What motor function is impaired with an ulnar nerve injury?

An ulnar nerve injury primarily affects the motor function of the muscles innervated by the ulnar nerve, which includes the intrinsic muscles of the hand responsible for fine motor skills and dexterity. One of the key muscles affected by this injury is the abductor digiti minimi, which is responsible for the abduction of the little finger. When the ulnar nerve is compromised, this muscle does not function properly, leading to impaired ability to spread the little finger away from the ring finger.

In contrast, the other choices involve muscles and movements that are either not innervated by the ulnar nerve or less directly impacted by an ulnar nerve injury. For instance, the opposition of the thumb is largely controlled by the median nerve, while wrist extension primarily relies on the radial nerve. Adduction of the thumb involves both the ulnar and median nerves but is more significantly impacted by the median nerve due to the control it exerts over the adductor pollicis muscle. Thus, the most accurate function impaired by an ulnar nerve injury is the abduction of the little finger.