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TBR GI and Kidney Psg XI Q 71

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erythrocyte666

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Answer is D

I was stuck between A and D.

Reasoning for A: low calcium à increased PTH release à digesting bone to release more Ca and phosphate à increased serum phosphate

Reasoning for D: because serum phosphate now increases due to above reasoning, its reabsorption in tubule will decrease

My question is how would I know to choose the right one? I’ve frequently encountered this sort of problem before, where one choice ends the line of reasoning early on but another choice can be seen as a continuation in reasoning from that previous choice.
 

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AlteredScale

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Reason A is wrong because PTH inhibits phosphate reabsorption.

So if [Ca] is low, PTH is release. One of the effects of PTH release is inhibition of tubular resorption of phosphate (D) which will eventually decrease [phosphate].

I think the biggest issue here was that you did not fully understand the affects of PTH to answer the question correctly.
 

erythrocyte666

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Reason A is wrong because PTH inhibits phosphate reabsorption.

So if [Ca] is low, PTH is release. One of the effects of PTH release is inhibition of tubular resorption of phosphate (D) which will eventually decrease [phosphate].

I think the biggest issue here was that you did not fully understand the affects of PTH to answer the question correctly.

But when PTH causes digestion of bone matrix, aren't both Ca and phosphate released into bloodstream? And then when it's filtered through the kidney, PTH would prevent its reabsorption at the renal tubule because its serum concentration is high, no?
I'm guessing my reasoning of temporally separated PTH effects is wrong.
 

AlteredScale

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But when PTH causes digestion of bone matrix, aren't both Ca and phosphate released into bloodstream? And then when it's filtered through the kidney, PTH would prevent its reabsorption at the renal tubule because its serum concentration is high, no?
I'm guessing my reasoning of temporally separated PTH effects is wrong.

You are correct in that aspect (the MoA of PTH) but then you need to think about where the majority of [phosphate] is derived, from the breakdown of the bone matrix? or from renal absorption at the proximal tubule.

In my viewpoint, renal regulation plays a larger role in the overall serum [PO4] than PO4 release by PTH. So the overall affects of PTH cause a systemic decrease in serum PO4.

Hopefully that didn't confuse you.
 
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erythrocyte666

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Ah so I'm also wrong in assuming that PTH will cause simultaneous release of both Ca and phosphate when it stimulates osteoclasts to digest more matrix?
 

Cawolf

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You are correct, it does liberate phosphate from the matrix.

The effect of decreased phosphate reabsorption is slightly more appreciable though, so that this effect exhibits the larger influence over phosphate levels.

So an increase in PTH levels results in a slight decrease in plasma phosphate levels.
 
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