What motor function is primarily affected by a musculocutaneous nerve injury?

A musculocutaneous nerve injury primarily affects the biceps function. The musculocutaneous nerve innervates the biceps brachii muscle, which is responsible for flexing the elbow and supinating the forearm. When this nerve is injured, the individual may have significant difficulty in performing these actions effectively. The biceps are key muscles for lifting and carrying objects, so impairment can substantially affect daily activities.

The other choices involve motor functions served by different nerves. Wrist extension is primarily controlled by the radial nerve, finger flexion is mediated by the median and ulnar nerves, and shoulder abduction involves the axillary and suprascapular nerves. Therefore, recognizing the musculocutaneous nerve's specific role in elbow flexion clarifies why biceps function is the correct answer to this question.