Which of the following is not a mechanical barrier to infection?

• A. Secretions
• B. Skin
• C. Mucous membranes
• D. Ciliary action

Answer: (A)

The key idea is distinguishing physical blocks from chemical defenses. Mechanical barriers are physical structures or processes that block or remove pathogens, such as the skin forming a solid wall, mucous membranes lining passages to trap microbes, and ciliary action in the airways that sweeps mucus and trapped organisms out of the body. Secretions, while they can help flush pathogens away, are mainly a chemical defense because they contain antimicrobial substances (like enzymes and low pH in some secretions) that inhibit or kill microbes rather than serving as a primary physical barrier. So secretions are not considered a mechanical barrier to infection.