Pneumoperitoneum

A 58-year-old man with chronic NSAID use presents to the emergency department with sudden-onset severe generalised abdominal pain that began 4 hours ago. He lies completely still, is shocked (BP 88/52, HR 124), and has a rigid, board-like abdomen with generalised tenderness, guarding, and rebound tenderness. Bloods show WBC 24,000/mm³, lactate 5.8 mmol/L, and metabolic acidosis. An erect posteroanterior chest radiograph is obtained. The structure marked **A** in the diagram shows crescentic free air beneath both hemidiaphragms. What is the most likely diagnosis and primary pathology?