Final answer:
The scaphoid bone articulates with the base of the radius when the hand is in PA position with ulnar flexion. It is one of the three carpal bones that contribute to the radiocarpal joint, the others being the lunate and triquetrum.
Step-by-step explanation:
The carpal bone that articulates with the base of the radius when the hand is in PA position with ulnar flexion is the scaphoid. Within the carpal bones, the scaphoid and lunate directly articulate with the distal end of the radius forming the radiocarpal joint. These carpal bones form the base of the hand and are visible in a radiograph. When performing ulnar flexion, the scaphoid comes into closer proximity with the radius and is therefore the bone that articulates with the base of the radius.
Other relevant details include that the carpal bones are organized into a proximal row (scaphoid, lunate, triquetrum, and pisiform) and a distal row (trapezium, trapezoid, capitate, and hamate). However, it is the scaphoid bone that is the focus when discussing articulation with the radius during the specific hand positioning mentioned.