**Core Concept**
The ability to adduct the thumb depends on the function of specific muscles and nerves that control thumb movement. Thumb adduction involves the opposition and movement of the thumb towards the palmar surface of the hand.

**Why the Correct Answer is Right**
The primary muscles responsible for thumb adduction are the adductor pollicis muscle and the deep head of the flexor pollicis brevis muscle. These muscles are innervated by the ulnar nerve. The ulnar nerve supplies motor innervation to the muscles of the hand, including the adductor pollicis, allowing for thumb adduction. Damage to the ulnar nerve can lead to weakness or paralysis of these muscles, resulting in the inability to adduct the thumb.

**Why Each Wrong Option is Incorrect**
**Option A:** Injury to the median nerve would affect the flexor pollicis longus and flexor pollicis brevis muscles, leading to thumb flexion weakness rather than adduction weakness.
**Option B:** Injury to the radial nerve would affect the extensor pollicis longus and extensor pollicis brevis muscles, leading to thumb extension weakness rather than adduction weakness.
**Option C:** Injury to the musculocutaneous nerve would affect the biceps brachii and brachialis muscles, leading to elbow flexion weakness rather than thumb adduction weakness.

**Clinical Pearl / High-Yield Fact**
The ulnar nerve is often tested by assessing the ability to adduct the thumb and to from an "OK" sign with the hand.

**Correct Answer: C. Injury to the musculocutaneous nerve would affect the biceps brachii and brachialis muscles, leading to elbow flexion weakness rather than thumb adduction weakness.