Pneumoperitoneum — Air Under the Diaphragm

A 56-year-old man with untreated H. pylori infection and chronic NSAID use presents with sudden-onset severe epigastric pain that became generalized 3 hours ago. He lies motionless, unable to tolerate any movement. On examination, his abdomen is rigid with diffuse involuntary guarding, rebound tenderness, and absent bowel sounds. An erect PA chest radiograph is obtained. The structure marked **A** in the diagram represents a crescentic lucent collection beneath the right hemidiaphragm. What is the most likely diagnosis?