A 68-year-old man with hypertension and smoking history presents with a 2-week history of transient monocular vision loss. Carotid duplex ultrasonography is performed. The structure marked **D** in the diagram shows a peak systolic velocity (PSV) of 245 cm/s in the internal carotid artery. According to current consensus criteria for carotid stenosis grading, what degree of stenosis does this finding represent, and what is the most appropriate next step in management?