A 58-year-old man with a 2-year history of progressive vision loss presents with a pale optic disc on fundoscopy. The structure marked **C** in the diagram shows reduced disc vasculature. On examination, the disc margins are sharp, and the cup-to-disc ratio is normal. Visual acuity is 6/36 in both eyes, and a relative afferent pupillary defect (RAPD) is present bilaterally. The patient reports jaw claudication and scalp tenderness over the past month. Which of the following is the most critical next step in management to prevent irreversible vision loss in the fellow eye?