A patient loses the ability to flex his forefinger. The nerve that supplies the muscles that cause this action is formed from which of the following cord(s) of the brachial plexus?
**Core Concept**
The question tests the understanding of the anatomy of the brachial plexus and its association with the motor function of the forearm and hand. The brachial plexus is a complex network of nerves that arise from the spinal cord and supply the muscles of the upper limb.
**Why the Correct Answer is Right**
The flexion of the forefinger is primarily caused by the flexor digitorum profundus muscle, which is innervated by the ulnar nerve. The ulnar nerve arises from the C8 and T1 roots of the brachial plexus. These roots are part of the medial cord of the brachial plexus. The ulnar nerve supplies the muscles of the forearm and hand, including the flexor digitorum profundus, which is responsible for flexing the fingers.
**Why Each Wrong Option is Incorrect**
* **Option B:** The musculocutaneous nerve, which arises from the lateral cord of the brachial plexus, supplies the biceps brachii, brachialis, and coracobrachialis muscles, which are involved in elbow flexion and forearm supination, but not finger flexion.
* **Option C:** The radial nerve, which arises from the posterior cord of the brachial plexus, supplies the muscles of the extensor forearm and wrist, including the extensor digitorum communis, which is responsible for extending the fingers, not flexing them.
**Clinical Pearl / High-Yield Fact**
The ulnar nerve is commonly compressed at the elbow, leading to a condition known as cubital tunnel syndrome, which can cause weakness and numbness in the hand.
**Correct Answer:** D. C8 and T1. The ulnar nerve is formed from the C8 and T1 roots of the brachial plexus, which are part of the medial cord.