What was atypical about the 2009 H1N1 pandemic?

Correct Answer: B

Rationale: The correct answer is B because the 2009 H1N1 pandemic was caused by a newly assorted virus unrelated to existing human strains. This is atypical as typical pandemics are caused by animal strains. Option A is incorrect because the 2009 H1N1 pandemic did not emerge entirely as an assortment of human strains. Option C is incorrect as the 2009 H1N1 virus was not a variant of the 1918 H1N1 strain circulating in pigs. Option D is incorrect because the 2009 H1N1 pandemic did affect individuals across all age groups, including the very young.

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: C

Rationale: The correct answer is C because in response to Leishmania parasites, the activation of Th cells into Th1 effectors that secrete IFNγ is crucial for activating macrophages to eliminate the intracellular parasites. IFNγ enhances the microbicidal activity of macrophages, helping to control the infection. Choice A is incorrect because IL-4 is not required to induce the appropriate antibody response against Leishmania parasites. Choice B is incorrect because Th2 effectors secreting IL-4 would not activate macrophages effectively against Leishmania, which is an intracellular pathogen. Choice D is incorrect because in the case of Leishmania, a Th1-mediated response is most effective due to the need for macrophage activation by IFNγ.

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.