Pathogenic microorganisms cannot penetrate which of the following barriers?

Intact skin serves as a primary barrier to pathogenic microorganisms. It is composed of multiple layers of cells that are tightly packed together, creating a physical shield that prevents pathogens from entering the body. This barrier is crucial in maintaining the body’s defenses against infections. When the skin is intact, it provides an effective protective layer that pathogens find difficult to breach.

In contrast, open wounds compromise the skin's integrity, allowing microorganisms direct access to underlying tissues. Mucous membranes, while also a protective barrier, can be more susceptible to pathogen entry due to their moisture and location in the respiratory, gastrointestinal, and urogenital tracts. The respiratory tract, with its mucous and cilia, plays a role in trapping and expelling pathogens but is still vulnerable, particularly when allergens or irritants are present.

Thus, intact skin is the correct choice because it remains an effective barrier against pathogenic microorganisms, while the other options represent areas where pathogens can enter the body.