Which of the following is a characteristic of axenic animals?

• A. Their blood cholesterol level is higher than in normal animals.
• B. They have larger thymuses than normal animals.
• C. They have difficulty absorbing the fats they ingest.
• D. Their immune system is more developed than in normal animals.

Answer: (A)

Axenic animals lack normal gut microbiota, so their bodies develop and metabolize differently than conventional animals. Microbes normally help process lipids and regulate bile acids, which influences how cholesterol is handled and cleared from the body. Without these microbial activities, cholesterol metabolism is disrupted, often resulting in higher levels of cholesterol in the blood compared with animals that have a normal microbiota. This makes elevated blood cholesterol a distinguishing, documented difference in axenic animals. In contrast, axenic animals typically have underdeveloped immune tissues due to lack of microbial exposure, so they do not have a more developed immune system; thymus size is often reduced rather than enlarged. Fat absorption can be affected, but it’s not the defining characteristic tied to axenic status; the key point is the impact on lipid metabolism and immune development, with higher blood cholesterol being the best-supported feature.