Which of the following is not a physiological barrier

Options

A. Acid in stomach
B. Tears from eyes
C. Saliva in the mouth
D. mucus lining

Answer

To determine which of the following options is not a physiological barrier, we need to analyze each option provided:

1. **Acid in stomach**: This is a physiological barrier. The hydrochloric acid (HCl) in the stomach creates an acidic environment that helps to kill bacteria and other pathogens that enter the digestive system.

2. **Tears from eyes**: This is also a physiological barrier. Tears help to wash away foreign particles and contain enzymes that can break down bacteria, providing a defense mechanism for the eyes.

3. **Saliva in the mouth**: This is another physiological barrier. Saliva contains enzymes like lysozyme that can kill bacteria and helps to maintain oral hygiene.

4. **Mucus lining**: This is not a physiological barrier; it is classified as a physical barrier. The mucus lining traps pathogens and particles, preventing them from entering the body, particularly in the respiratory tract.

Based on this analysis, the option that is not a physiological barrier is **mucus lining**.

### Final Answer:
**Mucus lining** is not a physiological barrier.