<prompt> Based on the appearance of these two siblings, which pharyngeal arch appears to be most affected?Pharyngeal arch 1Pharyngeal arch 2Pharyngeal arch 3Pharyngeal arch 4  <response>: Here both siblings show mandibular hypoplasia. Mandible develops from 1st pharyngeal arch Pierre Robin sequence and Treacher Collins syndrome are both associated with mandibular hypoplasia. 