Which arches are involved in Treacher-Collins syndrome according to neural crest migration defects?

• A. First arch only
• B. Second arch only
• C. First and second arches
• D. Third arch

Answer: (C)

The main idea is that Treacher-Collins syndrome arises from a failure of neural crest cells to migrate into specific pharyngeal arches during early development. Neural crest cells populate the first and second pharyngeal arches to form much of the midface skeleton and many ear structures. When this migration is defective, those arches don’t develop properly, leading to underdevelopment of the midface, zygomatic bones, portions of the maxilla and mandible, and ear anomalies that are characteristic of Treacher-Collins.

So, the problem best fits involvement of the first and second arches. The third arch is not the primary source of the facial features seen in this condition, since its derivatives—parts of the hyoid bone and some laryngeal structures—do not explain the typical midface hypoplasia and ear abnormalities.