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NBME 21 Answers

nbme21/Block 2/Question#47

A 55-year-old woman comes to the physician because ...

Increased parathyroid hormone

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submitted by hayayah(416),

Secondary hyperparathyroidism (usually d/t chronic renal failure).

Lab findings include ↑ PTH (response to low calcium), ↓ serum calcium (renal failure), ↑ serum phosphate (renal failure), and ↑ alkaline phosphatase (PTH activating osteoBlasts).

haliburton  also remember that in renal failure, 1-alpha-hydroxylase activity is down, so there will be less activation of 25-hydroxycholecalciferol to 1,25-hydroxycholecalciferol, which is a key mechanism causing hypocalcemia. +1  
cr  why not increased 25-hydroxycholecalciferol?, with the same logic haliburton explain +  
nala_ula  Increased phosphate, since the kidneys aren't working well, leads to the release of fibroblast growth factor 23 from bone, which decreases calcitriol production and decreased calcium absorption. The increase in phosphate and the decrease in calcium lead to secondary hyperparathyroidism. +1  
privatejoker  Probably a dumb question but how do we definitively know that the ALP is elevated if they give us no reference range in the lab values or Q stem? Everything stated above definitely makes sense from a physiological standpoint, I was just curious. +  




 +1  upvote downvote
submitted by cr(1),

why not increased 25-hydroxycholecalciferol?, with the same logic haliburton explain +

usmle11a  i believe increase 25OHcholecalciferol indicates the storage capacity of vit D, which wont be affected in case of CKD. it goes like this, kidneys wont respond to regular PTH, loses Ca and cant execrete PO4, PTH gets made and tries to burn bone to produce Ca, resulting in elevated levels of Ca and PO4, Ca will bind the calcium and go to kidneys, yet same story all over. add to that the fact that 1 oh hydroxylase wont be able to function in a dead kidney. +  




Remember PTH as Phosphate Trashing Hormone.

↑Phosphorus -> ↑PTH secretion -> ↑bone resorption -> ↑ALP and bone pain.