**Core Concept:** The brachial plexus is a network of nerves formed by the union of spinal nerves C5, C6, C7, C8, and T1. It supplies motor and sensory innervation to the upper limb. Infraclavicular branches of the brachial plexus include: radial, ulnar, musculocutaneous, and median nerves.

**Why the Correct Answer is Right:** The correct answer, D (the musculocutaneous nerve), is indeed an infraclavicular branch of the brachial plexus. It arises from the lateral cord of the brachial plexus and supplies motor innervation to the lateral and medial cutaneous nerves of the arm and forearm, as well as the coracobrachialis muscle.

**Why Each Wrong Option is Incorrect:**

A. The radial nerve is incorrect because it is a supraclavicular branch of the brachial plexus, arising from the posterior cord.
B. The ulnar nerve is also incorrect, as it is a supraclavicular branch arising from the posterior cord.
C. The median nerve is a supraclavicular branch, originating from the anterior cord, not infraclavicular.

**Clinical Pearl:** Understanding the anatomy of the brachial plexus and its branches is crucial for understanding clinical presentations, such as Horner's syndrome, where a lesion in the sympathetic chain can result in ptosis, miosis, and anhidrosis due to damage to the sympathetic fibers in the brachial plexus.

**Correct Answer:** **D. Musculocutaneous Nerve**