A 68-year-old man presents to the emergency department with severe abdominal pain, headache, chest pain, and shortness of breath for the past three days. He has a history of coronary artery disease, asthma, and recently diagnosed hypertension that has been resistant to treatment with hydrochlorothiazide and lisinopril. Over the past month, his glomerular filtration rate (GFR) has rapidly declined. On physical examination, his blood pressure is 210/115 mmHg, and he has diffuse pulmonary crackles bilaterally and an abdominal bruit on the left side. A chest X-ray shows bilateral fluffy pulmonary infiltrates. Renal artery stenosis is suspected. What is the gold standard for diagnosing renal artery stenosis?
