NBME 24 Lower motoneurons (NBME Answers)

Retired NBME 24 Answers

nbme24/Block 2/Question#35 (reveal difficulty score)
A 55-year-old woman comes to the physician ...
Lower motoneurons 🔍 / 📺 / 🌳 / 📖
tags: neuro

+12 upvote downvote
submitted by ∗therealslimshady(42)

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Here is my personal way how I reasoned to the correct answer, if it helps anyone. First off, I recognized she probably has ALS, as she has purely motor symptoms (no sensory symptoms) that include both UMN (3-week history of muscle cramps) and LMN lesions (fasciculations, atrophy). In ALS, you have damage to anything that is composed of UMNs and LMNs. UMNs are found in (A) cerebral white matter, (B) corticospinal tract, and (C) internal capsule. I picked (D) for two reasons: (1) she has more LMN symptoms than UMN symptoms (the only UMN symptom was muscle cramps), and (2) choices A, B, and C are all UMNs, and so they can't all be right.

That's just what was going on through my brain. Very convoluted I know and might have errors, but if I helped at least 1 person then I did my job.

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xmen done, thank you +

weirdmed51 Thank you...this taught me so much! +

+8 upvote downvote
submitted by ∗tinydoc(276)

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I don't think it matters whether or not this was ALS ( which I still think it is). But theres fasiculations and Weakness and atrophy those are LMNL signs. Theres sooo many sneaky questions in this exam, this isnt one of them.

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+5 upvote downvote
submitted by ∗sattanki(82)

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Does anyone have any idea on this question? Thought it was ALS.

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ankistruggles I thought it was ALS too (and I think it still could be?) but my thought process was that a lower motor neuron lesion would be the more specific answer. +2

sattanki Yeah makes sense, just threw me off cause ALS is both lower and upper motor neuron problems. Corticospinal tract would have been a better answer if they described more upper motor neuron symptoms, but as you said, they only describe lower motor neuron symptoms. Thanks! +6

mousie Agree I thought ALS too but eliminated Peripheral nerves and LMN because I guess I thought they were the same thing ....??? Am I way off here or could someone maybe explain how they are different? Thanks! +1

baconpies peripheral nerves would include motor & sensory, whereas LMN would be just motor +16

seagull Also, a LMN damage wouldn't include both hand and LE unless it was somehow diffuse as in Guil-barre syndrome. It would likely be specific to part of a body. right??? +1

charcot_bouchard No. if it was a peri nerve it would be limited to a particular muscle or muscles. but since its lower motor neuron it is affecting more diffusely. Like u need to take down only few Lumbo sacral neuron to get lower extremity weakness. but if it was sciatic or CFN (peri nerve) it would be specific & symptom include Sensory. +1

vulcania I think it's ALS too. The correct answer choice here seems more based on specific wording: the answer choice "Corticospinal tract in the spinal cord" wouldn't explain the tongue symptoms, since tongue motor innervation doesn't involve the corticospinal tract or the spinal cord (it's corticobulbar tract). This is a situation of "BEST answer choice," not "only correct answer choice." +

+3 upvote downvote
submitted by ∗rainlad(33)

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I was thrown off because I didn't realize lower motoneuron = lower motor neuron face palm

"LMN deficits... dysarthria, dysphagia, asymmetric limb weakness, fasciculations, atrophy

UMN deficits... pseudobulbar palsy (i.e. dysarthria, dysphagia, emotional lability, spastic gait, clonus])"

-- FA 2019 p.518

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+1 upvote downvote
submitted by ∗guillo12(58)

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"The loss of lower motor neurons leads to weakness, twitching of muscle (fasciculation), and loss of muscle mass (muscle atrophy)".

https://www.medicinenet.com/script/main/art.asp?articlekey=33870

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+1 upvote downvote
submitted by ∗an_improved_me(91)

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I figured since the answer was mis-spelled, it couldn't be the answer.

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madamestep Definitely not a good move for NBME. They love a good typo/obscure name. +

+0 upvote downvote
submitted by ∗tissue creep(133)

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I treated this like Werdning Hoffman/SMA ("tongue fasciculations"), and just kind of accepted it being weird to be in a 55-year-old.

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dr.xx SMA4 (Adult-onset)? +

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