NBME 16 Area labeled by the letter 'E' (NBME Answers)

Retired NBME 16 Answers

nbme16/Block 1/Question#26 (reveal difficulty score)
A 16-year-old boy is brought to the emergency ...
Area labeled by the letter 'E' 🔍 / 📺 / 🌳 / 📖
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submitted by ∗bingcentipede(359)

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Talking about a boy who lost voluntary movement in his extremities. Thus, his corticospinal tract is injured. It decussates in the medulla, so contralateral limbs are affected. Cross out anything on the left side.

The FA picture is really good with this. Just gotta know E (and F) are the lateral corticospinal tracts.

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cassdawg Tracts are FA2020 p508. +5

wonkyhonky69 Right, but it said he is unable to move his right side, while the answer was right corticospinal tract. I think we had to realize that this slice was below the level of the pyramidal deccusation. +11

dermgirl How can we realize that the lesion was below the level of pyramidal decussation? +1

pruvs Pyramidal decussation occurs in the caudal medulla. The cross-section given was of the spinal cord (you can see the ventral/dorsal horns). While you wouldn't know where the lesion actually happened, the only plausible answer on the spinal cord section which would result in loss of voluntary movement completely would be the LCSTs (E and F). +2

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submitted by time2swim(2)

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I understand that damage to the area labeled E would cause issues with the corticospinal tract. However, wouldnt this damage cause UMN findings as it is before the transition to LMN in the anterior horn? In the stem we read that the pnt has LMN finding of "unable to move" So why are we seeing LMN and not UMN findings? thanks in advance

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skilledboyb Paralysis is a finding common both the UMN and LMN. The distinguishing findings are things like: atrophy (LMN): 30 minute onset not long enough to see this reflex changes, spasticity, etc: not mentioned in the stem +2

fatboyslim I chose G too thinking it is the ventral horn but G is the anterior spinothalamic tract. To answer your UMN vs LMN question, maybe it's because in acute spinal trauma you get a spinal shock which leads to flaccid paralysis, even if the lesion is supposed to cause UMN sx (like in this case affecting corticospinal tract). And then with time, the patient will develop UMN lesion sx (e.g. spastic paralysis, spasticity, etc.) +

fatboyslim ^Correction: G is the lateral spinothalamic +

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submitted by ∗helen(0)

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I chose G. How do you know it's corticospinal tract deficit? Why the deficit not in the ventral horn? somebody plz explain.

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neurotic999 That is a good doubt. The thought process probably should be that since both upper and lower extremities are involved the lesion is higher up (section of spinal chord also seems to be cervical). If it was the ventral horn area, the paralysis would probably be limited to a smaller area supplied by the nerve. However, CST lesion makes it more likely to have a more widespread area of affection. I think! +1

neurotic999 Also, G & H most likely are to be indicating the AST. Ventral horn would be within the gray matter. +1

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submitted by azzacel(1)

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You need to identify first that this a corticospinal tract issue and then you need to know that the lateral CST goes to the extremities while the anterior goes to the trunk

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