A 28-year-old woman presents with a 24-hour history of periumbilical pain that migrated to the right lower quadrant, associated with nausea and mild fever (37.5°C). On examination, she has right lower quadrant tenderness and mild rebound tenderness. Her leukocyte count is 11,200/μL with a left shift. Her Alvarado score is calculated as 5. The structure marked **B** in the diagram indicates the management approach for this score range. Which of the following is the most appropriate next step in her management?