What type of blood vessel is characterized by having only an endothelium membrane where nutrient and waste exchange occurs?

The correct answer is capillary. Capillaries are the smallest blood vessels in the body and are specifically designed for the exchange of nutrients, gases, and waste products between the blood and the tissues. They consist of a very thin wall that is composed solely of the endothelium, which is a single layer of endothelial cells. This thin structure allows for easy diffusion and transport of substances directly to and from cells, facilitating efficient metabolic exchanges.

In contrast, other blood vessels have more complex walls that include multiple layers of cells and connective tissue, which are not conducive to such direct exchange of materials. For example, veins and arterioles have thicker walls containing muscle and elastic tissue, which serve different functions related to blood flow and pressure regulation, rather than direct nutrient and waste exchange like the capillaries.