A person presents with their fourth and fifth fingers unable to flex. Which nerve might be affected?

The ulnar nerve is responsible for innervating the muscles that control the flexion of the fourth and fifth fingers, specifically the intrinsic muscles of the hand such as the flexor digitorum profundus (medial part) and the interossei muscles. If a person is unable to flex these fingers, it suggests a dysfunction in the ulnar nerve, which can be caused by various conditions, such as cubital tunnel syndrome, where the nerve gets compressed at the elbow.

The median nerve primarily innervates the muscles responsible for flexing the thumb and index fingers, and the radial nerve is largely responsible for extending the wrist and fingers. The musculocutaneous nerve primarily innervates the muscles in the anterior compartment of the arm, which do not significantly contribute to finger flexion. Therefore, the symptoms described strongly point to an issue with the ulnar nerve, making it the most likely affected nerve in this scenario.