Which of the following is not a constituent of the physiological barr…

The correct answer is Option (4) → Mucus coating of the epithelium lining the respiratory tract

Innate immunity is the non-specific defence present from birth. It works through four types of barriers: physical, physiological, cellular and cytokine barriers. Physical barriers block the entry of microbes, while physiological barriers are secretions that prevent microbial growth.

Option (1) Acid in the stomach — A physiological barrier. The acid in the stomach kills or stops the growth of microbes that enter with food.

Option (2) Saliva in the mouth — A physiological barrier. Saliva in the mouth is one of the secretions the book lists as a physiological barrier.

Option (3) Tears from eyes — A physiological barrier. Tears from the eyes also prevent microbial growth, so they belong to the physiological barriers.

Option (4) Mucus coating of the epithelium lining the respiratory tract — Not a physiological barrier. The mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts traps microbes. It is a physical barrier, grouped with the skin.

NCERT Extract:
Innate Immunity – “… Innate immunity consist of four types of barriers. These are —
(i) Physical barriers : Skin on our body is the main barrier which prevents entry of the micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.
(ii) Physiological barriers : Acid in the stomach, saliva in the mouth, tears from eyes–all prevent microbial growth.”