Structure of Capillary Walls

Steps:

• Capillaries enable nutrient exchange, requiring thin walls.
• Walls consist only of a single layer of endothelial cells.
• No smooth muscle or elastic fibers are needed for their function.
• Collagen fibers are minimal or absent in the basic wall structure.

Why A is correct:

• Capillary walls are defined as a simple endothelium layer without smooth muscle, elastic tissue, or collagen fibers, per basic vascular histology.

Why the others are wrong:

• B: Includes collagen fibers, which are not a key component of capillary walls.
• C: Omits endothelium, the essential lining of all blood vessels.
• D: Lacks endothelium and wrongly adds elastic fibers.

Final answer: A