Which layer serves as the outermost protective barrier of the skin?

The correct choice is the stratum corneum, which is indeed the outermost layer of the skin. This layer consists of dead, flattened keratinocytes that have lost their nuclei and are filled with keratin, a fibrous protein that provides both structural strength and waterproofing. The stratum corneum serves as a critical protective barrier against environmental factors such as pathogens, chemicals, and physical abrasions, while also helping to retain moisture within the underlying layers of the skin.

In addition, the stratum corneum undergoes a continual process of desquamation, where older cells are shed and replaced by newer cells from deeper layers. This process is essential for maintaining healthy skin and ensuring protection is always provided to the underlying tissues. The stratum corneum's composition and structure are tailored for its role as a barrier, outlining its primary function in skin health and protection.