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Retired NBME 20 Answers

nbme20/Block 2/Question#32 (reveal difficulty score)
A 12-year-old girl is brought to the ...
Osteoblasts ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
tags: msk repeat

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 +30  upvote downvote
submitted by โˆ—hayayah(1212)
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A big thing here too is noticing that the ALP is decreased. Osteoblast activity is measured by bone ALP. I think that was the main focus here and not that you necessarily need to know the CBFA1 gene mutation.

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sympathetikey  Exactly. That's the only way I got to the answer. +4
pakimd  isnt increased alk phos consistent with increased osteoblastic activity? +1
champagnesupernova3  A defect with chondrocytes would cause an short limbs like in achondroplasia so those are ruled out +1
pg32  Exactly. Can also be helpful if you remember that the clavicles are formed by intramembranous ossification rather than endochondral; that allows you to rule out the chondroblast/cyte answer choices. +7



 +22  upvote downvote
submitted by โˆ—monoloco(155)
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This is a conditional called craniocleidodysplasia. The kid on Stranger Things with the lisp has the disorder. No collar bones, too many teeth, frontal bossing => craniocleidodysplasia. CBFA1 is a gene highly implicated in osteoblast function.

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submitted by โˆ—usmleuser007(464)
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To answer this question is to note that: 1) alkaline phosphatase activity is linked with osteoblasts.

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submitted by thomas(3)
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They tell you that the kid has no clavicle. This means the defect is in membranous ossification, NOT endochondral, so the pathology is NOT going to involve the chondro-whatever cells. decreased ALK is consistent with osteoblast defect.

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pakimd  isnt increased alk phos consistent with increased osteoblastic activity? +
eacv  @pakimd Yes ! ALK phos is a measure of osteoblast work, if the are not working is LOW as in thix px. +1



 +0  upvote downvote
submitted by โˆ—namesthegame22(13)
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  • The CBFA1 gene mutation in cleidocranial dysplasia results in impaired osteoblast differentiation with resultant impaired ossification in membranous bone.

  • Features include frontal bossing, facial bone malformations, supernumerary teeth, and absent or diminutive clavicles.

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