Renal physiology question / scenario I believe to be wrong

[tony montana]

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A healthy 32-year-old male on a hi potassium diet for the past two weeks comes up with a urine concentration higher than his plasma concentration. One of the following scnearios best describes the percentage of the filtered potassium in the tubular fluid as it flows along this patient's nephron:


BC PCT LOH DCT CD

A) 100% 30% 23% 15% 150%
B) 100% 30% 23% 150% 150%
C) 100% 150% 150% 150% 150%
D) 100% 100% 100% 150% 150%
D) 100% 100% 100% 120% 150%


Some important people say it's A
I believe it's B since the DCT is impermeable to water and this dude is on a high K+ diet, what do you think?


BC= bowman's capsule
PCT = proximal convoluted tubule
LOH = loop of henle
DCT = distal convoluted tubule
CD = collecting duct

 

[alpha06]

I say "A".

Even if the DCT is impermeable to water, it still regulates K.

I think....

 

[tony montana]

I say "A".

Even if the DCT is impermeable to water, it still regulates K.

I think....

Certainly it does, but given a high potassium diet it should secrete K+ not reabsorb it. So how can the [K+] be lower in the DCT in this patient? 😕

 

[SOUNDMAN]

BRS Physiology states the K+ secretion occurs in both the Distal Tubule and collecting duct. So obviously A is correct, but the question I have is B "more correct?"

Would there be any "reabsorption" of K+ in the DCT if you have a high diet, it would have to be via the alpha-intercalated cells would it not? And BRS states that is active only in a LOW K+ diet.

However I think the priniciple cells would be secreting K+ at this point due to the electrochemical forces driving K+ into the lumen (on a high K+ diet). That would support an answer of B in my mind.

I think A would be the answer in the case of "excess aldosterone" would it not? Just food for thought I guess.

 

[HIJay]

A for sure. i seen this qs somewhere

and the intercalated and principal cells are not really found in the DCT and more in the CD where aldosterone has its effect on these cells

the DCT is basically the Na/Cl channel, what the thiazides has effect on

 

[InternationlDoc]

What I think:
it could be one of them ghey questions where they really mean "FILTERED" and NOT SECRETED ...hence A

ofcourse, that is an idiot of a question to base a question on such a wordplay. Just a thought people. Other than that, if they just meant % of potassium (including secretion) then B should be the correct answer.

What BSS book thinks:
Also people, I've done a similar question in BSS book and their explaination was:

"When excess K+ must be eliminated from the body, the rate of K+ excretion (mg/min) can increase to at least equal the initial filtered load (mg/min) of K+ because of K+ secretion."

they also said this "...by the time the tubular fluid reaches the begining of the distal tubule, the amount of K+ present in tubule has fallen to ~ 8%"
AHA! so combining those statements should be = 123%? uhh..B looks the right answer then.

 

[InternationlDoc]

What I think:
it could be one of them ghey questions where they really mean "FILTERED" and NOT SECRETED ...hence A

ofcourse, that is an idiot of a question to base a question on such a wordplay. Just a thought people. Other than that, if they just meant % of potassium (including secretion) then B should be the correct answer.

What BSS book thinks:
Also people, I've done a similar question in BSS book and their explaination was:

"When excess K+ must be eliminated from the body, the rate of K+ excretion (mg/min) can increase to at least equal the initial filtered load (mg/min) of K+ because of K+ secretion."

they also said this "...by the time the tubular fluid reaches the begining of the distal tubule, the amount of K+ present in tubule has fallen to ~ 8%"

AHA! so combining those statements should be = 123%? uhh..B looks the right answer then.

 

[Blesbok]

I dunno, I think it depends on your breakdown of the anatomy of a nephron. I have always been taught that the DCT contains thiazide sensitive NaCl transporters and the cortical collecting duct is the portion with the aldosterone sensitive Na/K antiporters. Then the medullary collecting duct has V2 receptors,aquaporins, and K/H antiporters. This may be wrong, but it would explain A being the answer.

 

[Blesbok]

What I think:
it could be one of them ghey questions where they really mean "FILTERED" and NOT SECRETED ...hence A

ofcourse, that is an idiot of a question to base a question on such a wordplay. Just a thought people. Other than that, if they just meant % of potassium (including secretion) then B should be the correct answer.

What BSS book thinks:
Also people, I've done a similar question in BSS book and their explaination was:

"When excess K+ must be eliminated from the body, the rate of K+ excretion (mg/min) can increase to at least equal the initial filtered load (mg/min) of K+ because of K+ secretion."

they also said this "...by the time the tubular fluid reaches the begining of the distal tubule, the amount of K+ present in tubule has fallen to ~ 8%"

AHA! so combining those statements should be = 123%? uhh..B looks the right answer then.

It doesn't say WHERE it is secreted is the problem. I have always heard it is in the cortical collecting duct.

 

[InternationlDoc]

just checked FA - Aldosterone activity in Collecting tubules NOT DCT

I'll believe FA.

 

[SOUNDMAN]

There are cells in the DCT according to BRS Physiology and Wikipedia the medical school source greater than all...LOL that K+ secretion is involved in the DCT.

http://en.wikipedia.org/wiki/Distal_convoluted_tubule

I believe that K+ can be secreted in the DCT and obviously the CD system. Especially in the case of high K+ diet.

Remember wiki > all

 

[SOUNDMAN]

You know looking at this question and just on guts I would go with A, if I'd seen this question without any of this conversation going on, as my initial answer I think.

 

[bgreet]

the question is directly from usmleworld and the explanation they gave stated that K excretion occurs in the CD

 

[tony montana]

the question is directly from usmleworld and the explanation they gave stated that K excretion occurs in the CD

They state K+ regulation is mainly there (CD), but do nothing to explain why the K+ concentration drops to 15% at the DCT being that the regulation is at the CD. Particularly given that the pt is on a high potassium diet.

 

[HIJay]

i think its bc that the CD is the MAJOR area where K is can and actively secreted under aldosterone, and everywhere else, such as DCT, it is not actively secreted, and rather it is being absorbed somewhat bc of the absorption of Na, thus dropping it to 15%. and if there were "secretion" it is not as significant as the CD

 

[tony montana]

i think its bc that the CD is the MAJOR area where K is can and actively secreted under aldosterone, and everywhere else, such as DCT, it is not actively secreted, and rather it is being absorbed somewhat bc of the absorption of Na, thus dropping it to 15%. and if there were "secretion" it is not as significant as the CD

HIJAY under a high potassium diet how could the DCT reabsorb more potassium?

BRS phys states the following regarding K+ reabsorption in the distal tubule and collecting duct:

Distal tubule and collecting duct
- either reabsorb or secrete K., depending on dietary K+ intake.
a. Reabsorption of K+
- involves a H+/K+-ATPase in the luminal membrane of the a-intercalated
cells.
- occurs only on a Iow-K+ diet (K+ depletion). Under these conditions,
K+ excretion can be as low as 1% of the filtered load because the
kidney conserves as much K+ as possible.

 

[Blesbok]

HIJAY under a high potassium diet how could the DCT reabsorb more potassium?

BRS phys states the following regarding K+ reabsorption in the distal tubule and collecting duct:

Distal tubule and collecting duct
- either reabsorb or secrete K., depending on dietary K+ intake.
a. Reabsorption of K+
- involves a H+/K+-ATPase in the luminal membrane of the a-intercalated
cells.
- occurs only on a Iow-K+ diet (K+ depletion). Under these conditions,
K+ excretion can be as low as 1% of the filtered load because the
kidney conserves as much K+ as possible.

It will excrete less through the aldosterone sensitive Na/K pumps and then whatever is left in the medullary collecting duct lumen, it will exchange for H+.

 

[tony montana]

It will excrete less through the aldosterone sensitive Na/K pumps and then whatever is left in the medullary collecting duct lumen, it will exchange for H+.

Sure, but that does not explain why the K+ % went from 23% in the LOH to 15% in the DCT, that would imply K+ leaving the DCT lumen (reabsorption), no?

 

[RussianJoo]

I would have to say that at first I would have picked A but after reading all this and looking at my physio class notes B is a much better answer. There are cells that are influenced by aldosterone in the distal portion of the distal convoluted tubule and collectiong duct, and under high K+ concentration will cause K+ to be secreted thus increase it's excretion..

What is the correct answer? I can't imagine it being A.. A sounds like a distractor all the way.. Especially if the intake of K is high. Where did you get this question? What did they say was the right answer?

 

[tony montana]

I would have to say that at first I would have picked A but after reading all this and looking at my physio class notes B is a much better answer. There are cells that are influenced by aldosterone in the distal portion of the distal convoluted tubule and collectiong duct, and under high K+ concentration will cause K+ to be secreted thus increase it's excretion..

What is the correct answer? I can't imagine it being A.. A sounds like a distractor all the way.. Especially if the intake of K is high. Where did you get this question? What did they say was the right answer?

They (begreet above IDed the source) say the answer is A. I disagree for the same reasons you stated.

 

[RussianJoo]

They (begreet above IDed the source) say the answer is A. I disagree for the same reasons you stated.

yeah USMLE world isn't god, i have found one or two other errors in their questions.. B is definitely the correct answer especially when taking into consideration the high K diet.. I even emailed UW with a question about one other question about a month and they haven't gotten back to me.. So I say just chalk it up to error on their part..

 

[tony montana]

yeah USMLE world isn't god, i have found one or two other errors in their questions.. B is definitely the correct answer especially when taking into consideration the high K diet.. I even emailed UW with a question about one other question about a month and they haven't gotten back to me.. So I say just chalk it up to error on their part..

Cool, yeah just wanted to get some agreement on it and made sure I wasn't way off here 👍

 

[78222]

The problem with answer B is that it assumes that all secretion must occur in the DCT since it is 150% in the DCT and still 150% in the CD. Most K+ secretion occurs in the CD so it doesn't make any sense for the secreted percentage of K+ to remain constant between the DCT and CD in a high K+ diet.

 

[78222]

Also, since it's impermeable to H2O, it will dilute the percentage of K+ making it decrease.

 

[RussianJoo]

The problem with answer B is that it assumes that all secretion must occur in the DCT since it is 150% in the DCT and still 150% in the CD. Most K+ secretion occurs in the CD so it doesn't make any sense for the secreted percentage of K+ to remain constant between the DCT and CD in a high K+ diet.

ok I agree and the problem with A is that the % goes from 23 to 15, and that's more incorrect than B, since we know that excretion not reabsorption should take place in the dct. I guess a the best answer choice would be 100%, 30%, 23%, 60%, 150%. or a number above 23% but below 150% for the part in the dct.

 

[RussianJoo]

Also, since it's impermeable to H2O, it will dilute the percentage of K+ making it decrease.

i guess but since K is secreted, the % at best should stay the same.. not really decrease.. Idk just hope you don't get this question on the step..

 

[78222]

ok I agree and the problem with A is that the % goes from 23 to 15, and that's more incorrect than B, since we know that excretion not reabsorption should take place in the dct. I guess a the best answer choice would be 100%, 30%, 23%, 60%, 150%. or a number above 23% but below 150% for the part in the dct.

The DCT is the diluting segment. It will retain water while filtering other substances, so it would be expected for the %to drop in relation to the water content.

 

[78222]

I don't think that matter because it's talking about % of total K not concentration of K. if it said concentration then maybe that would be correct.

Maybe, but I have a feeling some K+ is being reabsorbed in the DCT since Thiazide diuretics - which act in the DCT - cause hypokalemia.

 

[RussianJoo]

Maybe, but I have a feeling some K+ is being reabsorbed in the DCT since Thiazide diuretics - which act in the DCT - cause hypokalemia.

they cause hypo kalemia for the same reason all the other non K+ sparing diuretics cause hypokalemia, that's cause there's too much Na going to the collecting duct and in the collecting duct Aldosterone acts to reabsorb Na and waste K. Sure that happens in the late DCT too but that would cause wasting of K+ in the urine thus secretion thus low levels of K in the blood (hypokalemia). reabsorption would cause hyperkalemia, or hypokaluria.