Which statement best describes routine respiratory flora as part of the body's first-line defenses?

• A. They are primarily pathogenic bacteria that invade respiratory epithelium to stimulate antibody production.
• B. They are transient microbes that normally sterilize the lower airways by releasing large amounts of hydrochloric acid.
• C. They are viruses that permanently replace the host's epithelial cells to prevent bacterial attachment.
• D. They are normal microbiota that can inhibit pathogen colonization through competition and production of antimicrobial substances.

Answer: (D)

Verified step by step guidance

1. Understand the concept of 'routine respiratory flora' as part of the body's first-line defenses, which refers to the normal microbiota residing in the respiratory tract.
2. Recognize that these normal microbiota are not harmful pathogens but instead play a protective role by occupying space and resources, thereby preventing harmful pathogens from colonizing the respiratory epithelium.
3. Recall that these microbes can produce antimicrobial substances, such as bacteriocins, which inhibit the growth of potential pathogens.
4. Eliminate incorrect options by noting that routine respiratory flora are not primarily pathogenic bacteria, do not sterilize the lower airways with hydrochloric acid (which is a function of the stomach), and are not viruses that replace host cells.
5. Conclude that the best description is that routine respiratory flora are normal microbiota that inhibit pathogen colonization through competition and production of antimicrobial substances.