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NBME 24 Answers

nbme24/Block 1/Question#20 (50.6 difficulty score)
Immediately after a spontaneous vaginal ...
Fusion of the sclerotomesπŸ”
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submitted by lsmarshall(396),
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If hte lareun betu efladi to dpvleeo hnte teh oelhw CNS odwlu ont eahv .edpveodle

olkY sca si reietlarnv to shti .tnepati

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sympathetikey  Exactly. I knew it had to due with fusion of the neuropores but had never heard of sclerotomes. Thanks for the explanation. +13  
hungrybox  Fuck I picked "Formation of neural tube" but yea that makes sense... that would affect the whole CNS +3  
ruready4this  I also never heard of sclerotomes and I chose that and then switched it to formation of the neural tube because I thought that was close enough ugh close enough is not the right answer +  

submitted by peridot(59),

Paraxial mesoderm means mesoderm surrounding the axis, aka the neural tube. They eventually form a pair of somites on either side. Somites then differentiate into:

  • sclerotomes -> vertebrae, rib cage, occipital bone (part)
  • myotomes -> skeletal muscle of the back, ribs, limbs
  • syndetomes -> cartilage, tendons
  • dermatomes -> skin of the back


In this case, the patient is lacking spinous processes on top of having spina bifida. Maybe you were thinking spina bifida = neuropore defect, but what does this have to do with sclerotomes? This is what I found:

"Failure of closure of the caudal neuropore during embryogenesis will lead to spina bifida. This condition is always marked by a local lack of osteogenesis, or bone growth. The reason is that the correct differentiation and placement of underlying tissues (the spinal chord and spinal nerves and associated tissues) induce the osteogenesis, even small flaws in the underlying tissues can lead to a problem in the formation of the spine (vertebrae)." Source

So I'm guessing that when the question asks for "cause of the defect" they are talking about the vertebrae, not exactly the cause of spina bifida, which would be failure of neuropores to fuse. If that was an answer choice then def pick that, but the lack of vertebrae is due to lack of sclerotome fusion (because spina bifida could not induce it properly).

submitted by titanesxvi(78),
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