A 58-year-old man presents to the emergency department with acute onset of severe pain, coldness, and pallor of the right lower limb for 4 hours. On examination, the limb is mottled, sensory loss is present in the foot, and dorsalis pedis pulse is absent. The patient has a history of atrial fibrillation not on anticoagulation. Which is the investigation of choice to confirm the diagnosis and guide immediate management?