The structures you listed - hair, fingernails, tendons, and connective tissue - are primarily made from proteins.

Here's why:

* Hair and fingernails: These are made of a protein called keratin. Keratin is a fibrous protein that provides strength and structure.

* Tendons: These are made of collagen, another fibrous protein that gives tendons their strength and flexibility.

* Connective tissue: While connective tissue is diverse, many types rely heavily on collagen for structure and support.

While other organic molecules play supporting roles in these structures, proteins are the key building blocks that provide the necessary strength and form.