This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Abdomen and pelvis, Subject name: Anatomy
What best defines vasa vasorum ? 
Select and explain. 
A. Small blood vessels supplying walls of large blood vessels, B. Small blood vessels supplying nerves, C. Vessels accompanying aery, D. Vessels accompanying nerves
A: Small blood vessels supplying walls of large blood vessels
Answer: A: Small blood vessels supplying walls of large blood vessels 
Explanation: The vasa vasorum is a network of small blood vessels that supply the walls of large blood vessels, such as elastic aeries (aoa) and large veins (vena cava). Depending on the type of vasa vasorum, it penetrates the vessel wall staing at the intimal layer or the adventitial layer. Due to higher radial and circumferential pressures within the vessel wall layers closer to the main lumen of the aery, vasa vasorum externa cannot perfuse these regions of the vessel wall. imageref-sciencedirect.com