Pierre Robin sequence is caused by defect in development of which pharyngeal arch?

**Core Concept**
The Pierre Robin sequence is a congenital disorder characterized by micrognathia (small lower jaw), glossoptosis (retrodisplacement of the tongue), and upper airway obstruction. This condition is often associated with cleft palate and other developmental anomalies.

**Why the Correct Answer is Right**
The Pierre Robin sequence is primarily caused by a defect in the development of the first pharyngeal arch. The first pharyngeal arch gives rise to the lower jaw, mandible, and surrounding structures. A defect in this arch can lead to micrognathia and other associated anomalies. The mandible is formed by the fusion of two mesenchymal swellings, the lateral and medial components, which arise from the first pharyngeal arch.

**Why Each Wrong Option is Incorrect**
* **Option A:** The second pharyngeal arch is primarily responsible for the development of the hyoid bone, styloid process, and the stylohyoid ligament. Defects in the second pharyngeal arch are not typically associated with the Pierre Robin sequence.
* **Option B:** The third pharyngeal arch contributes to the development of the greater horn and upper part of the body of the hyoid bone, but it is not directly involved in the formation of the lower jaw or the Pierre Robin sequence.
* **Option C:** The fourth pharyngeal arch is primarily responsible for the development of the laryngeal cartilages and the epiglottis. Defects in this arch are not typically associated with the Pierre Robin sequence.

**Clinical Pearl / High-Yield Fact**
The Pierre Robin sequence is often associated with other congenital anomalies, such as cleft palate, cardiac defects, and chromosomal abnormalities. A high index of suspicion is necessary for early diagnosis and management.

**Correct Answer: C. The first pharyngeal arch.**