Arterial wall composition in transverse section

Steps:

• Arteries consist of three layers: tunica intima (endothelium, elastic, collagen), tunica media (elastic fibers, smooth muscle, collagen), and tunica adventitia (collagen, fibroblasts).
• Layer X in a transverse section typically represents the tunica media, the thick middle layer responsible for contraction and elasticity.
• The tunica media contains circumferentially arranged smooth muscle cells for vasoconstriction.
• It also includes elastic fibers for recoil and collagen fibers for tensile strength, with no other major tissues dominating.

Why D is correct:

• The tunica media is defined in histology as containing elastic fibers for stretch, collagen for support, and smooth muscle for regulation, matching the open list in D.

Why the others are wrong:

• A: Omits elastic fibers, which are essential for arterial elasticity.
• B: Excludes collagen, which reinforces the media against pressure.
• C: The "only" restriction ignores minor components like ground substance or fibroblasts present in the layer.

Final answer: D