Which nerves arise from the posterior cord of the brachial plexus?

The correct answer is that the radial and axillary nerves arise from the posterior cord of the brachial plexus.

The posterior cord is formed by the contributions from the upper, middle, and lower trunks of the brachial plexus, which are largely responsible for innervating the extensor muscles of the arm and forearm. The radial nerve innervates the majority of the posterior compartments, including the triceps brachii, and is crucial for extending the elbow and wrist. The axillary nerve is responsible for innervating the deltoid and teres minor muscles, and also provides sensory innervation to the skin over the lateral aspect of the shoulder.

The other options do not correctly reflect the anatomy of the brachial plexus. The ulnar and musculocutaneous nerves originate from different parts of the plexus. The ulnar nerve typically arises from the medial cord, while the musculocutaneous nerve arises from the lateral cord. Similarly, the lateral and medial pectoral nerves are also branches from the lateral and medial cords respectively, rather than the posterior cord, and they primarily innervate the pectoral muscles. Thus, the selection of radial and axillary nerves accurately identifies the