Final answer:
The nerve likely severed causing shoulder sag and difficulty in arm abduction is the spinal accessory nerve, which innervates the trapezius and sternocleidomastoid muscles. This nerve plays a key role in shoulder elevation and arm abduction movements.
Step-by-step explanation:
The nerve that appears to have been severed, resulting in a sagging shoulder (scapula) and difficulty in fully abducting the arm after receiving a laceration along the anterior border of the trapezius muscle, is the spinal accessory nerve. The spinal accessory nerve, also known as the accessory nerve, innervates both the sternocleidomastoid and trapezius muscles. Damage to this nerve can lead to an inability to elevate the scapula, which is necessary for full abduction of the arm as performed by the deltoid muscle. Furthermore, the trapezius muscle's role in stabilizing and positioning the scapula is compromised, leading to a dropped or sagging appearance of the shoulder.
To confirm the involvement of the accessory nerve, strength tests such as shrugging the shoulders against resistance can be performed. A deficit on one side compared to the other would indicate damage to the nerve on the weaker side. Therefore, based on the symptoms described, it is likely that the spinal accessory nerve has been affected by the laceration.