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NBME 16 Answers

nbme16/Block 4/Question#34 (14.2 difficulty score)
A 43-year-old woman comes to the physician ...
Degeneration of motoneurons of the lumbar cordπŸ”

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submitted by aoa05(22),

Weakness with decreased muscle bulk implies problems that include the lower motor neuron system. Decreased DTRs implies a disrupted reflex loop but the absence of sensory loss implies that it is on the motor side of the reflex loop. Of the available choices, B is the best fit. "A" is arguably true because a strictly motor polyneuropathy (such as in lead poisoning) could account for the findings, but a 3-month course could hardly be called "acute."

itsalwayslupus  I was able to deduce the right answer, but what is the specific reason against "demyelination of the corticospinal pathways"? just out of curiousity +1  
lsp1992  I believe it's because damage to the corticospinal tract would be considered UMN damage, while degeneration of motorneurons is LMN damage. LMN damage causes decreased reflexes. UMN disease would cause hyperreflexia....I think. That's how I reasoned through it at least +11  
nbmeanswersownersucks  I also think you can rule out peripheral neuropathy because typically that includes both motor and sensory +2  
saqeer  yes but is not Achilles an S1 reflex (sacral cord) ? how does the degeneration in lumbar cord affects it ? i rule it out first thing because of this :S +  
meryen13  i think she just had a dics herniation. there can be problem with temperature and sensation in some case but those are usually very severe herniations. not sure tho... but it can on your differentials. +  
djeffs1  I assumed that "motoneurons of the lumbar chord" means upper and not peripheral +