In order to check the pulse of a child whose forearm is in a cast, the pediatrician presses her finger into the depth of the "anatomical snuffbox". The tendon lying immediately medial (ulnar) to the physician's finger belongs to what muscle?

The anatomical snuff box is bounded on the ulnar side by the tendon of extensor pollicis longus. It is bounded radially by the tendons of abductor pollicis longus and extensor pollicis brevis. The radial aery lies in the floor of the snuff box, and scaphoid can be felt in the floor of the anatomical snuff box. Extensor carpi radialis longus and brevis attach their tendons to the base of the second and third metacarpals, respectively. Brachioradialis inses its tendon on the lateral side of the base of the styloid process of the radius.