Final answer:
The primary cause of an external effect on the skin is UV radiation, which leads to premature aging, immune system suppression, and an increased risk of skin cancer. Limiting sun exposure and using sunscreen can help reduce these harmful effects.
Step-by-step explanation:
A primary cause of an external effect on the skin is C) UV radiation. Exposure to UV radiation can lead to a variety of harmful effects on the skin, such as premature aging, suppression of the immune system, and the development of skin cancer. All UV radiation has the potential to damage collagen fibers, which accelerates the aging process and results in the formation of wrinkles. Furthermore, excessive and repeated exposure to UV radiation can result in sunburn and increase the risk of skin cancer, including melanoma. While sunburn typically affects the epidermis, the damage from UV exposure extends deeper and can lead to a breakdown of deeper layers of the skin.
It is important to be aware that these adverse effects of UV radiation can be reduced by limiting exposure to the sun and by taking protective measures such as using sunscreen and wearing protective clothing when spending time outdoors. Additionally, while the question focuses on external factors affecting the skin, it is worth noting that some skin concerns like hormonal imbalance, genetics, and dehydration do play a role in the skin's health; however, they are not considered primary external effects.