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AAMC 3 BS #125

Discussion in 'MCAT Study Question Q&A' started by sshah92, Jan 13, 2014.

  1. sshah92

    sshah92 5+ Year Member

    Apr 25, 2011
    (Passage not needed to answer the question.)

    A lower than normal blood pressure will cause which of the following effects on the rate of plasma clearance of Substance A?
    A) An increase, because the concentration of Substance A in the urine will increase
    B) An increase, because the ADH levels will be very low
    C) A decrease, because the decreased rate of urine output will allow more reabsorption by the kidney
    D) A decrease, because ADH levels will be very high

    So the answer is C but I chose D. The answer says that there is not enough information in the passage to go with D -- HOWEVER, it does state: "The effect of ADH on the plasma clearance was tested. The volunteer was given two dosages of aDH during separate trials. The first dosage was within the normal physiological range of concentrations and affected the renal concentration of water. The second dosage, which was much higher than the normal physiological concentration, affected urine output by increasing blood pressure."

    Even if the tester doesn't know the effect of ADH (which we obviously should) on BP, there's enough info in the passage to ascertain this. Meanwhile C seems kinda shaky since the decreased rate of urine output is an EFFECT of decreased reabsorption by the kidney... not the other way around.

    What gives?!?!
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  3. sshah92

    sshah92 5+ Year Member

    Apr 25, 2011
  4. dillweed


    Jul 24, 2013
    You actually do answer this using the passage. "The "Tm" is the maximum rate of transport at which a substance can be reabsorbed by the kidney." Basically, if the kidney has more time with the filtrate, it will be able to reabsorb more of a particular substance, like Substance A, and there will be less plasma clearance because more will be reabsorbed into the blood. If the blood pressure is lowered, then the filtration rate is lowered, and there will be a decreased rate of urine output which will give more time for reabsorption.
    ahisma likes this.
  5. QuinnTheEskimo


    Jan 9, 2014
    D is a bad answer because ADH levels will not be "very high" under physiological conditions.

    Also, I don't see how you believe increased ADH would cause clearance of substance A to decrease.

    ADH affects the nephron at the end of the tubule. Substances are usually re-absorbed more towards the beginning (before the loop of henle).

    By "urine output" I believe they are referring to filtration at the glomerulus. When bp is low, there less pressure on the glomerulus, and less fluid gets into the nephron.
  6. eradicator

    eradicator 5+ Year Member

    Jun 25, 2010
    A high BP increases the glomerullus filtration rate of the nephrons, so a low BP decreases the filtration rate...allowing the "rate of urine output" to also slow down...which allows for more time for Substance A to be reabsorbed at the proximal tubule. ADH is a hormone that actually works at the collecting ducts to reabsorb water.
  7. jonhamm

    jonhamm 2+ Year Member

    Jul 14, 2013
    Why does the amount of urine in the nephron/tubule dictate how long that urine will remain in the tubule? i..e. I don't understand why less urine in the nephron means more time for reabsorption.
  8. Arayh

    Arayh Resident 2+ Year Member

    Jul 18, 2013
    If you think about the nephron like a long tube that connects to the collecting duct, more fluid will increase the speed of that fluid inside the tube, assuming that you don't want your nephron to distend infinitely and explode out of your kidney.
  9. Messier83


    Mar 3, 2014
    Just to summarize your points: (correct me if I am wrong)
    *Higher blood pressure = increased filtration rate = increased output = increased plasma clearance = decreased reabsorption.

    *Lower blood pressure = decreased filtration rate = decreased output = decreased plasma clearance = increased reabsorption.

    I am curious how would ADH fit into all of this. I agree that it increases reabsorption at the DCT's but wouldnt that still somehow decrease plasma clearance?
    Last edited: Mar 23, 2014
  10. Next Step Tutor

    Next Step Tutor MCAT Guru 2+ Year Member

    Sep 30, 2013
    Tucson, AZ
  11. remedy23

    remedy23 2+ Year Member

    Jun 17, 2014
    Just did this question - and yes, I definitely agree that C is a "shaky" answer, and picked D as well. It was tricky.

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