A 58-year-old male with a history of chronic NSAID use for osteoarthritis presents to the emergency department with sudden onset severe epigastric pain and board-like abdominal rigidity. An erect chest X-ray is obtained. The structure marked **A** in the diagram—a lucent crescent of air under the right hemidiaphragm—is identified. Which of the following is the most likely diagnosis and the primary etiology of this radiological finding?