Which of the following muscles does NOT flex the carpus?

The extensor carpi radialis muscle is primarily responsible for extending the carpus, which is the joint where the forearm meets the hand. It plays a crucial role in extending the wrist joint, allowing for movements such as lifting the hand upward. This action contrasts with flexion, which involves decreasing the angle at the joint to bring the hand closer to the forearm.

In comparison, the other muscles listed—flexor carpi ulnaris, flexor carpi radialis, and superficial digital flexor—are all involved in the flexion of the carpus. These muscles contract to pull the carpals closer to the ulna and radius, thus decreasing the angle at the wrist and facilitating movements like grasping or holding an object. Therefore, the extensor carpi radialis is correctly identified as not contributing to the flexion of the carpus, solidifying its role as an extensor instead.