renal question (loss of 50% of nephrons)

[Bunjagraham]

I have spent way too much time obsessing over this question, and don't want to send the professor who wrote it any more than the two emails I have already sent him trying to understand the answer. Anyone interested in taking a stab at it?

Following a crush injury in an auto accident a 20-year-old male is forced to undergo a leg amputation. Owing to use of an antibiotic with significant renal toxicity to combat a post-surgical infection, he loses half his original number of functional nephrons. Six months later (all renal parameters are in steady state) which of the measurements of renal function would be least changed compared to before the accident?

A. creatinine excretion
B. creatinine clearance
C. Na+ excretion
D. plasma concentration of urea
E. glucose reabsorption

 

[masterofmonkeys]

I would GUESS D.

This based on the fact that I've always been told that you have to lose 70-80% of your kidney function before you see a chance in plasma Cr or BUN

 

[thechad]

Probably C because the other kidney can increase its urine output to maintain electrolyte balance.

 

[Bunjagraham]

C is the correct answer. Good job, thechad! I'm impressed. (I had thought the correct answer was E.)

 

[coolioyo]

GFR will change because there are less glomeruli, plasma concentration of urea will increase because there is less urea being filtered (GFR*plasma concentration= excretion). Because creatinine clearance is GFR, it will also change. Glucose reabsorption is decreased due to the same logic, less GFR less excretion and thus less reabsorption. Sodium excretion is dependent on blood pressure, initially sodium excretion will be decreased but there will be compensation by the remaining tubules to maintain sodium excretion, so it would not change. However, I think A could also be right because creatinine excretion is dependent on creatinine creation which is constant, but it might have changed because of the auto accident. Yes, it did change because of the loss of the leg. Thanks for the question.

 

[IHeartNerds]

Remember that the kidney can vary its Na excretion over an extraordinary wide range depending on dietary intake. This is why C is correct -- hell, you could have 25% of kidney function and maintain normal Na excretion.

All the other may or may not change, but C is the least likely to.

Questions like this are why we are moving away from "all BUT" and "everything true EXCEPT" formatting...

 

[MattD]

Right, the real reason for answer c is that Na transporters are typically nowhere NEAR saturation, so they have the greatest ability to compensate.