Final answer:
A PA with radial flexion best demonstrates the radial collateral ligament and parts of the elbow joint. This radiological view is particularly useful for examining the lateral structures of the elbow and the proximal radioulnar joint, helping in assessing stability and joint mechanics.
Step-by-step explanation:
The anatomical structures that a PA (posteroanterior) with radial flexion imaging view best demonstrates are primarily those of the elbow, including the radial collateral ligament and the proximal and distal arm bones. This specific radiological view allows clinicians to assess the lateral aspect of the elbow joint, where the lateral epicondyle of the humerus, the annular ligament, and the head of the radius are located. It provides detailed visualization of the elbow's supporting structures like the radial collateral ligament that runs from the lateral epicondyle of the humerus to merge with the annular ligament, ensuring stability to the joint. Moreover, it can help in assessing the interactions at the proximal radioulnar joint, where the head of the radius articulates with the radial notch of the ulna allowing for supination and pronation of the forearm.