A 53 years old man presents to the emergency room complaining of severe mid-epigastric abdominal pain that radiates to the back. The pain improves when the patient leans forward or assumes the fetal position and worsens with deep inspiration and movement. He also complains of nausea, vomiting and anorexia, and gives a history of heavy alcohol intake this past week. He is tachycardic, tachypneic, and febrile with hypotension. He is slightly agitated and confused. He is diaphoretic with decreased breath sounds over the base of the left lung. What is the most likely diagnosis?