What causes a callus to form on the skin?

A callus forms on the skin primarily due to excessive friction. When an area of skin is subjected to repeated pressure or rubbing, the body responds by thickening the outer layer of the skin (the stratum corneum) in an attempt to protect itself from injury. This process creates a hard, protective layer known as a callus, often found on areas of the feet, hands, or other parts of the body that experience frequent abrasion.

Excessive moisture can lead to other skin conditions, such as maceration, rather than the thickening that characterizes a callus. Infection typically results in symptoms like inflammation, redness, and pain, rather than the hardening of skin. Allergies may cause rashes or other skin reactions, but they do not contribute to the formation of calluses. Thus, the conditions leading directly to the development of a callus are firmly rooted in friction, making it the correct answer.