Correct Answer: C

Rationale: Correct Answer: C - Immunological tolerance to the pathogen Rationale: 1. Immunological tolerance is when the immune system does not mount a response against specific antigens to prevent autoimmunity. 2. In the context of infection, tolerance to the pathogen means the immune system does not recognize the pathogen as a threat and does not initiate a protective response. 3. This lack of response leaves the host vulnerable to infection as there is no specific immune defense mechanism activated against the pathogen. Summary of incorrect choices: A: Opsonic action of antibody - enhances phagocytosis of pathogens B: Viral neutralization by specific antibody - prevents viral entry into host cells D: Neutralization of protein exotoxin - prevents the harmful effects of toxins produced by pathogens.

Correct Answer: D

Rationale: The correct answer is D because antibody diversity is generated by rearrangements of DNA encoding variable regions of heavy and light chains (choice A) and by the combination of different heavy and light chains that form the antigen binding site (choice B). Step 1: DNA rearrangements create different variable regions in heavy and light chains. Step 2: Combination of different heavy and light chains leads to unique antigen binding sites. Step 3: This diversity allows antibodies to bind to different epitopes on the same antigen. Step 4: Choice C is incorrect as antibodies do not physically change shape to bind different epitopes.

Correct Answer: C

Rationale: The correct answer is C: Carry genes for antibiotic resistance. Bacterial plasmids are small, circular DNA molecules that can be transferred between bacteria, carrying genes such as antibiotic resistance. This allows bacteria to survive in the presence of antibiotics. Plasmids do not directly play a role in protein synthesis (choice A) or DNA replication (choice B). Plasmids are not responsible for motility (choice D) as that function is usually carried out by flagella or other cellular structures.

Correct Answer: C

Rationale: The bacterial capsule is composed of polysaccharides. This outer layer helps bacteria evade the host immune system, protect against desiccation, and aid in attachment to host cells. Polysaccharides are long chains of sugar molecules, providing a protective barrier for the bacteria. Lipopolysaccharides (choice A) are found in the outer membrane of Gram-negative bacteria and are not a major component of the capsule. Teichoic acid (choice B) is a component of the cell wall in Gram-positive bacteria, not the capsule. Peptidoglycan (choice D) is another component of the cell wall but is not part of the capsule.

Correct Answer: C

Rationale: The correct answer is C: All answers are correct. Pneumonia can be caused by various pathogens, including Legionella pneumophila and Mycoplasma pneumoniae. Choice C is correct because both A and B are known causative agents of pneumonia. Choice D is incorrect because we know that Legionella pneumophila and Mycoplasma pneumoniae can cause pneumonia, so the statement "None of the above" is not accurate in this context.