Which of the following statements is true?

Correct Answer: C

Rationale: The correct answer is C because a given MHC molecule can bind only one peptide at a time. This is essential for the proper presentation of antigens to immune cells. MHC class I and class II molecules do not consist of two transmembrane protein chains (A), MHC class I molecules have one peptide-binding site while MHC class II molecules have one peptide-binding site (B), and MHC molecules on the cell surface typically have peptides bound to their peptide-binding sites, even in the absence of infection (D).

Correct Answer: B

Rationale: Step-by-step rationale: 1. Mucins are heavily glycosylated proteins that form a gel-like barrier on mucosal surfaces. 2. This barrier helps protect the underlying tissues from pathogens and physical damage. 3. Mucins are primarily produced by goblet cells in epithelial tissues. 4. Choice B correctly describes the function and structure of mucins. Summary: - Choice A is incorrect because not all epithelial cells produce mucins. - Choice C is incorrect as mucins are not natural antibiotics. - Choice D is incorrect as mucins are present in various mucosal surfaces, not exclusively in the respiratory tract.

Correct Answer: D

Rationale: The correct answer is D because both TLR4 and NOD1 are pattern recognition receptors (PRRs) that can activate the NF-κB pathway. TLR4 recognizes bacterial lipopolysaccharides, while NOD1 detects bacterial peptidoglycan fragments. NLRP3, choice C, is an inflammasome receptor that activates caspase-1, not NF-κB. Therefore, choices A, B, and C are incorrect as they do not directly activate NF-κB like TLR4 and NOD1.

Correct Answer: D

Rationale: The correct answer is D because all three choices are valid triggers for complement activation. A is correct as opsonins like MBL can bind to microbes and activate the complement system. B is correct as direct recognition of microbes can also trigger complement activation. C is correct as the formation of immune complexes can lead to complement activation through the classical pathway. Therefore, all of the above choices are triggers for complement activation.

Correct Answer: D

Rationale: The correct answer is D because vertebrate eyes have single-lens eyes where light is focused by the curved cornea and lens (A), and changing the shape of the lens allows for focusing at different distances (B). Additionally, photoreceptor cells in the retina detect light (C). Therefore, all of the above statements are true, making D the correct choice. Choice A is incorrect because it does not encompass the entire process of vision in vertebrate eyes, missing the role of changing the lens shape. Choice B is incorrect as it only addresses the lens shape without mentioning the role of the cornea in focusing light. Choice C is incorrect as it only focuses on the function of photoreceptor cells in the retina without considering the lens and cornea's role in focusing light.