This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: miscellaneous, Subject name: Pathology
Based on the appearance of these two siblings, which pharyngeal arch appears to be most affected? 
Select and explain. 
A. Pharyngeal arch 1, B. Pharyngeal arch 2, C. Pharyngeal arch 3, D. Pharyngeal arch 4
A: Pharyngeal arch 1
Answer: A: Pharyngeal arch 1 
Explanation: 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.