A 28-year-old female with a stab wound to the left flank is brought to the emergency department. She is conscious, BP 92/60 mmHg, HR 115/min, and complaining of left-sided abdominal pain. Physical examination reveals a single stab wound with minimal external bleeding. The abdomen is soft with mild tenderness around the wound. Two units of O-negative blood are ordered. Which investigation is most appropriate to determine the need for exploratory laparotomy in this potentially penetrating abdominal trauma case?