Which of the following can cause skin conditions?

The presence of allergens or irritants is a well-documented cause of various skin conditions. When the skin comes into contact with substances that provoke an immune response or physical irritation, it can lead to conditions like eczema, contact dermatitis, or allergic reactions. Allergens are substances that cause an allergic reaction, while irritants can damage the skin barrier or provoke inflammatory responses. The skin's response can manifest in various ways, including redness, itching, swelling, and rashes.

On the other hand, stress relief, excessive hydration, and normal aging do not directly cause skin conditions. Stress relief typically helps improve skin health by reducing stress-related hormonal fluctuations that can exacerbate certain skin issues. Excessive hydration is generally beneficial for the skin, promoting moisture retention and overall skin health. Normal aging, while it may lead to changes in skin texture and elasticity, is not classified as a cause for skin conditions in the same sense as allergens or irritants.