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NBME 24 Answers

nbme24/Block 4/Question#16

A 24-year-old woman who has diffuse toxic goiter ...

Phosphate: increased;
Parathyroid hormone: decreased;
25-hydroxyvitamin D: normal

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 +1  upvote downvote
submitted by trazabone(4),

Kidney makes 1-25, hydroxy vitamin D. (calcitriol) 25-hydroxy vit D (calcidiol) is made in the liver, and hypoparathyroidism would not decrease its levels as it acts to increase 1-alpha hydroxylase in the kidney to increase calcitriol concentrations --> Ca/phosphate reabsorption from the bone and small intestine.

queezyfish  I'm confused about the phosphate level in questions like these. Decreased calcitriol would decrease phosphate absorption while PTH decrease lowers phosphate excretion. I'm assuming that the PTH decrease has the greater effect with serum phosphate levels? +1  
mousie  PTH = "Phosphate trashing hormone" if PTH is high Phosphate must be low - they are always opposite (unless d/t renal failure then Phosphate will be high - kidneys will be unable to get rid of phosphate) So low Ca d/t low PTH does not effect 25 H. Vit D ... only 1,25 H Vit D (active Calcitriol)? +4  
haliburton  Clarification because I was confused: PTH stimulates kidney to produce 1,25-(OH)2 D3 (calcitriol) via 1α-hydroxylase in proximal convoluted tubule. Therefore, without parathyroid glands, low PTH, 25,D is not converted and therefore not down (normal or up). phosphate "trashed" by PTH as eloquently stated above. +  
zbird  Here the primary defect is high up from the parathyroid gland, there is decresed or no PTH which normally trashes phosphate but not in this case so serum PHOSPHATE INCREASES and the serum calcium is low because PTH should have prevented the urine calcium so there is calciuria and no resorption from bone-LOW CALCIUM, Vitamin-D is independent of PTH so stays NORMAL +  

 +1  upvote downvote
submitted by tinydoc(39),

This question is very sneaky, but in essence this is whats happening.

The accidental removal of the PTH glands during thyroidectomy ⇒ ↓ PTH

PTH normally: --in bone: ↑ removal of Ca²⁺ and Phophate from bone --in kidneys: ↑ Ca²⁺ reabsorption and ↓ PO₄³⁻ reabsorption --↑ conversion of 25, Hydroxyvitamin D to 1,25 Hydroxyvitamin D (Calcitriol - active form) via ↑ activity of 1-a Hydroxylase deficiency

Therefore a ↓ PTH would lead to:

⇒ ↑ PO₄³⁻ ⇒ ↓ Ca²⁺ ⇒ ↓ 1,25 Hydroxyvitamin D

The question is sneaky (much like the rest of this exam) because someone who isnt focusing really hard or in a rush might pick the option C where phophate is ↑ and PTH is ↓ BUT ↓ 25 hydroxyvitamin D

This is wrong as only 1,25 hydroxyvitamin D would be decreased, the conversions before this are done by the skin (sunlight) and liver.

I really wish they would stop making the questions confusing PURELY for the sake of making them confusing. Isnt it enough that we have to know this ridiculous amount of information, without having them intentionally making it harder by pointing you to 1 answer choice but changing a minute detail to make you answer wrong. Or using a random ass nomenclature for a disease to avoid making it too simple (PSGN = "proliferative GN")

tinydoc  I literally got this wrong because I had the font zoomed in and assumed the 1 was on the line above like on uworld when it tries to squish the whole title in the same space x_X +  

 +0  upvote downvote
submitted by dentist(1),

Follow the Calcium and work backwards. ↓Ca means ↓ Phosphate resorption in the PCT (which pulls Ca with it)

25-hydrovitD normal b/c its unrelated to PTH.