Which of the following constitutes a true vascular ring?

B. Double aortic arch.

DISCUSSION: Only the double aortic arch secondary to persistence of the right and left fourth aortic arches forms a true vascular ring. Pulmonary artery sling may cause symptoms that are due to compression of the trachea, and an anomalous right subclavian may cause dysphagia, but these anomalies do not constitute complete rings. Cervical aortic arch, which is thought to be secondary to persistence of the third aortic arch, is not a complete ring and usually is asymptomatic.