Weakness of finger flexion without sensory impairment is characteristic of which nerve injury?

Weakness of finger flexion without sensory impairment is a hallmark of injury to the anterior interosseous nerve. This nerve is a branch of the median nerve and primarily innervates the flexor pollicis longus, the flexor digitorum profundus to the index finger, and the pronator quadratus. When the anterior interosseous nerve is damaged, it leads to weakness in flexing the thumb and the index finger, but since the sensory supply is preserved (because the sensory fibers of the median nerve remain intact), there are no sensory deficits associated with this specific injury.

In contrast, injuries to the ulnar and median nerves typically result in both motor weakness and sensory loss due to their roles in innervating hand muscles and providing sensation to the fingers. The musculocutaneous nerve primarily affects the flexion of the elbow and sensation in the forearm, without typically impacting finger flexion directly, so it does not align with the weakness described in the question.

Thus, the unique presentation of finger flexion weakness, particularly in the thumb and index finger, alongside preserved sensation, distinctly points to damage of the anterior interosseous nerve.