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NBME 23 Answers

nbme23/Block 2/Question#25

A 44-year-old man with invasive pancreatic cancer ...

Area labeled ‘H’

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We're viewing this slice of the spinal cord from the bottom right? The left right labels always mix me up.

mcl  Yeah, I just stared at this again for a solid 5 minutes straight up dying. I think if we're going with the L/R labels as they are shown on the image, imagine the patient lying on their stomach with their feet pointed towards you and it should make sense. +5  

Pain & temperature fibers for the right side come in on the dorsal right side, cross at the anterior white commisure, and travel up in the Spinothalamic tract.


The thing is, the spinothalamic tract crosses 2 vertebral levels up and then decussates at the anterior white commissure to get from the right to the left, so how do I know which vertebral level I'd be working on this guy?

chris07  I think the assumption here is that we are dealing with the cord section at the level of the problem. The picture is incredibly misleading. You have to orient yourself. The dorsal columns F, E, A, B are facing the patient's posterior. Once you properly orient it in 3D space, you know that what's labeled "right" is actually the patient's left, and what's labeled "left" is his right side. Super confusing. +2  
sne  The input arises in a limb/part of body at the level of lesion, enters through the dorsal root (pictured between A and B), decussates and ascends at the anterior commissure, and finally synapses on the second order neuron in the lateral spinothalamic tract. So the spinothalamic tract is responsible for contralateral pain and temperature sensation. So AT THE LEVEL technically would be in the dorsal column +4  
nwinkelmann  also, @chris07, I think you're wrong about the labels being wrong on the image. Becuase the spinothalamic tract = contralateral pain and temperature, and the patient's pain is on the right side, you would want to target the left spinothalamic tract for pain relief, i.e. the area labeled H. The area labeled D would be the right spinothalamic, purely because that is how the image is labeled. If you assume the label is different, you will get it wrong. +8  
kpjk  @sne I don't think entering from the dorsal root would be between A and B. It would be part of the gray matter so, lateral to B and F +  

If you assume that because the pt has R-sided back and ab pain somewhere in the lower thoracic, and you know that this slice of cord is above that because it has a posterior intermediate sulcus present (i.e. has both a gracilus and cuneatus track), then you know that the spinal cord slice is likely above where the pain is, therefore the spinothalamic has crossed over already.

+3/- spow(5),

What is the area labeled 'G' and 'C'? And more characters?

alexxxx30  lateral corticospinal i think +