A 15-year-old boy presents to the emergency department with sudden-onset severe left scrotal pain that woke him from sleep 4 hours ago, accompanied by nausea and vomiting. On examination, the left testis is **high-riding and lies horizontally** (as marked **C** in the diagram), and the cremasteric reflex is absent bilaterally. Prehn sign is negative. What is the most appropriate next step in management?