What causes the appearance of stretch marks on the skin?

The appearance of stretch marks on the skin is primarily caused by the degeneration of connective tissue. When the skin undergoes rapid stretching, which can occur during pregnancy, puberty, significant weight gain or loss, the elastic fibers in the dermis can break down. This breakdown is a result of the skin being stretched beyond its capacity, leading to tears in the connective tissue. As the skin attempts to heal and regain its structure, it can form visible lines or streaks known as stretch marks.

In the context of the other options, while hormonal changes can contribute to skin conditions and may affect the elasticity of the skin, they do not directly cause the degeneration of connective tissue associated with the formation of stretch marks. Similarly, excess sun exposure can damage the skin and contribute to other skin issues such as aging or pigmentation changes, but it does not lead to stretch marks. Increased hydration is beneficial for skin health overall but is not linked to the formation of stretch marks. Thus, the degeneration of connective tissue is the best explanation for the appearance of stretch marks.