Compared to naïve T cells, memory T cells:

Correct Answer: D

Rationale: Memory T cells, compared to naïve T cells, take less time to become activated as they have previously encountered the antigen. They also have less need for costimulation during activation due to their enhanced sensitivity. Furthermore, memory T cells have a much longer life span, allowing for a more rapid and robust immune response upon re-exposure to the antigen. Therefore, all of the statements in choices A, B, and C are correct, making option D the correct answer.

Correct Answer: A

Rationale: Step 1: TLR4 is the receptor for lipopolysaccharide (LPS), a component of gram-negative bacteria. Step 2: The question states that normal mice injected with high doses of LPS die, indicating TLR4 activation. Step 3: Mutations in TLR4 can confer resistance to LPS-induced septic shock. Step 4: Therefore, the correct answer is A: TLR4, as it is directly involved in recognizing LPS and triggering septic shock. Summary: B: TLR5 does not specifically bind LPS. C: NOD1 and D: NOD2 are involved in intracellular recognition of bacterial components, not LPS.

Correct Answer: B

Rationale: The correct answer is B: NK cells. NK cells are part of the innate immune system and can directly kill virus-infected cells, providing early protection against viral infections. They do not require prior exposure to the virus to initiate their killing mechanism, making them crucial for early defense. Eosinophils (A), Neutrophils (C), and Mast cells (D) are not primarily involved in killing virus-infected cells. Eosinophils are more associated with parasitic infections, Neutrophils are primarily involved in bacterial infections, and Mast cells are involved in allergic reactions.

Correct Answer: A

Rationale: Step 1: Hypervariable regions are responsible for antigen binding specificity. Step 2: Complementary Determining Regions (CDRs) are the specific locations within the hypervariable regions. Step 3: As CDRs are part of the hypervariable regions, choice A is correct. Summary: Choices B, C, and D are incorrect because hypervariable regions are not in the constant region, are present in secreted antibodies, and are found in both heavy and light chains.

Correct Answer: D

Rationale: The correct answer is D because cardiac output refers to the amount of blood pumped by the heart into the systemic circulation per minute. It is a crucial measure of heart function. Option A, coronary insufficient circulation, specifically refers to inadequate blood flow to the heart muscle itself, not overall cardiac output. Option B, a defect in heart valves, would affect the efficiency of blood flow but not necessarily the total amount of blood pumped. Option C, a disorder in heart rhythm, relates to irregular heartbeat patterns rather than the volume of blood pumped. Therefore, D is the correct choice as it directly relates to the definition of cardiac output.