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