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NBME 20 Answers

nbme20/Block 2/Question#32

A 12-year-old girl is brought to the physician by ...


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 +14  upvote downvote
submitted by hayayah(505),

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.

sympathetikey  Exactly. That's the only way I got to the answer. +  
pakimd  isnt increased alk phos consistent with increased osteoblastic activity? +  
champagnesupernova3  A defect with chondrocytes would cause an short limbs like in achondroplasia so those are ruled out +  
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. +  

 +11  upvote downvote
submitted by monoloco(75),

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.

 +1  upvote downvote
submitted by thomas(4),

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.

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. +  

To answer this question is to note that: 1) alkaline phosphatase activity is linked with osteoblasts.