A man is unable to hold typing paper between his index and middle fingers. Which of the following nerves was likely injured?
**Core Concept**
The question is testing the clinical application of motor nerve function, specifically the function of the median nerve in the hand. The median nerve is responsible for innervating the muscles of the anterior forearm and the thenar eminence, including the flexor digitorum profundus and superficialis muscles, which control finger flexion and opposition.
**Why the Correct Answer is Right**
The inability to hold typing paper between the index and middle fingers suggests an impairment in the adductor pollicis muscle, which is responsible for thumb adduction and opposition. The median nerve, specifically the anterior interosseous branch, innervates the flexor digitorum profundus muscles of the index and middle fingers, which are involved in thumb opposition. Damage to the median nerve at the wrist or forearm level would lead to weakness or paralysis of these muscles, resulting in the inability to perform fine motor tasks such as holding paper between the index and middle fingers.
**Why Each Wrong Option is Incorrect**
**Option A:** The ulnar nerve primarily innervates the muscles of the medial forearm and the intrinsic muscles of the hand, including the flexor digitorum profundus and interosseous muscles of the little and ring fingers. Damage to the ulnar nerve would not affect the index and middle finger flexion or opposition.
**Option B:** The radial nerve primarily innervates the muscles of the posterior forearm and is responsible for wrist extension and finger extension. Damage to the radial nerve would not affect the index and middle finger flexion or opposition.
**Option C:** The musculocutaneous nerve primarily innervates the muscles of the anterior forearm, including the biceps and brachialis muscles. Damage to the musculocutaneous nerve would not affect the index and middle finger flexion or opposition.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the median nerve is responsible for the "median" muscles of the forearm, including the flexor digitorum profundus and superficialis muscles, which control finger flexion and opposition. Damage to the median nerve can lead to weakness or paralysis of these muscles, resulting in impaired fine motor tasks.
**Correct Answer:** C. Radial nerve