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#1
02-23-2012
 USMLE Forums Addict Steps History: Step 1 Only Posts: 106 Threads: 9 Thanked 73 Times in 43 Posts Reputation: 83
calculation without calculation ???

hey guys! i have some difficulties with this questing. i know how to calculate the clearance but the explanation of this question drives me nuts...mark to view: Because in this patient the urin solute concentration is half the plasma concentration, the amount of plasma concentration must be half the urine volume per day. Alternatively, you can calculate clearance: cl=(6L/d * 150 mosm/L) /300 moms/l = 3 L/d
(where the hell does the 6L/d come from????)

A patient with a history of unexplained polydipsia is admitted for evaluation of possible diabetes insipidus. After several hours of fluid deprivation, the following measurements are made:
plasma osmolarity: 300mosm/L
urin osmolarity: 150mosm/L
urin solute output: 900mosm/d
What is the solute clearance?

a) 1.5 L/d
b) 2.0 L/d
c) 3.0 L/d
d) 6.0 L/d
e) unable to be calculated unless plasma volume is also known

#2
02-24-2012
 Banned Steps History: Not yet Posts: 176 Threads: 34 Thanked 117 Times in 69 Posts Reputation: 127

no idea
#3
02-24-2012
 USMLE Forums Addict Steps History: Step 1 Only Posts: 106 Threads: 9 Thanked 73 Times in 43 Posts Reputation: 83

ok...i got it....showering really helps thinking....y think i can shower during my exam?

here we go:

900mosm/d and 150 moms/L -> 6L/d 'excretion'

#4
02-24-2012
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 216 Threads: 23 Thanked 180 Times in 72 Posts Reputation: 190

Haha, nevermind. We weren't born to be mathematicians after all were we!
#5
02-24-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 538 Times in 375 Posts Reputation: 548

Quote:
 Originally Posted by MedPharm ok...i got it....showering really helps thinking....y think i can shower during my exam? here we go: 900mosm/d and 150 moms/L -> 6L/d 'excretion' calc: cl=(150*6)/300 =3 tadddaaaaa.......

did not get it still why 900 * 150 = 600 ?? could you please explain in a little detail ...

thanks
#6
02-24-2012
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 216 Threads: 23 Thanked 180 Times in 72 Posts Reputation: 190

Quote:
 Originally Posted by Hitman did not get it still why 900 * 150 = 600 ?? could you please explain in a little detail ... thanks
They've given the urine output as 900 mosm/day (which sounds pretty ridiculous, btw!)
And the "osmolality" of urine is 150/L (i.e 150 milli osmoles in 1 litre.)
So how many "litres" would have 900 milli osmoles?

Do that math!

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