A 32-year-old man is brought to the emergency department following a high-speed motor vehicle collision. On arrival, he is alert and oriented, blood pressure 110/72 mmHg, heart rate 98/min, respiratory rate 20/min. Primary survey reveals no airway obstruction, bilateral breath sounds present, and no external bleeding. During secondary survey, the trauma surgeon palpates the abdomen and notes mild tenderness in the left upper quadrant without guarding or rigidity. The patient denies pain elsewhere. What is the most appropriate next step in the management of this patient?