virgola82
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The clearance of several substances was mesured at a constant GFR and constant urine flow rate, but at increasing plasma concentrations of the substance. Under these conditions, clearance will increase at high plasma concentrations for which of the following sustances
A. creatinine
B. mannitol
C. penicillin
D. phosphate
E. urea
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| Luckyall
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Penicillin is like PAH so at high plasma conc---> low CL
Mannitol--> is like inulin--> either incr/decr plasma conc----> we clear same Volume
Answer Creatinine since it exhitits secretion---> incr CL at ct GFR
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| new_n_lost
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E. urea
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| Luckyall
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i thought urea is partialy reabsorbed( passive mechanism)-----> CL decr
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| monam84
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All I know is that it has to be sumthin that is reabsorbed by a Tm system like glucose for example, the answer is not creatinine because as you increase the conc of it the clearance decreases and approaches GFR. Also, mannitol as you said is like inulin so its clearnace stays the same. So that limits me to C,D,E. Urea is reabsorbed passively so Im not sure if thats affected. My guess is C or D.
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| Luckyall
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Phosphate is reabsorbed through Tm system... at high plasma conc..it shall follow Glucose story, totally agree, but the Qs stem says "clearance of several substances was mesured at a CONSTANT GFR CONSTANT urine flow rate"
constant GFR from more plasma conce--- more PH filtered... yeah, it could be Hight plasma Phosphate saturating Tm system---> spilled over in urine & incr CL agree
PNC i dont think so....it should follow PAH high levels rule
Creatinine exhibits small secretion... i still go for creatinine, though I agree with phosphate following high Glucose rule since it exhibits TM system...
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| monam84
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Uggh, im so curious, whats the answer, I say phosphate (i didnt know it was reabsorbed) great question.
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| virgola82
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When I did it I said urea and got it wrong.
The clearance of a substance can change with increasing plasma conc of that substance only if it is either reabsorbed or secreted by a carrier in the tubule. If the conc increases saturating the carrier each further increase in conentration will result in a change in clearance. So Luckyall was right and the ansewr is Phosphate. 80% of phosphate is reabsorbed in prox tubule by a Na/P cotransporter which can be saturated increasing plasma phosphorus
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| Luckyall
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just curious... whas the source of the Qs...
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| virgola82
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kaplan q book
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| Luckyall
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thanks 
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| lq2006
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D
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