Which active substance is paired correctly with its mechanism of action?

The active substance polyethylene glycol is indeed paired correctly with its mechanism of action, which is to protect and lubricate mucous membrane surfaces. Polyethylene glycol is a hydrophilic polymer that acts as a lubricant, providing moisture and a protective barrier to mucous membranes, particularly in formulations designed for ophthalmic use. This property makes it effective for alleviating dryness and irritation in the eyes, enhancing patient comfort.

In contrast, the other options present mismatches between substance and mechanism. For example, lifitegrast is primarily known as a lymphocyte function-associated antigen-1 (LFA-1) antagonist used in treating dry eye disease, and does not specifically prevent bacterial contamination of artificial tears. Carboxymethylcellulose, while a lubricant, does not function as a calcineurin inhibitor. Benzalkonium chloride is an antimicrobial agent commonly used as a preservative in eye drops, but it primarily functions by disrupting cell membranes rather than precipitating protein to clear mucus. Thus, polyethylene glycol's designation as a protector and lubricant accurately reflects its intended use in mucous membrane applications.