In the determination of serum insulin levels by radioimmunoassay, which one of the following is NOT needed?

Correct Answer: D

Rationale: The correct answer is D because in the determination of serum insulin levels by radioimmunoassay, we need isotope-labeled insulin (choice A) as a tracer for detection, anti-insulin antibody made in goats (choice B) to bind to insulin, and anti-goat gamma globulin made in rabbits (choice C) to detect the antibody bound to insulin. Isotope-labeled anti-insulin antibody made in goats (choice D) is not needed as it is not typically used in this assay. This choice would not contribute to the accurate measurement of serum insulin levels.

Correct Answer: C

Rationale: The correct answer is C because the alternative complement pathway can be initiated independently of C3b fragments. It is activated by foreign surfaces directly, such as certain bacteria or viruses, without the need for prior activation of C3b. This is a key feature that distinguishes the alternative pathway from the classical pathway. Choices A, B, and D are incorrect because A is true - the alternative pathway can be triggered by infectious agents in the absence of antibodies, B is true - it does not require C1, C2, or C4, and D is false - the alternative pathway has a different terminal sequence of events compared to the classic pathway.

Correct Answer: A

Rationale: The correct answer is A: Streptococcal antigens induce antibodies cross-reactive with heart tissue. This is known as molecular mimicry, where antibodies produced against the streptococcal antigens also react with antigens present in heart tissue, leading to carditis. This mechanism is the basis for the development of rheumatic heart disease following a streptococcal infection. Choice B is incorrect because it does not explain the specific relationship between streptococcal infection and carditis. Choice C is incorrect as it describes a mechanism related to IgE and histamine release, which is not the primary mechanism involved in carditis. Choice D is incorrect as it focuses on neutrophils and proteases, which are not the primary mediators of carditis in the context of a streptococcal infection.

Correct Answer: D

Rationale: The correct answer is D because class I MHC proteins are primarily involved in presenting antigens to cytotoxic T cells to trigger an immune response against infected or abnormal cells. A is correct as class I MHC proteins are found on most cells. B is correct as they are indeed involved in T cell recognition. C is correct as they are expressed from both alleles. Choice D is incorrect because class I MHC proteins are not directly involved in the skin test response to Mycobacterium tuberculosis, which is primarily mediated by the immune response involving macrophages and T cells in the skin.