USMLE STEP 1 QUESTION

CONCEPT: Aortic arch embryology

A 3-year-old boy is brought to the pediatrician by his mother due to recurrent episodes of difficulty swallowing and breathing. On physical examination, his vital signs are as follows: Temperature 98.6°F (37°C), pulse 95/min, respirations 25/minute, and blood pressure 95/60 mmHg. His oxygen saturation is 97% on room air. The physician orders a chest X-ray and observes a vascular ring compressing the trachea and esophagus. Further imaging confirms the presence of a double aortic arch. Which of the following embryologic derivatives is most closely related to the formation of this anomaly?

A. First aortic arch

B. Second aortic arch

C. Third aortic arch

D. Fourth aortic arch

E. Fifth aortic arch

F. Sixth aortic arch

G. Ductus arteriosus

What’s going on here?

This question is about a double aortic arch, which is a vascular anomaly that results from the persistence and development of both the right and left embryonic aortic arches. This condition can cause compression of the trachea and esophagus, leading to symptoms like dysphagia and dyspnea, as described in the patient.

Let's dissect each option to understand their roles in embryological development and relate them to the correct answer.

A. First aortic arch

• Role in Development: The first aortic arch contributes to the formation of the maxillary artery, a branch of the external carotid.

• Relevance to the Condition: It does not directly contribute to the formation of the aortic arch; hence, it is not related to the double aortic arch anomaly.

B. Second aortic arch

• Role in Development: The second aortic arch regresses and its remnants contribute to the formation of the hyoid and stapedial arteries.

• Relevance to the Condition: Like the first aortic arch, it does not play a role in the formation of the aortic arch, making it unrelated to the condition in question.

C. Third aortic arch

• Role in Development: The third aortic arch is involved in the formation of the common carotid arteries and the proximal part of the internal carotid arteries.

• Relevance to the Condition: Although important for vascular development, the third aortic arch does not contribute to the aortic arch structure and thus is not responsible for the double aortic arch anomaly.

D. Fourth aortic arch

• Role in Development: The fourth aortic arch has a significant role in cardiovascular development. On the right, it contributes to the formation of the right subclavian artery, and on the left, it is a major contributor to the aortic arch.

• Relevance to the Condition: The double aortic arch results from the persistence and development of both right and left fourth aortic arches, leading to the formation of two aortic arches that can encircle and compress the trachea and esophagus.

E. Fifth aortic arch

• Role in Development: The fifth aortic arch either does not form or regresses early in development and has no significant contribution to the adult vascular system.

• Relevance to the Condition: Due to its insignificant role or absence in normal development, it is not related to the double aortic arch anomaly.

F. Sixth aortic arch

• Role in Development: The sixth aortic arch is involved in the formation of the pulmonary arteries and, on the left, the ductus arteriosus.

• Relevance to the Condition: Although crucial for the development of the pulmonary vasculature and the ductus arteriosus, the sixth aortic arch does not contribute to the formation of the aortic arch itself, and thus is not related to this anomaly.

G. Ductus arteriosus

• Role in Development: The ductus arteriosus is a fetal blood vessel connecting the pulmonary artery to the descending aorta, allowing blood to bypass the non-functioning fetal lungs. It normally closes after birth, becoming the ligamentum arteriosum.

• Relevance to the Condition: While the ductus arteriosus is an important structure in fetal circulation, it does not contribute to the formation of the aortic arch structure and is not involved in the double aortic arch anomaly.

---

Correct Answer: D. Fourth aortic arch

The double aortic arch results from the development and persistence of both the right and left fourth aortic arches, which can lead to the vascular encirclement of the trachea and esophagus, causing symptoms like those seen in the patient. Understanding the embryological origin of this condition is crucial for medical professionals, especially when considering surgical correction or management strategies for alleviating the compressive symptoms.