A 58-year-old man presents to the emergency department with sudden onset of severe upper abdominal pain radiating to his right shoulder. He reports that the pain started abruptly 6 hours ago. He has a history of long-term NSAID use for chronic back pain. On examination, his abdomen is rigid with tenderness and guarding. His vital signs show BP 90/60 mmHg, HR 110 bpm, and Temp 100.2°F. What is the most likely diagnosis?
