A 55-year-old man presents with sudden onset, severe epigastric pain radiating to his back. He is nauseous and has vomited several times. He drinks approximately 40 units of alcohol per week. On examination, he is tender in the epigastrium with guarding. Observations: HR 110 bpm, BP 130/80 mmHg, Temp 37.8°C. Which investigation is most useful for confirming the diagnosis?