A 45-year-old male presents to an urgent care clinic with a 2-day history of severe, constant upper abdominal pain, which started acutely after a large meal and radiates to his back. He reports associated nausea, repeated episodes of bilious vomiting, and subjective fever. His medical history includes recurrent episodes of biliary colic. On examination, he is visibly distressed. Vitals show a temperature of 38.2C, blood pressure of 105/70 mmHg, pulse of 110/min, respiratory rate of 20/min, and oxygen saturation of 97% on room air. Abdominal assessment reveals marked tenderness and guarding in the epigastric region, with reduced bowel sounds. Further examination reveals subtle periumbilical discoloration (Cullen's sign) in addition to a faint bluish ecchymosis noted on his left flank. Given these clinical findings, what is the most likely diagnosis, and what immediate management step should be prioritized in the urgent care setting?