NBME 21 Answers : Block 4 : #37 = A 66-year-old right-handed woman develops the acute ...

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submitted by hungrybox(176),

Following a stroke, this patient had weakness of her left face and body, so the stroke must have affected the right side of her brain. B was the only choice on the right side of her brain.

Still confused? Read on...

The voluntary motor fibers (corticospinal tract) descend from the primary motor cortex, cross (decussate) at the medullary pyramids, and then synapse at the anterior motor horn of the spinal level.

Because of decussation at the medullary pyramids, you should make a note of where any stroke occurs. Is it above the medullary pyramids? Then it will affect the side opposite the stroke (contralateral). Is it below the medullary pyramids? Then it will affect the same side as the stroke (ipsilateral).

hungrybox Woops, E is also on the right side (also remember that imaging is looking up at someone, feet first). But a cerebellar stroke would have caused ataxia. +

mnemonia Very nice!! +

usmleuser007 What gets me is that they mention that Left 2/3 of face is affected. This should indicate a non cortical innervation as most of the cranial nuclei are bilaterally innervated from the left and right hemisphere. If left 2/3 of the face is affected then it should also mean that the lesion is after CN5 nuclei. +1

yotsubato @hungrybox Thats not the cerebellum thats the occipital lobe. You would see leftsided homonymous hemianopsia in that lesion +

mrsmac To my mind, it is simpler to consider the question first in terms of blood supply distribution. Left sided hemiparesis and weakness of lower 2/3 of face are both indicative of a MCA rupture/stroke (First Aid 2018 pg. 498). Furthermore, since the injury has affected motor function we would be considering the descending tract i.e. lateral corticospinal which courses through the ipsilateral posterior limb of the internal capsule then decussates in the caudal medulla. +1

mrsmac You're considering the wrong CN here. CN5 motor function involves muscles of mastication and lower 2/3 of tongue. The nerve in question in this case is CN7/VII Facial n. CNVII UMN injury affects the contralateral side, whereas LMN injury affects ipsilateral (First Aid 2018 pg. 516). i.e. before and after the nucleus in pons respectively. I hope this helps. +1

nala_ula Spastic means UMN lesion, since they also don't specify if there is arm or leg weakness, I didn't assume it was MCA stroke. I went with the reasoning that for there to be spastic hemiparesis, there must be damaged to the UMNs and therefore the internal capsule is where these tracts are. +