Which of the following best describes the role of gastrointestinal (GI) bacteria in the human body?

• A. They function only in oxygen production.
• B. They aid in digestion and synthesis of certain vitamins.
• C. They are always pathogenic and cause disease.
• D. They prevent absorption of nutrients.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the role of gastrointestinal (GI) bacteria by considering their interaction with the human body, especially in the digestive system.
2. Step 2: Recognize that GI bacteria are mostly beneficial and contribute to breaking down complex food substances that the human digestive enzymes cannot process alone.
3. Step 3: Know that these bacteria synthesize certain vitamins, such as vitamin K and some B vitamins, which are essential for human health.
4. Step 4: Eliminate incorrect options by recalling that GI bacteria do not produce oxygen, are not always pathogenic, and do not prevent nutrient absorption; rather, they support nutrient absorption.
5. Step 5: Conclude that the best description of GI bacteria's role is that they aid in digestion and synthesis of certain vitamins, highlighting their beneficial symbiotic relationship with the host.