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NBME 22 Answers

nbme22/Block 2/Question#5 (reveal difficulty score)
A 74-year-old man with emphysema and lung ...
Negative nitrogen balance 🔍 / 📺 / 🌳 / 📖
tags: biochem

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 +35  upvote downvote
submitted by keycompany(351)
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Nitrogen balance is a measurement of protein metabolism in the body. A negative nitrogen balance indicates muscle loss, as increased amounts of amino acids are being metabolized to produce energy. This increases the amount of nitrogen secreted from the body. Because the amount of nitrogen you are taking in is less than the amount of nitrogen you are secreting, you have a negative nitrogen balance.

This man is malnourished, edematous, cachetic, and has hypoalbuminemia. These clinical findings point to protein malnutrition (Kawashkior Disease), which causes edema due to decreased serum oncotic pressure. Low oncotic pressure in this case is due to protein loss, and hence a negative nitrogen balance.

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drdoom  Nice! +23
dubywow  I knew your last sentence and suspected Kwashiorkor. It's just everything else I did not know. I have not heard or thought of muscle/protein changes in terms of "nitrogen balance" before... and that's why I got this wrong. Nice explanation! +3
macrohphage95  I agree with you in first part but i dont think it has any relation to kwashirkor. It is simply due to cachexia which causes muscle destruction through the proteasome pathway .. +5
zevvyt  also, it says that his albumin is low. +
ankigravity  A few other points related to nitrogen balance. Any patient who is growing (i.e., children, people working out and building muscle) are in positive nitrogen balance and arginine becomes an essential amino acid. +1
drdoom  @ankigravity Wow, this is technically correct! (Didn’t realize “essential” doesn't mean “cannot be synthesized by the body” but rather ”cannot be synthesized in sufficient amounts”; so what qualifies as essential is a bit of a moving target! Thanks!) +
drdoom  Here's the definition from MeSH: “Amino acids that are not synthesized by the human body in amounts sufficient to carry out physiological functions. They are obtained from dietary foodstuffs.” +
imgdoc  I don't think this has to do with kwashiokor like macrohphage95 suggested. This is cachexia secondary to lung cancer, TNF-alpha increases and causes proteasomal destruction of proteins. This guy is also not intaking enough proteins so negative nitrogen balance is the answer explained above +

 +2  upvote downvote
submitted by bking(2)
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I understand the edema is from decreased protein- why is Starvation Ketosis wrong? Couldn't his proteins be degraded through this process: Protein--Amino acids-- Acetyl CoA-- Ketones

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waterloo  negative nitrogen balance better explains the edema. +
courtney  I might be totally wrong but I don't think you can technically be in ketosis if you have glucose around and in this case, the patient eats only carbs +1
chaosawaits  You can't be in ketosis if you have insulin around, not glucose. Hence, T1DM can have DKA but T2DM typically does not. Both still have glucose, like everyone living. +

 +0  upvote downvote
submitted by pparalpha(93)
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Can someone please explain why it would not be glycogen depletion? I thought the question was talking about the Warburg phenomenon... so why not breakdown of glycogen to glucose?

I guess it would not explain the edema?

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hello  Glygocen stores are depleted within 24h. This person has signs and symptoms of longterm nutritional deficiences. +2
raffff  it would not explain the edema, yes +
drzed  Also the warburg phenomenon has to do with cancers preferentially taking up glucose; there is no indication that he has cancer. +
haydenelise  The first sentence says that he has lung cancer. +2

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