Treacher-Collins syndrome is mainly associated with which pharyngeal arch?

• A. First pharyngeal arch
• B. Second pharyngeal arch
• C. Third pharyngeal arch
• D. Fourth pharyngeal arch

Answer: (A)

Treacher-Collins syndrome reflects defective development of structures that arise from the first pharyngeal arch. The first arch forms the jaw (maxilla and mandible), the malleus and incus in the middle ear, and parts of the cheek and orbit. When neural crest cell migration or survival to this arch is impaired, these derivatives become hypoplastic, producing the characteristic facial features: micrognathia (small jaw), malar (cheekbone) hypoplasia, downslanting palpebral fissures, and external ear anomalies. The other arches contribute different structures (for example, the second arch mainly affects the stapes and facial muscles), so the pattern fits best with involvement of the first arch.