USMLE UMN vs LMN UW question

[PremedSurvivor]

For intellectual property reasons, I'll briefly paraphrase:

Middle-aged man comes in with a 2 month history of "left arm clumsiness and weakness". Also recently "developed problems with his gait". No recent head injury, HAs, bowel/bladder symptoms. Exam revels reduced strength in left upper extremity and MRI shows a lesion in the corticospinal tract. What additional sign do you expect? Answer = UMN sign

I guess my question is, how does this stem at all indicate a UMN lesion? MRI just shows a corticospinal tract lesion, but how does that suggest whether its above or below the level of the anterior horn (i.e. LMN or UMN)? Is this just a poorly worded question or am I missing something?

Thanks in advance!

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[hopefulERdoc251]

Just took neuro so this is my 0.02

Since the lesion has to be somewhere around the L C-spine (since it's only effecting LUE), then everything below the level of the lesion will exhibit UMN signs, thereby I think answering the question)

I get where you're coming at though, this is just my attempt to reason it

 

[karling]

The corticospinal tract is made up of UMNs originating from the cerebral cortex. Individual axons leave the corticospinal tract at the appropriate level to synapse at the ventral horn onto LMNs. If the stem says the MRI shows a lesion in the corticospinal tract, then that's UMN.