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

Weakness of finger flexion without sensory impairment is characteristic of anterior interosseous nerve injury. This branch of the median nerve is responsible for the innervation of specific muscles in the forearm that contribute to flexing the fingers, particularly the flexor pollicis longus and the lateral half of the flexor digitorum profundus.

When there is an injury to the anterior interosseous nerve, patients may present with weakness in flexing the thumb and the index finger (the "ok" sign is affected), while the sensation in the hand remains intact, as this nerve does not carry sensory fibers. This distinguishes it from injuries to the median nerve as a whole, which typically present with both motor and sensory deficits.

In contrast, injuries to the ulnar nerve would affect the intrinsic muscles of the hand, leading to weakness and sensory changes in the ring and little fingers. The musculocutaneous nerve primarily affects elbow flexion and sensation over the lateral forearm, while the median nerve damage would usually present with sensory symptoms due to its broad sensory distribution. Thus, the anterior interosseous nerve is the most precise answer, focusing specifically on the motor deficit without accompanying sensory loss.