Arterial wall layer identification

Steps:

• Identify layer W as the middle layer in a blood vessel cross-section based on its position and staining.
• Note the presence of circularly arranged nuclei indicating smooth muscle cells.
• Observe wavy, refractile bands characteristic of elastic fibers.
• Confirm this matches the tunica media structure in mammalian arteries.

Why C is correct:

• The tunica media of arteries is defined as consisting mainly of smooth muscle cells for vasoconstriction and elastic fibers for recoil, per histological standards.

Why the others are wrong:

• A: Collagen fibers dominate the outer tunica externa, not the middle layer.
• B: Elastic and collagen fibers together form the tunica adventitia, lacking muscle.
• D: Squamous endothelium lines the inner tunica intima, not a thick middle layer.

Final answer: C