**Core Concept**
The double arch aorta is a rare congenital anomaly characterized by the presence of two separate aortic arches, which is a deviation from the normal single aortic arch. This condition arises due to the abnormal development of the aortic arches during embryogenesis.

**Why the Correct Answer is Right**
The double arch aorta is derived from the persistence of the left 4th aortic arch. Normally, the left 4th aortic arch contributes to the formation of the adult aortic arch, but in the case of the double arch aorta, this arch persists and remains separate from the rest of the aortic arch. This results in a double-arched structure that can be identified on imaging studies. The left 4th aortic arch is a critical structure in the development of the aortic arch, and its abnormal persistence leads to the formation of the double arch aorta.

**Why Each Wrong Option is Incorrect**
**Option A:** The right 6th aortic arch is not directly involved in the formation of the double arch aorta. The 6th aortic arches are typically involved in the formation of the pulmonary arteries.
**Option B:** The left 6th aortic arch is not a recognized contributor to the formation of the double arch aorta. The left 6th aortic arch is not a typical structure in human embryology.
**Option C:** The right 4th aortic arch is not involved in the formation of the double arch aorta. The right 4th aortic arch typically regresses during embryogenesis and does not contribute to the adult aortic arch.

**Clinical Pearl / High-Yield Fact**
The double arch aorta is a rare congenital anomaly that can be identified on imaging studies, such as CT angiography or MRI. It is essential to recognize this condition to avoid misdiagnosis and to provide appropriate management and follow-up care.

**✓ Correct Answer: D. Left 4th aortic arch**