What should be done to the arm to ensure proper positioning during the external rotation of the shoulder?

Ensuring that the elbow epicondyles are perpendicular to the bucky is crucial for proper positioning during external rotation of the shoulder. This orientation allows for an optimal projection of the shoulder joint, minimizing distortion and ensuring accurate representation of the anatomical structures involved. When the elbow epicondyles are perpendicular to the bucky, it helps to correctly orient the humerus in relation to the x-ray beam, facilitating a clearer visualization of the shoulder joint. This positioning is particularly important for distinguishing between various structures and pathologies.

Moreover, maintaining this specific orientation assists in achieving consistent and reproducible results in imaging, which is essential for accurate diagnosis and comparison in future studies. The other choices might not provide the same level of clarity and accuracy needed for effective imaging of the shoulder joint.