A 34-year-old man sustains a mid-shaft fracture of the humerus following a road traffic accident. During the axillary surgical approach for fixation, the operating surgeon identifies and tags a nerve (arrowed in Image 2) based on its intraoperative course and relationships — not solely on the fracture pattern. Postoperatively, the patient has weakness of elbow flexion and loss of sensation over the lateral forearm, consistent with injury to the tagged nerve. The nerve was identified intraoperatively as it pierced a muscle of the anterior axillary wall, and the image shows this tagged nerve with a clear arrow at the point of entry. Which statement best describes the course of the tagged nerve that accounts for both the motor and sensory deficits seen in this patient?

Question

Accepted Answer

B. It arises from the lateral cord, pierces coracobrachialis, and continues as the lateral cutaneous nerve of the forearm. ***B. It arises from the lateral cord, pierces coracobrachialis, and continues as the lateral cutaneous nerve of the forearm***
- The **musculocutaneous nerve** is the only nerve of the brachial plexus that typically **pierces the coracobrachialis muscle**, which serves as its defining surgical landmark.
- It provides motor supply to the **anterior compartment of the arm** (biceps, brachialis, coracobrachialis) and carries sensory fibers that become the **lateral cutaneous nerve of the forearm**.

*A. It arises from the medial cord, passes posterior to coracobrachialis, and terminates as the medial cutaneous nerve of the forearm*
- The musculocutaneous nerve originates from the **lateral cord** (C5-C7), whereas the **medial cord** gives rise to the ulnar and medial cutaneous nerves.
- The **medial cutaneous nerve of the forearm** is purely sensory and does not pierce muscles or supply the flexors of the elbow.

*C. It arises from the posterior cord, passes anterior to the subscapularis, and supplies the lateral compartment of the arm*
- The **posterior cord** gives rise to the **radial and axillary nerves**, which are responsible for extension and shoulder abduction, not primarily elbow flexion.
- There is no "lateral compartment of the arm"; muscles of the anterior arm are supplied by the **musculocutaneous nerve** and posterior muscles by the radial nerve.

*D. It arises from the lateral cord, passes superficial to coracobrachialis, and terminates as the posterior cutaneous nerve of the forearm*
- The musculocutaneous nerve **pierces** rather than passes superficial to the coracobrachialis, making this description anatomically inaccurate.
- It terminates specifically as the **lateral cutaneous nerve**, while the **posterior cutaneous nerve of the forearm** is a branch of the radial nerve. [Practice more Anatomy questions on OnCourse]

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