A 38-year-old male presents with 10 hours of persistent severe epigastric pain, accompanied by nausea and vomiting. The pain initially localized to the left upper abdomen but gradually spread to the entire abdomen. Physical exam: temperature 39°C, BP 90/60 mmHg, flat abdomen with marked upper abdominal muscle rigidity, significant tenderness, and mild rebound tenderness. Abdominal X-ray shows no free air under the diaphragm. Ultrasound reveals gallstones. Lab findings: WBC 16×10⁹/L (16,000/mm³), neutrophils 90%, serum amylase 2460 U (Somogyi method). The most likely diagnosis is: