Question

Paramedics bring an 82-year-old resident from a skilled nursing facility to the emergency room following a sudden episode of intractable abdominal cramping. The patient has a documented medical history of persistent atrial fibrillation and currently takes metoprolol, but receives no anticoagulant therapy due to a prior severe gastrointestinal bleed. Vital signs demonstrate an irregular heart rate of 118 beats per minute, a respiratory rate of 22 breaths per minute, and a blood pressure of 105/65 mm Hg. On clinical evaluation, the abdomen is remarkably soft, non-distended, and lacks rebound tenderness, despite the patient writhing in agony. Initial laboratory analysis reveals a serum lactate level of 4.2 mmol/L (normal: 0.5-2.2 mmol/L). Which of the following is the most appropriate initial imaging modality to establish the diagnosis?