Absence of the right common carotid artery.

In a 33-year-old male patient with multiple sclerosis, brain magnetic resonance imaging incidentally showed a narrow right intracranial internal carotid artery. Gadolinium-enhanced magnetic resonance angiography showed absence of the right common carotid artery. Right external and internal carotid arteries arose separately from the ipsilateral innominate artery and subclavian artery, respectively. Color-Doppler ultrasound confirmed this finding. Embriologically, the case could be interpreted as the disappearance of the third aortic arch with persistence of the carotid duct connecting the third and fourth aortic arches.