Vein Wall Layers: Thin Media, Thick Adventitia

Steps:

• Recall blood vessel walls have three tunics: intima (inner endothelium), media (smooth muscle/elastic), adventitia (outer connective tissue).
• Note major veins have thin tunica media for flexibility and thick tunica adventitia for support.
• Distinguish from arteries (thick media, thin adventitia) and capillaries (single endothelial layer).
• Match diagram showing thin middle layer and thick outer layer to vein structure.

Why D is correct:

• Diagram D depicts thin tunica media and thick tunica adventitia, matching the histological definition of major vein walls for low-pressure flow.

Why the others are wrong:

• A shows thick media and thin adventitia, characteristic of arteries.
• B illustrates a single endothelial layer, typical of capillaries.
• C features equal or prominent media, resembling arterioles.

Final answer: D