Final answer:
Basal cell carcinoma is a common type of skin cancer that affects the epidermis. It is primarily caused by excessive exposure to UV rays.
Step-by-step explanation:
Basal cell carcinoma is a form of skin cancer that affects the mitotically active stem cells in the stratum basale of the epidermis. It is the most common type of skin cancer and is often found in people with chronically sun-damaged skin. Excessive exposure to UV rays is the main cause of this cancer, but other factors like radiation and arsenic exposure can also contribute.