**Core Concept**
The musculocutaneous nerve is a branch of the lateral cord of the brachial plexus that supplies the biceps brachii, brachialis, and coracobrachialis muscles. Injury to this nerve at the lateral cord would lead to weakness or paralysis of these muscles.

**Why the Correct Answer is Right**
When the musculocutaneous nerve is injured at the lateral cord, the primary clinical findings would be related to the muscles it supplies. The biceps brachii is involved in elbow flexion and forearm supination, while the brachialis is involved in elbow flexion. The coracobrachialis assists in arm flexion and adduction. A patient with a musculocutaneous nerve injury at the lateral cord would exhibit weakness or paralysis of these movements, particularly difficulty in flexing the elbow and supinating the forearm.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the radial nerve, not the musculocutaneous nerve, is responsible for wrist extension and finger spread. Injury to the musculocutaneous nerve would not primarily affect these movements.
* **Option B:** This option is incorrect because the ulnar nerve, not the musculocutaneous nerve, is responsible for finger abduction and adduction, as well as intrinsic hand muscle function. Injury to the musculocutaneous nerve would not primarily affect these movements.
* **Option D:** This option is incorrect because the median nerve, not the musculocutaneous nerve, is responsible for thumb opposition and flexion, as well as forearm pronation. Injury to the musculocutaneous nerve would not primarily affect these movements.

**Clinical Pearl / High-Yield Fact**
The musculocutaneous nerve is often tested by assessing the strength of elbow flexion and forearm supination, as these movements are primarily affected by the biceps brachii and brachialis muscles. A patient with a suspected musculocutaneous nerve injury should be carefully examined for weakness or paralysis of these movements.

**Correct Answer: C. Weakness of elbow flexion and forearm supination.**