The defining microscopic feature of collagen is its unique triple helix structure. Collagen is a fibrous protein that provides strength and support to various connective tissues in the body, such as skin, bone, tendons, and cartilage.

- The triple helix is composed of three polypeptide chains wound tightly around each other, forming a robust and stable structure.
- This configuration gives collagen its characteristic tensile strength and resistance to stretching.
- Each chain primarily consists of repeating sequences of amino acids, commonly glycine-proline-hydroxyproline, which facilitate the tight helical winding.

In contrast:
- Beta-pleated sheets are a feature of some globular proteins and certain fibrous proteins like silk fibroin, but not collagen.
- Alpha-keratin filaments are found in hair and nails, composed of alpha-helical structures, but they differ structurally and functionally from collagen.
- Elastic fiber networks are rich in elastin, which provides elasticity rather than tensile strength; they co-exist with collagen fibers in connective tissues but differ in microscopic structure.

Therefore, the triple helix is the hallmark of collagen’s molecular structure, providing it with its distinctive physical and biological properties.

Reference: Gray's Anatomy, 41st Edition, Chapter 3, Histology of Connective Tissue