Layers of blood vessel walls from deep to superficial.

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The main idea tested is the correct inside-to-outside arrangement of the vessel wall layers. The innermost layer is the tunica intima, which lines the lumen with endothelial cells. Surrounding that is the tunica media, composed mainly of smooth muscle (and elastic fibers) that acts to regulate vessel diameter through contraction and relaxation. The outermost layer is the tunica externa (adventitia), a connective tissue sheath that provides structural support and anchors the vessel to surrounding tissues. This deep-to-superficial order—intima, then media, then externa—is the standard organization for arteries and veins. If the layers were arranged differently, the vessel would lose the functional arrangement that allows the endothelium to interface with the lumen, the muscular layer to control flow, and the outer connective tissue to stabilize and connect the vessel.