Which of the following is considered an external factor that can lead to cancer?

Chemicals and radiation are considered external factors that can lead to cancer because they are environmental carcinogens that can cause damage to the DNA in cells, leading to mutations and the uncontrolled growth characteristic of cancer. For instance, exposure to certain chemicals, such as those found in tobacco smoke, can result in specific types of cancer. Similarly, ionizing radiation, such as that from ultraviolet rays from the sun or radon gas, can also increase the risk of developing cancer by causing direct DNA damage.

In contrast, genetics refers to inherited traits and mutations that may predispose individuals to cancer, making them an internal factor rather than an external one. Hormones, while they can influence cancer development (such as hormone-receptor-positive breast cancer), are also considered more of a biological factor than an external one. Finally, immune disorders are related to the body's internal functioning and can affect its ability to detect and fight off cancerous cells but do not fit in the category of external factors that lead to cancer.