Final answer:
The 45-degree Semi-Supinated Oblique Wrist is used to best demonstrate the distal radioulnar joint, which is emphasized by the semi-supinated position of the radius and ulna.
Step-by-step explanation:
The structure best demonstrated on a 45-degree Semi-Supinated Oblique Wrist is the distal radioulnar joint. During semi-supination, the radius and ulna are in an intermediary position between full pronation and full supination, which is useful for visualizing certain structures in the wrist. Supination is the motion used when turning the forearm so that the palm faces up, as though scooping soup with a spoon. This motion separates the radius and ulna, making them parallel, and is contrasted by pronation, where the radius crosses over the ulna, forming an X-shape. The superficial anterior compartment of the forearm is responsible for bending the wrist towards the body and tilting the hand to the side away from the body. Conversely, the superficial posterior compartment of the forearm straightens the wrist away from the body and performs the same side-tilting action.