**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. It also provides sensory innervation to the lateral surface of the forearm.

**Why the Correct Answer is Right**
The musculocutaneous nerve is responsible for innervating the muscles involved in flexion and supination of the forearm. Damage to this nerve would lead to weakness in these movements. Additionally, the musculocutaneous nerve provides sensory innervation to the lateral surface of the forearm, which is consistent with the patient's loss of sensation in this area. The musculocutaneous nerve arises from the lateral cord of the brachial plexus, which is formed by the C5-C7 nerve roots.

**Why Each Wrong Option is Incorrect**
**Option A:** The radial nerve is responsible for innervating the muscles involved in extension of the wrist and fingers, as well as providing sensory innervation to the back of the hand and the lateral aspect of the forearm. Damage to the radial nerve would not explain the patient's weakness in flexion and supination.

**Option C:** The median nerve is responsible for innervating the muscles involved in flexion of the wrist and fingers, as well as providing sensory innervation to the palmar surface of the hand. Damage to the median nerve would not explain the patient's weakness in supination or the loss of sensation on the lateral surface of the forearm.

**Option D:** The lateral cord of the brachial plexus gives rise to the musculocutaneous nerve, but it is not the nerve itself that has been injured. Damage to the lateral cord would result in a more complex pattern of weakness and sensory loss.

**Clinical Pearl / High-Yield Fact**
The musculocutaneous nerve is often injured in fractures of the humerus, particularly those that occur in the mid-shaft region. This is because the nerve passes through the coracobrachialis muscle, which is located near the fracture site.

✓ Correct Answer: B. Musculocutaneous.