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NBME 21 Answers

nbme21/Block 3/Question#40 (34.4 difficulty score)
A 65-year-old woman undergoes surgical repair ...
Area labeled ‘A’ (Cranial nerve 3)🔍
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 +7  upvote downvote
submitted by hayayah(983),
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:Note hTe bcasdune .n si taclalyu eht erven omts lkiley to be admgade by na deinanpxg einantlr acodrti nreamyus ni hte ancersvou inuss tub eyth veig uyo ifpecsci CN3 ntinocuf in sith q.ouintse

hungrybox  One pupil larger than the other indicates damage to the pupillary light reflex - afferent: CN II, efferent: CN III. +12  
cienfuegos  A little more info regarding other sxs (via UW): -cavernous carotid aneurysm: small usually asx, enlargement can cause u/l throbbing HA &/or CN deficits. VI most common thus ipsilateral lateral rectus weakness, can cause esotropia = inward eye deviation & horizontal diplopia worse when looking toward lesion -can also damage III, IV and V1/2 -can occasionally compress optic nerve or chiasm thus ipsilateral monoocular vision loss or non-specific visual acuity decrease +2  
lovebug  There are in FA2019, page 530. +  





 +1  upvote downvote
submitted by docshrek(5),

Question stem says: PE shows Right pupil is larger than the left pupil. (Note: It is not light reflex). And weakness with movement of eye (that means most movements).

CN III palsy - Opthalmoplegia, Ptosis, Mydriasis and loss of accommodation. Ptosis, Mydriasis and loss of accommodation are due to - Parasympathetics travel outside of CN III. So more likely CN III is damaged and not others in the cavernous sinus.




 +0  upvote downvote
submitted by dr.xx(125),
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 +0  upvote downvote
submitted by azibird(63),

What if you know it's CNIII and remember CN's III, IV, V1, V2, and VI all run through the cavernous sinus, but don't remember which is which in the picture?

CN's IV and VI only control one muscle each and are therefore very small in diameter. V1 and V2 serve a lot of functions and are therefore large. III is intermediate.

Also they are organized in an ascending manner - III above IV above V1 above V2, and VI to the side.