Callus formation is caused by which of the following?

Callus formation occurs primarily due to the hypertrophy (thickening) of the stratum corneum, which is the outermost layer of the epidermis. This thickening is a protective response of the skin to repetitive friction, pressure, or irritation. When the skin experiences consistent trauma, it compensates by growing additional layers of skin cells, leading to the formation of a callus. This process helps to protect the underlying tissues and prevent injury, making it an adaptive mechanism of the skin.

The other options do not accurately describe the cause of callus formation. Thinning of skin layers does not typically lead to callus development, as calluses are a response to thickness instead. Infection may cause changes in the skin, but it does not lead to the distinct thickening characteristic of a callus. Similarly, while sun exposure can affect skin health and lead to changes such as sunburn or tanning, it does not result in the formation of calluses through the mechanism of epidermal hypertrophy.