**Core Concept**
A double aortic arch is a rare congenital anomaly where the aortic arch is formed by both the right and left fourth aortic arches. This condition occurs during embryonic development when the normal regression of the right fourth aortic arch fails to occur.

**Why the Correct Answer is Right**
In a normal embryonic development, the right fourth aortic arch typically regresses, leaving the left fourth aortic arch to form the aortic arch. However, in the case of a double aortic arch, the right fourth aortic arch persists, forming a second arch that surrounds the trachea and esophagus. This persistent right fourth aortic arch gives rise to the characteristic double aortic arch seen in this condition. The left fourth aortic arch, on the other hand, forms the typical aortic arch.

**Why Each Wrong Option is Incorrect**
**Option B:** The right sixth aortic arch is not involved in the formation of the aortic arch. It is actually the left sixth aortic arch that contributes to the pulmonary artery.
**Option C:** The left fourth aortic arch forms the typical aortic arch, not the double aortic arch.
**Option D:** The left sixth aortic arch is not involved in the formation of the aortic arch, and it does not contribute to the double aortic arch.

**Clinical Pearl / High-Yield Fact**
In a double aortic arch, the aortic arch is formed by both the right and left fourth aortic arches, which can lead to compression of the trachea and esophagus, potentially causing respiratory and gastrointestinal symptoms.

**✓ Correct Answer: A. Right 4th arch**