Arm muscle not supplied by musculocutaneous nerve-
**Core Concept**
The musculocutaneous nerve is a branch of the brachial plexus that innervates the muscles of the anterior compartment of the arm, including the biceps brachii, brachialis, and coracobrachialis. The musculocutaneous nerve is responsible for motor innervation to these muscles, which are involved in elbow flexion and forearm supination.
**Why the Correct Answer is Right**
The correct answer is the arm muscle that is not innervated by the musculocutaneous nerve. This is because the musculocutaneous nerve does not supply the triceps brachii muscle, which is responsible for elbow extension. The triceps brachii is a large muscle in the posterior compartment of the arm, and it is innervated by the radial nerve. The radial nerve is a separate branch of the brachial plexus that innervates the muscles of the posterior compartment of the arm.
**Why Each Wrong Option is Incorrect**
* **Option A:** The biceps brachii is actually supplied by the musculocutaneous nerve, making this option incorrect.
* **Option B:** The brachialis muscle is also supplied by the musculocutaneous nerve, so this option is incorrect as well.
* **Option D:** The coracobrachialis muscle is innervated by the musculocutaneous nerve, making this option incorrect.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the musculocutaneous nerve is responsible for innervating the muscles of the anterior compartment of the arm, while the radial nerve innervates the muscles of the posterior compartment. This distinction is crucial for understanding the anatomy and innervation of the arm muscles.
**Correct Answer: C. Triceps brachii.**