Which component is NOT part of the tear film?

The tear film is a complex structure primarily composed of three essential components: lipids, aqueous, and mucin. Each of these plays a critical role in maintaining the health and comfort of the ocular surface.

Lipids form the outermost layer of the tear film, providing a barrier to prevent the evaporation of the underlying aqueous layer. The aqueous layer is the thickest part and contains water, electrolytes, and various proteins, providing hydration and delivering nutrients to the corneal surface. Mucin is found in the innermost layer and helps to stabilize the tear film by allowing it to adhere to the corneal epithelium, enhancing wetting and spreading of the tear film over the eye surface.

Although proteins are present in the tear film and do contribute to its functions, they are not classified as one of the three primary components like lipids, aqueous, and mucin. Therefore, identifying proteins as a separate, non-essential component in the context of the basic structure of the tear film justifies why this choice is the correct answer to the question.