Final answer:
The most likely diagnosis for a person stabbed in the neck with an expanding mass and a GCS of 15 is a carotid artery aneurysm, a bulging area in the carotid artery wall.
Step-by-step explanation:
If a person is stabbed in the neck and presents with an expanding mass in the lateral neck alongside a Glasgow Coma Scale (GCS) of 15, the most likely diagnosis is a carotid artery aneurysm. This condition refers to a weakened area in the wall of the carotid artery, which has become abnormally enlarged or bulged out. Given that the person is conscious and alert (as indicated by a GCS of 15), it is less likely that a complete rupture or dissection has occurred, since these conditions would likely lead to a more rapid deterioration in mental status. However, in the case of a traumatic injury such as a stabbing, a partial tear leading to an aneurysm could create an expanding hematoma without immediately compromising consciousness. Carotid artery thrombosis, although serious, would typically present with neurological deficits rather than an expanding mass.