Which observation after nerve injury is correctly paired with the appropriate nerve?

Which observation after nerve injury is correctly paired with the appropriate nerve?

• A. Inability to flex the forearm—ulnar nerve
• B. Numbness in the thumb—radial nerve
• C. Inability to extend the forearm—musculocutaneous nerve
• D. Numbness in the little finger—median nerve

Answer: (B)

Nerve injuries produce deficits that mirror the nerve’s normal motor and sensory roles. The radial nerve supplies the extensor muscles of the forearm and provides sensation to the dorsolateral hand, including the dorsal aspect of the thumb. So numbness in the thumb fits radial nerve involvement. In contrast, elbow flexion is mainly performed by the musculocutaneous nerve (biceps, brachialis), so inability to flex the forearm would point to musculocutaneous injury, not ulnar. Extending the forearm is a function of the radial nerve, not the musculocutaneous. Finally, the little finger is primarily innervated by the ulnar nerve, not the median nerve. Therefore, numbness in the thumb corresponding to the radial nerve is the best match.

Nerve injuries produce deficits that mirror the nerve’s normal motor and sensory roles. The radial nerve supplies the extensor muscles of the forearm and provides sensation to the dorsolateral hand, including the dorsal aspect of the thumb. So numbness in the thumb fits radial nerve involvement. In contrast, elbow flexion is mainly performed by the musculocutaneous nerve (biceps, brachialis), so inability to flex the forearm would point to musculocutaneous injury, not ulnar. Extending the forearm is a function of the radial nerve, not the musculocutaneous. Finally, the little finger is primarily innervated by the ulnar nerve, not the median nerve. Therefore, numbness in the thumb corresponding to the radial nerve is the best match.