NBME 21 Hyponatremia (NBME Answers)

NBME 21 Answers

nbme21/Block 4/Question#10 (reveal difficulty score)
A 64-year-old man is evaluated for cough, ...
Hyponatremia 🔍 / 📺 / 🌳 / 📖
tags: renal SIADH hyponatremia

+13 upvote downvote
submitted by ∗kchakhabar(52)

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What threw me off in this question is the phrase "cells with little cytoplasm that are twice the size of lymphocytes." I though "small cell carcinoma" cannot be that big.

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nala_ula Omg literally the same thought process I had, that phrase through me off! +

nala_ula threw* +1

jesusisking Super counter-intuitive but apparently SCC cells are 1.5-4x the size of lymphocytes: https://www.ncbi.nlm.nih.gov/pubmed/1313679 +1

skonys How can it be SCC if it says there isnt squamous organization. I thought of pulmonary carcionoids because it's the only other pulmonary tumor that causes paraneoplastic SIADH. +

epiglotitties @skonys in this case they're abbreviating Small Cell Carcinoma to SCC not Squamous Cell Carcinoma +

+10 upvote downvote
submitted by ∗haliburton(225)

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Small cell lung cancer causes SIADH. Location + exclusionary clues.

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mcl To expand, SIADH may also result in euvolemic hyponatremia. This is because, as we know, ADH increases absorption of water and therefore initially results in an increased circulating volume. However, this results in increased stretch of the atria and subsequent secretion of ANP. ANP (atrial natriuretic peptide) then results in loss of sodium and water. +11

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submitted by ∗hello(429)

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The patient has "cells arranged in infiltrating sheets" --> indicates malignant cancer cells.

The cells are neither glandular nor squamous --> the cancer is neither adenocarcinoma nor squamous cell lung cancer, respectively.

This leaves small oat cell lung cancer. SIADH is the most common paraneoplastic syndrome for small oat cell lung cancer --> leads to hyponatremia.

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+3 upvote downvote
submitted by ∗ergogenic22(401)

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why is hyperlipidemia secondary to cushing syndrome not a possibility?

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hello SIADH = MC paraneoplastic syndrome of small oat cell lung cancer. Also, Cushing syndrome would cause would weight gain, skin hyperpigmentation, and hypokalemia. Not, lyperlipidemia. +1

charmrooftops You do get hyperlipidemia in cushing though? https://www.amboss.com/us/knowledge/Cushing_syndrome So still unsure why this is not a possibility. Is it just a "more common" thing for SIADH? +2

peridot I was debating the same thing. But yeah I guess the SIADH association is just supposed to be stronger somehow and "more common"? +

+0 upvote downvote
submitted by ∗an_improved_me(91)

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My dumbass thought it was Large Cell cancer, for which i had no associated paraneoplastic syndrome. Yay.

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