**Question:** 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?

A. Pharyngeal arch 1
B. Pharyngeal arch 2
C. Pharyngeal arch 3
D. Pharyngeal arch 4

**Correct Answer:** **D. Pharyngeal arch 4**

**Core Concept:** The pharyngeal arches are a series of embryonic structures that contribute to the formation of the head and neck region. There are four pharyngeal arches: I, II, III, and IV. Each arch gives rise to different structures, including bones, cartilage, muscles, and blood vessels.

**Why the Correct Answer is Right:** In this case, the patient exhibits symptoms of hypoplasia of the mandible, underdevelopment of the bones of the face, and deformed external ears, which are consistent with abnormalities arising from the fourth pharyngeal arch (Pharyngeal arch 4). The fourth pharyngeal arch contributes to the development of the mandible (lower jaw), the malleus, incus, and stapes bones of the middle ear, and some parts of the external ear.

**Why Each Wrong Option is Incorrect:**

A. Pharyngeal arch 1 (mandibular branch) is responsible for the development of the mandible, not the structures mentioned in the question.
B. Pharyngeal arch 2 (maxillary branch) contributes to the development of the maxilla (upper jaw), which is not related to the symptoms described.
C. Pharyngeal arch 3 (hyoid branch) forms the hyoid bone and muscles of the thyroid gland, which are not involved in the symptoms presented.
D. Pharyngeal arch 4 (mandibular branch) is the correct answer provided above, contributing to the development of the mandible and structures involved in the symptoms.

**Clinical Pearl:** Understanding the development of the pharyngeal arches and their derivatives is essential for understanding congenital anomalies affecting the head and neck region. A knowledge of these structures helps in diagnosing and treating related medical conditions.