A person receives a shallow knife wound just behind the sternocleidomastoid muscle and about 1 1/2 inches above the clavicle. There is an immediate numbness of the skin below the wound and over the acromion and clavicle. The nerve most likely severed is?
**Core Concept:** The correct answer is based on the understanding of the anatomy and pathophysiology of the affected area. The numbness of the skin below the wound and over the acromion and clavicle indicates that the injury is to a nerve that supplies sensory innervation to these regions. The relevant nerves in this context are the upper trunk of the brachial plexus and the suprascapular nerve.
**Why the Correct Answer is Right:** The correct answer is based on the understanding of the anatomy and pathophysiology of the affected area. The injury is to a nerve that supplies sensory innervation to these regions. In this case, the suprascapular nerve is severed. The suprascapular nerve arises from the upper trunk of the brachial plexus, which is formed by the union of the C5 and C6 spinal nerves. The suprascapular nerve provides sensory innervation to the skin over the acromion and clavicle, as well as motor innervation to the supraspinatus muscle.
**Why Each Wrong Option is Incorrect:**
A. The spinal accessory nerve (C1-C4) supplies motor innervation to the sternocleidomastoid muscle but does not innervate the acromion or clavicle. Therefore, option A is incorrect.
B. The axillary nerve (C5, C6, and C7) supplies motor innervation to the deltoid muscle, but does not innervate the acromion or clavicle. Hence, option B is incorrect.
C. The lateral pectoral nerve (C5 and C6) innervates the pectoralis major muscle but does not innervate the acromion or clavicle. Therefore, option C is incorrect.
D. The suprascapular nerve (C5, C6, and a branch of the axillary nerve) supplies both motor and sensory innervation to the acromion and clavicle. Hence, option D is incorrect.
**Clinical Pearls:**
The suprascapular nerve is a crucial structure in the upper limb and shoulder girdle region. Its damage can lead to a significant loss of function and sensation. In this case, the injury to the suprascapular nerve results in the loss of sensation to the acromion and clavicle, as well as weakness of the supraspinatus muscle. This can be a clinical pearl for students to remember the function of the suprascapular nerve and its clinical implications.