A 45-year-old male presents with a painful, deformed right shoulder after a motor vehicle accident. On examination, the examiner attempts to palpate the humeral head anteriorly but finds an empty glenoid. The arm is held in internal rotation and adduction, and there is a prominent coracoid process. Which of the following radiographic signs is most characteristic of this presentation?

Explanation

The clinical presentation of the arm held in internal rotation and adduction, along with an empty glenoid anteriorly, is highly suggestive of a posterior shoulder dislocation. The 'lightbulb sign' is a classic radiographic finding on an AP view of the shoulder in posterior dislocation, where the humeral head appears symmetrically rounded due to fixed internal rotation, resembling a lightbulb. While a 'squared-off shoulder' is a general sign of dislocation, the specific arm position and the 'lightbulb sign' are pathognomonic for posterior dislocation. Hill-Sachs and Bankart lesions are associated bony/labral injuries, not primary radiographic signs of the dislocation itself, though they are often seen with anterior dislocations.