hear what you're saying: 2 MOLES of ions x 150 mOsm = 300 mOsm, then B would be ~isotonic, right?

I can understand that A is hypotonic, but I just want to know that the above logic is right.

Nothing like pulling a little gen chem back out of the closet.

 

[Cwing99]

Yes, B is considered isotonic, so no water would flow. A is the only hypotonic answer.

 

[HelpPleaseMD]

Wait...if I hear what you're saying: 2 MOLES of ions x 150 mOsm = 300 mOsm, then B would be ~isotonic, right?

I can understand that A is hypotonic, but I just want to know that the above logic is right.

Nothing like pulling a little gen chem back out of the closet.

there are quite a bit of gen chem and some surprise orgo questions on the uslme. this is something that is covered in physiology though and yes I did mean hypertonic (though only slightly).

 

[thinkenergy]

Solution would be hypotonic, in this case, half of that of RBC, so 300/2 = 150, but keep in mind, every one NaCl formula unit breaks down into 2 ions: Na+ & Cl-, so 150/2 = 75.

 

[Phloston]

Yeah, I realize the simplicity of the question now. Kind of a joke, really, when you think about it.