Which option correctly ranks the normal vessel diameters from largest to smallest (ICA, ECA, vertebral)?

• A. ICA > ECA > Vertebral
• B. Vertebral > ICA > ECA
• C. ECA > ICA > Vertebral
• D. ICA > Vertebral > ECA

Answer: (B)

The main idea is that vessel size reflects function and flow needs. The vertebral arteries carry the posterior circulation and feed the brain through the basilar system, typically maintaining a relatively large lumen. The internal carotid arteries provide the major anterior cerebral circulation and also require a substantial, steady flow, so their diameters are usually larger than those of the arteries supplying extracranial tissues. The external carotid arteries mainly supply the face and scalp through many branches and do not need to carry as much cerebral blood flow, so their lumens are smaller.

Therefore, in normal adults, the vertebral arteries are the largest, followed by the internal carotid arteries, with the external carotid arteries being the smallest.