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NBME 22 Answers

nbme22/Block 1/Question#37

A 28-year-old woman is brought to the physician ...

Area labeled ‘C’ (Abducens nucleus, right)

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 +14  upvote downvote
submitted by imgdoc(72),

I think its the right MLF (area C), not the right abducens nucleus that is lesioned on the cross section. If the abducens nucleus were lesioned she wouldn't have abduction in her left eye. The MLF mediates cross talk between the abducens nucleus on both sides to the MLF on the opposite side (2 abducens nuclei, 2 MLF one on each side). In her case, her right MLF wasn't functioning hence why she was gazing left but her right middle rectus wasn't contacting to mediate leftward gaze.

This picture helps: http://what-when-how.com/wp-content/uploads/2012/04/tmp15F9_thumb.jpg

imgdoc  *medial rectus +  
quip13  This shows the MLF pathway in a saggital cut: https://ars.els-cdn.com/content/image/3-s2.0-B9781437719260100128-f12-08-9781437719260.jpg +  

 +4  upvote downvote
submitted by lnsetick(61),

This is internuclear ophthalmoplegia. In INO you would see impaired adduction during horizontal gaze due to a lesion of the ipsilateral MLF. However, since the MLF is not involved in convergence, the affected eye is still able to converge normally.

 +3  upvote downvote
submitted by ahd_ve(4),

This imagen is a transverse section through the pons at the level of trigeminal nuclei: A and B: Superior Cerebellar peduncle. C and D: Medial longinutidinal fasciculus. E and F: Bundles of corticospinal and cortinuclear fibers.

 +2  upvote downvote
submitted by drdoom(369),

Here’s another very nice one that superimposes the pathway onto a simplified brainstem drawing (nice for the anatomical relations):


Source article:


To see even more, try google image search on “medial longitudinal fasciculus”:


endochondral1  what is A and B in this pic? i knew it was dorsal pons ipsilateral but i just didn't know what part that was on the pic? +  
nwinkelmann  A and B are the superior cerebellar peduncles.http://what-when-how.com/wp-content/uploads/2012/04/tmp15F2.jpg +  

so the lesion is in the Right MLF right? If so I'm just about to memorize the eye see SAME MiLF lol its the MLF on the same side of the eye keep it simple i hope that's what yall are saying lol

 +0  upvote downvote
submitted by drdoom(369),

Nice schematic of how horizontal gaze is coordinated through the abducens/MLF/oculomotor pathway:


In the diagram, the system is coordinating gaze toward pt’s left, which (conveniently) is the same as in the stem.

Source article: https://n.neurology.org/content/70/17/e57

hello  Oops meant https://imgur.com/pD4amBJ +