Which of the following is not an external defense mechanism of natural immunity?

The skin is a primary component of the body's external defense mechanisms, playing a crucial role in the natural immune response. It acts as a physical barrier, preventing pathogens from entering the body. In the context of the question, the correct response highlights options that do not fit into the category of external defense mechanisms.

The Bursa of Fabricius is associated with the development and maturation of B cells in birds rather than serving as an external barrier, which is why it is identified as not being a component of external defense.

Mucus serves as an important protective layer that traps pathogens and particles, while various body fluids, such as saliva and tears, contain antimicrobial proteins and factors that neutralize threats. Together, mucus and body fluids actively contribute to the external defenses of the immune system.

The distinction lies in understanding the role of each option within the context of natural immunity, where the Bursa of Fabricius does not directly function as an external barrier against pathogens like the skin, mucus, or body fluids do.