Identify the option that represents the correct order of events in naïve T cell activation:

Correct Answer: B

Rationale: The correct order of events in naïve T cell activation is: antigen recognition, clonal expansion, entry into the site of infection. First, T cells must recognize the specific antigen presented by antigen-presenting cells. This recognition triggers clonal expansion, where T cells proliferate and differentiate into effector cells. Finally, the activated T cells migrate to the site of infection to combat the pathogen. Option B is correct because it follows this sequential order. Options A, C, and D are incorrect because they do not adhere to the correct sequence of events in naïve T cell activation.

Correct Answer: C

Rationale: The correct answer is C: Natural killer cells. Natural killer cells are involved in cell-mediated immunity and have cytotoxic actions on tumor cells. They are part of the innate immune system and can directly kill infected or cancerous cells. Lysosomes (A) are organelles within cells that break down waste materials. Memory cells (B) are a type of immune cell that "remembers" previous infections to mount a faster response upon re-exposure. Histocompatibility cells (D) are involved in recognizing self from non-self in transplant situations, not in cytotoxic actions on tumor cells.

Correct Answer: D

Rationale: The correct answer is D, administration of immunoglobulin, for passive immunity. Immunoglobulin contains pre-formed antibodies that provide immediate protection against specific pathogens. Administering immunoglobulin transfers these antibodies to the recipient, offering temporary immunity. Choice A, having an acute disease, does not involve receiving external antibodies. Choice B, administration of a toxoid, induces active immunity against toxins, not passive immunity. Choice C, administration of a vaccine, stimulates active immunity by triggering the body to produce its antibodies, not providing immediate protection like passive immunity with immunoglobulin.

Correct Answer: C

Rationale: The correct answer is C: proteins. Gas vesicles are gas-filled structures found in certain bacteria and archaea to provide buoyancy. The membrane of gas vesicles is predominantly composed of proteins that form a semi-permeable barrier to trap gas. Proteins are essential for the structure and function of gas vesicles. Triglycerides (A) are not typically found in membrane structures, phospholipids (B) are more common in cell membranes, and lipopolysaccharides (D) are typically found in the outer membrane of Gram-negative bacteria, not in gas vesicle membranes.

Correct Answer: A

Rationale: Plaques are clear areas caused by virus infection in a lawn of cultured cells. This is because viruses infect cells, causing cell death and lysis, which results in clear areas. Choice B is incorrect because stained areas are not typically associated with plaques. Choices C and D are also incorrect as they refer to virus and bacterial colonies, respectively, not plaques.