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NBME 20 Answers

nbme20/Block 1/Question#16 (53.9 difficulty score)
A 30-year-old woman with multiple sclerosis ...

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submitted by thecatguy(16),

This is a very nitpicky question. As I see it, the 3 main concepts tested are:

  • This patient has trigeminal neuralgia (sharp, brief, episodic pain in the face), which is caused by a lesion to primary sensory fibers that carry pain sensation from the face.
  • Multiple sclerosis, which the patient has, is a demyelinating disease (i.e., it affects white matter).
  • The myelinated axons carrying pain sensation from the ipsilateral face enter the brainstem at the level of the pons and then descend (become the spinal tract of the trigeminal). These white matter fibers pass through the pons to synapse on the spinal nucleus of the trigeminal, which is in the medulla (nucleus = gray matter). (picture here). Therefore, a lesion in the white matter (i.e., plaque) in the pons could cause trigeminal neuralgia, and this phenomenon has been observed.

Above the level of the brainstem (thalamus & cerebral cortex), you have second order sensory neurons. Lesions in this part of the circuit are not generally in trigeminal neuralgia. I suppose they also want us to assume that once the spinal tract of the trigeminal enters the medulla, it's not myelinated anymore. I don't think this is completely true, but given the logic described above, pons would still be the better answer.

As people have pointed out, the primary sensory fibers carrying light touch sensation from the face synapse on the chief sensory nucleus in the pons immediately after they enter the pons. This question is not asking about those fibers though.

I got the question wrong too..

mightymito  Wow this is the best explanation yet! Thanks so much for very clearly walking us through a tricky question. +3  
lovebug  @thecatguy Are.... you a professor? thank you very much :) +  

submitted by whossayin(20),
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eTh minocnme I eikl ofr bgremnrieme teh oncloitas of hte lnaaicr nveers si eth 42,"4,,2 eur"l

evbAo etir=msbna CN I + iIndbr=a MiI CN ,III P= VoIsn CN V, IV, VI,I MuVIl lIaI=ed CN X,I ,X ,IX IIX

lovebug  @whossayin thanx so much!!! +  

submitted by sympathetikey(1248),
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rpPilneic eonysrS lcesNuu fo het elaiTgnmir is deoltca in the s,oPn sa si het roMot mneTgriila ucNeuls of eht .nops hisT repannsiotet si bbpralyo igeland erom hitw het riPpilcen Sseoynr uluecsN.

submitted by strugglebus(163),
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The npos sah enersv -58, os het rlteaiimgn dlwuo eb tdafecfe here

masonkingcobra  Thalamic pain syndrome would involve dysesthesias on the entire contralateral body so more than just the face. Also it occurs often after post-stroke. Additoinally, these dysesthesias appear weeks/months later +9  

submitted by feliperamirez(31),
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I od atresndnud ttha teh piacpnlir enusulc fo hte erniimtgla reven is cadotel ta hte pon.s uBt csine tish enpttai si hginav rmgileiatn reluiagna 'tlowdun uoy be nurtsiipdg teh htpaywa envldvoi ni ainp and aeem,utrprte ihhcw in tshi scae wldou be hte anlspi nesuluc aodle(ct ni eht l)admule?

siee,dBs I ufnod htsi at an aelirtc

A enetrc oshhesptiy ttestaibru teh pina of griiltmean alenragiu to a ncleart hiscmnmea ionivvngl teh pras orslia of hte saplni ngiliaetmr [n]us.cleu5


pg32  I literally had medulla selected the whole time and then changed it to pons simply because i felt the test writers were just seeing if we knew where the trigeminal nerve was located. bummer because I think your logic is way better. it's what i first thought when i read the case. +1  

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SM si a adtgeeyliimnn isse.aed tT'sha ywh I di'ndt cpik xcotre ro lhtsua;ma yhte rae egyr a.ttrme I cdikpe ulaemld aeubsec I eht innoptgaeimrils evners vserrtae teh med.lual I segus we era tsuj edoppsus ot uamses eth lnmidotinayee is niphgapen in het ?spon I todn' .konw

woodenspooninmymouth  Sorry, I meant to say that the pons would have the cell bodies for the pain/temp neurons, no? +  

submitted by mamed(14),

Per boards and beyond, in the lateral pons is there is the spinal V nucleus which carries contralateral pain and temperature. Knowing that + CN V arises it in the pons, I went with pons. The only other option I had left not crossed out was cerebral hemisphere but that seemed to broad.

vivarin  BB also said not to use the rule of 4's to localize CN V to the pons so that's why I crossed it out... +  

submitted by chandlerbas(95),
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etersh na tcsooanisai mdae ni 0127 whti airmnlietg lnuareiga and SM - ihhwc is hatw i hknti hist pt sha. yke hngit to eotn si that eiwlh all ortom adn yrsoesn eifsrb of NC 5 rteen at hte elvle of het onps oehe(wvr emos lsao do etenr ta the vlele of delulma dna veen teh cs vai het nlpais rttac fo 5 to aesnspy hwit hte lngo nroseys clueusn fo )5 se rhe hte knil of limeigrant laeairugn dna S :Mo.ii6.wewn7/Mt/P5wlshrc.Ccpth/cva3bn4ng/ml9.i//psmt4:

submitted by lilyo(69),
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I wsa nktngiih oalng hte lisne fo laiafc etaossinn whchi si mdadeeti yb het liigemarnt rveen dan eth afct htat the legnritmia veren is dealcto ni eht sop.n