The left recurrent laryngeal nerve loops under the ductus arteriosus while the right recurrent laryngeal nerve loops under the right subclavian artery. This asymmetry is explained by which embryological event?

• A Formation of the aorticopulmonary septum
• B Persistence of the distal part of the right sixth aortic arch artery
• C Regression of the distal part of the right sixth aortic arch artery ✓
• D Closure of the ductus venosus

Explanation

Both recurrent laryngeal nerves hook around the sixth arch arteries. On the left, the distal sixth arch persists as the ductus arteriosus, so the nerve remains hooked beneath it. On the right, the distal sixth arch artery regresses, allowing the nerve to slip upward and take its final course under the third arch derivative, the subclavian artery. The aorticopulmonary septum divides the outflow tract and plays no role in this asymmetry.

Reference: Langman's Medical Embryology, 14th ed.

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