A 3-day-old male infant has a noticeably small mandible. A CT scan and physical examinations reveal hypoplasia of the mandible, underdevelopment of the bones of the face, downward-slanting palpebral fissures, defects of the lower eyelids, and deformed external ears. Abnormal development of which of the pharyngeal arches will most likely produce such symptoms?

The first pharyngeal arch, which is often associated with the mandible, is responsible for development of Meckel's cartilage, malleus, incus, and mandible. Additionally, it is innervated by the trigeminal nerve, specifically the mandibular division that innervates the muscles of mastication. This patient presents with features characteristic of developmental defects in the first arch. The second pharyngeal arch gives rise to the stapes, styloid process, lesser cornu, Reichert's cartilage, and the upper half of the hyoid bone. It is innervated by the facial nerve. The third pharyngeal arch is responsible for formation of the greater cornu and the lower half of the hyoid bone and is innervated by the glossopharyngeal nerve. The fourth and sixth pharyngeal arches give rise to the laryngeal cartilages, in addition to being innervated by the vagus nerve.