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NBME 20 Answers

nbme20/Block 1/Question#35
A 70-year-old man dies of coronary artery ...
Spastic hemiparesis on the right🔍

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As stated below, the Left crus cerebri was damaged (see what it should normally look like below). This contains the corticospinal tract. Since the corticospinal tract decusates at the medulla, below the midbrain section we're looking at, you would see Contralateral (Right) Spastic Hemiparesis

hello  What identifies that a cross-section is medulla vs midbrain vs pons? +2  
kernicterusthefrog  @hello I like to pay attention to the Cerebral Aqueduct (diamond/spade shape seen mostly in Midbrain, and transitioning to 4th ventricle in rostral Pons), and then the shape and size of the 4th ventricle as you move down Pons to rostral&middle Medulla, and eventual closing and absence of fluid space at caudal Medulla. +8  
hello  @kernicterusthefrog Thank you. +  
mbourne  NGL, I thought the right side had the pathology lmao ty +8  

Is it appropriate to ask a question about the structure which is absent on the picture? It was super confusing! How can you say it was not a defect of the tissue cut?

drdoom  why did this get downvoted? +  

 +1  upvote downvote
submitted by strugglebus(129),

It affected the corticospinal tract in the crus cerebri on the L

The corticospinal tract runs from the cerebral cortex (Pre-Central Gyrus), through the posterior limb of the internal capsule, and into the posterior portion of the brainstem where they form highly myelinated medullary pyramids. These travel down the spinal cord, decussate and synapse of lower motor neurons in the ventral horn of the spinal cord.

A past history of cerebral infarction must have affected this patient's left middle cerebral artery and through Wallerian degeneration, the left medullary pyramid degenerated.

Destruction of the left corticospinal tract before decussation leads to contralateral spastic hemiparesis (since this is an upper motor neuron lesion.

 -6  upvote downvote
submitted by hayayah(879),

The damage is in the L midbrain in the area affecting the corticospinal tract. Because it is in the midbrain, decussation in the pyramids (medulla) so it will show ipsilateral dysfunctional motor signs.

Photo of midbrain and important areas: shorturl.at/myHLR

masonkingcobra  Just for clarification, on the left side, you see where he had the infarction 7 years ago and the tissue is gone. +1  
chefcurry  so is the dysfunction on the contralateral side? +  
praderwilli  If the decussation is in the pyramids of the medulla, shouldn't it be contralateral hemiparesis if the damage is on the right? It confuses me because of the labeling right and left at the top of the pictures. +  
endochondral1  that link isnt working @ hayayah....is there any good picture to look at to know where the tracts are on this section? +  

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