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NBME 24 Answers

nbme24/Block 1/Question#20

Immediately after a spontaneous vaginal delivery, a ...

Fusion of the sclerotomes

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submitted by lsmarshall(203),

Patient has Spina bifida occulta which is a neural tube defect (failure of fusion of the neuropores). Sclerotomes are the part of each somite in a vertebrate embryo giving rise to bone or other skeletal tissue. Since a part of this patient's spina bifida included "abscense of spinous process" then a sclerotome was involved. Knowing that neural tube defects are an issue with fusion should be enough to get to the right answer.

If the notochord failed to develop then the entire CNS would not develop as the notochord induces formation of neural plate.

If the neural tube failed to develop then the whole CNS would not have developed.

Yolk sac is irrelevant to this patient.

When neural crest cell it has different outcomes in different tissues. Failure of neural crest to migrate in heart can cause Transposition of great vessels, Tetralogy of Fallot, or Persistent truncus arteriosus. Failure of neural crests to migrate in GI can cause Hirschsprung disease (congenital megacolon). Treacher Collins Syndrome can occur when neural crest cells fail to migrate into 1st pharyngeal arch. Neural tube defects has nothing to do with failure of neural crest migration though.

sympathetikey  Exactly. I knew it had to due with fusion of the neuropores but had never heard of sclerotomes. Thanks for the explanation. +