Which layer lines the interior lumen of a blood vessel?

• A. Aorta
• B. Phlebitis
• C. Varicose veins
• D. Tunica intima

Answer: (D)

The inner lining that comes into contact with blood is the tunica intima. This innermost layer sits directly against the lumen and is made mainly of endothelial cells that create a smooth surface to reduce friction and help regulate blood flow and vessel function. The vessel wall as a whole has three layers: intima, media (smooth muscle), and adventitia (outer connective tissue). The other options describe a specific vessel (the aorta) or conditions affecting veins (phlebitis, varicose veins), not the lining itself. So the correct concept is the tunica intima.