In assessing the ulnar nerve function in a 9-year-old boy, which function should be checked?

To assess the function of the ulnar nerve, it is essential to evaluate the movements that are specifically innervated by this nerve. The ulnar nerve plays a crucial role in controlling the intrinsic muscles of the hand, particularly those responsible for finger abduction and adduction.

In this context, checking the abduction of the index, middle, ring, and small fingers is the correct approach. The ulnar nerve innervates the muscles responsible for these movements, including the dorsal interossei and the abductor digiti minimi, which are essential for spreading the fingers apart. This functionality is central to the ulnar nerve's role in hand dexterity and grip strength.

In contrast, the other options relate to movements primarily controlled by different nerves. Abduction of the thumb, for example, is mainly facilitated by the median nerve. Extension of the thumb and extension of the index and middle fingers are also predominantly governed by the radial nerve. Thus, assessing the function of the ulnar nerve focuses specifically on its unique contributions to finger abduction and the overall dexterity of the hand.