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NBME 23 Answers

nbme23/Block 2/Question#9

A 60-year-old man has had easy fatigability, loss ...

Lung

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I thought of this as squamous cell carcinoma of the lung causing increased PTHrP and hypercalcemia.

d_holles  I thought this was medullary thyroid cancer but demographically SCC works better. +  
smc213  Medullary thyroid carcinoma increases calcitonin levels leading to decreased serum Ca2+ by increasing Ca2+ renal excretion. So high levels of calcitonin secreted by the tumor may lead to hypOcalcemia. Source: Pathoma +6  




I understand why it's lung now, but I picked thyroid gland because often times thyroid tumors press on the parathyroid sitting above, which causes the parathyroids to secrete more Ca...can someone comment if they've read this too?

paulkarr  I personally have not read that, but I wouldn't be surprised by that fact. I think with these NBME problems though, if you can get the answer within one "step" that should be your choice. Here you can just go Squamous Cell Carcinoma with a direct action on serum calcium levels (via PTHrP). Thyroid requires a few more steps, (assuming your statement is true) so in the eyes of NBME, it ain't gonna be the right choice. Always follow the "KISS" logic! +  




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submitted by nwinkelmann(111),

So, in researching this further, I figured out a way to rule the other answeres out. Per this article, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4683803/, humoral hypercalcemia of malignancy is caused by 4 mechanisms with excess PTHrP secretion being MCC. Excess PTHrP secretion occurs in "squamous cell cancers, urinary tract cancers (renal cancer and bladder cancer), breast cancer, nonHodgkin's lymphoma, and ovarian cancer account for the majority of malignancies leading to hypercalcemia via PTHrP." The other 3 causes include hypercalcemia secondary to overproduction of cacitriol, osteolytic metastases, and PTH mediated hypercalcemia (parathyroid carcinoma and ectopic production). Also, FA only mentions PTHrP in regards to SCC of lung and renal cell carcinoma.

Prostate, colon, and pancreatic cancers are more commonly adenocarcinoma, and less likely to release PTHrP, and thyroid cancer is also less likely, though I did find one article that found PTHrP released by all of these cancers so I don't know. I guess I'm just going to go with SCC of lung being MCC of PTHrP humoral hypercalcemia of malignancy, plus patient's demographics (60's male).