A 35-year-old woman is brought to the emergency department 4 hours after an assault with a blunt object. She has a large contusion over the left flank with ecchymosis, but no abdominal distension, tenderness, or peritoneal signs. Vital signs are stable (BP 128/82, HR 88, RR 16). Urine dipstick shows no blood. What is the most appropriate next step in management?