Which of the following statements correctly describes the effect of the normal resident biota on host defenses?

• A. Normal resident biota compete with pathogens for nutrients and space, helping to prevent infection.
• B. Normal resident biota always weaken the host's immune system, increasing susceptibility to disease.
• C. Normal resident biota have no impact on the host's defenses against pathogens.
• D. Normal resident biota directly cause disease in healthy individuals by producing toxins.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the role of normal resident biota (also called normal flora) in the human body. These microorganisms live on and inside the host without causing disease under normal conditions.
2. Step 2: Recognize that normal resident biota contribute to host defenses primarily by competing with potential pathogens for nutrients and space, which limits the ability of harmful microbes to establish infections.
3. Step 3: Consider that normal biota can also produce substances that inhibit or kill pathogens, and they help stimulate the host's immune system, enhancing its readiness to fight infections.
4. Step 4: Evaluate the incorrect options: normal biota do not always weaken the immune system; they do have an impact on host defenses; and they do not typically cause disease in healthy individuals by producing toxins.
5. Step 5: Conclude that the correct description is that normal resident biota compete with pathogens for nutrients and space, thereby helping to prevent infection.