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NBME 18 Answers

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submitted by โˆ—aneurysmclip(209)
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basically polycystic kidneys won't work properly, they've hinted at this with the s.creat of 4mg/dl. thus the kidney won't do what its supposed to do. REMEMBER to check whether they are asking SERUM changes or URINE changes

Kidneys normal function - reabsorb HCO3- , its not doing that now > decreased HCO3

PTH - would cause increased Calcium reabsorption in kidneys, but kidneys aren't able to reabsorb calcium > PTH responds to low calcium levels and levels increase

PO4 ties into PTH as well, PTH acts be DECREASING PO4 reabsorption. Since kidneys aren't working ie: not responding to PTH > there would be increase in PO4

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pg32  Sometimes these questions are made more difficult by trying to decipher the order in which these changes happen (first cause). In my mind, whenever I see a question on renal insufficiency, I know that phosphate in the serum will increase. In response, Ca will decrease and in response to that, PTH will increase. Lastly and unrelated, HCO3 will decrease because the kidneys aren't absorbing HCO3 as they usually do. +15
baja_blast  I also think there is decreased 1alpha hydroxylation of Vitamin D, which would increase PTH secretion +3
freebie  This questions is testing the consequences of renal failure --> "MAD HUNGER" mnemonic in FirstAid +1

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submitted by konkon(3)
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I got this one wrong. I thought there is hyperkalemia which leads to alkalosis

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hellhellhell  Hmm +
itsalwayslupus  If you look at FA 2019 p. 590 under Consequences of Renal Failure ("MAD HUNGER"), there is hyperkalemia and a metabolic acidosis +2

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submitted by โˆ—cbrodo(77)
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Renal damage from PKD can ultimately cause chronic renal insufficiency. This impairs the ability of the kidney to excrete phosphorus and reabsorb HCO3. Elevated phosphate levels in the blood triggers release of FGF-23 from bone, which lowers vitamin D production and decreases calcium absorption in the intestine. The resulting hypocalcemia and hyperphosphatemia will lead to an increase in the secretion of PTH.

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