<s>[INST] context: Doppler ultrasonography (duplex) has become the best initial test for screening patients with carotid disease. It has become a highly accurate test, often obviating the need for carotid arteriography prior to carotid endarterectomy. Carotid arteriography remains the "gold standard" when quantifying the degree of carotid stenosis, but it is usually performed after noninvasive testing suggests significant stenosis. Spiral CT angiography is a new noninvasive modality that has been used to evaluate many segments of the vascular tree, but as yet its accuracy does not approach that of standard arteriography and it would certainly not be used in the initial evaluation of a patient with an asymptomatic bruit. Magnetic resonance arteriography (MRA) is also a relatively new modality that has enjoyed moderate success in the investigation of carotid disease. Although not quite as accurate as standard arteriography, it has been used in conjunction with the duplex as a complementary study. Once again, because of its cost, MRA would not be used as the primary screening modality. Transcranial Doppler studies are used to assess the intracranial vasculature. [/INST] question: A 75-year-old man is found by his internist to have an asymptomatic carotid bruit. The best initial diagnostic examination would be | options: A: Transcranial Doppler studies B: Doppler ultrasonography (duplex) C: Spiral CT angiography D: Arch aortogram with selective carotid artery injections | answer: B </s>