Which of the following layers is involved in the first line of defense against infection?

The correct answer is the stratum corneum. This outermost layer of the epidermis serves as the first line of defense against infection. Its primary role is to provide a tough, protective barrier that prevents pathogens such as bacteria, viruses, and fungi from penetrating the skin. The stratum corneum is made up of dead keratinized cells that are tightly packed together, creating a formidable obstacle against harmful microorganisms.

While the other layers mentioned also play important roles in skin health and function, they do not primarily serve as the first line of defense. The stratum lucidum is primarily found in thick skin areas, providing a little extra protection, but it is not the main barrier. The stratum granulosum contributes to the barrier function by producing keratin and lipids that help to strengthen the skin, while the stratum spinosum offers structural support and begins the process of keratinization but is not itself the first line of defense.

Overall, the stratum corneum is essential for protecting the body from external threats, making it the key layer in the skin's defense against infection.