Which of the following will put the humerus in a true AP projection?

To achieve a true anteroposterior (AP) projection of the humerus, the arm must be positioned with the humerus in external rotation. When the humerus is externally rotated, the epicondyles of the distal humerus become parallel to the film or detector, which is essential for a true AP view. This orientation allows for an accurate representation of the humerus without superimposition of other structures.

In AP external rotation, the thumb points outward, and the arm is rotated away from the body, which aligns the humeral head and greater tubercle correctly for optimal imaging. This rotational alignment is crucial for diagnosing conditions or evaluating the anatomy of the humerus effectively.

By contrast, in internal rotation, the arm is turned inward, which causes the epicondyles to be positioned at an angle to the film, leading to a distorted view. Neutral rotation does not properly align the humerus either, as it does not provide the necessary fit for the epicondyles parallel to the detector. Thus, only the external rotation achieves the desired anatomical position for a true AP projection of the humerus.