A woman had a high fever, hypotension, and a diffuse macular rash... a diagnosis of toxic shock syndrome was made... which one of the following is LEAST accurate?

Correct Answer: D

Rationale: The correct answer is D because toxic shock syndrome (TSS) is caused by superantigens, not A-B toxins. Superantigens directly activate a large number of T cells by binding to both the MHC class II protein and the T cell receptor simultaneously. This leads to massive cytokine release and the symptoms described. Choices A, B, and C are all accurate statements about the mechanism of action of superantigens in TSS. Choice D is incorrect because TSS toxins do not have an A-B subunit structure; they are superantigens that directly interact with T cells and do not enter cells to activate them.

Correct Answer: C

Rationale: The correct answer is C because complement lyses cells by forming a membrane attack complex (MAC) which involves the insertion of complement proteins into the cell membrane, leading to cell lysis. Choices A, B, and D are incorrect as complement does not lyse cells through enzymatic digestion, activation of adenylate cyclase, or inhibition of elongation factor 2. Complement-mediated lysis specifically involves the formation of MAC to disrupt the target cell membrane.

Correct Answer: D

Rationale: The correct answer is D because in autoimmune hemolytic anemia, activation of the complement system leads to the formation of membrane attack complexes (MAC) that directly lyse the red blood cells. This process is known as complement-mediated hemolysis. Perforins from cytotoxic T cells (Choice A) are involved in cell-mediated cytotoxicity but are not typically responsible for hemolysis of red blood cells. Neutrophils releasing proteases (Choice B) or interleukin-2 binding to its receptor on red cells (Choice C) are not mechanisms commonly associated with hemolysis in autoimmune hemolytic anemia.

Correct Answer: A

Rationale: The correct answer is A: They have the same amino acid sequence. This statement is incorrect because the variable regions of heavy chains and light chains in an antibody molecule have different amino acid sequences. The variable regions are responsible for binding to specific antigens, making choice B correct. The heavy and light chains are encoded by different genes on the same chromosome, making choice C incorrect. Both heavy and light chains contain hypervariable regions that contribute to antigen binding specificity, making choice D correct. Overall, the variable regions of heavy and light chains have distinct amino acid sequences, contributing to the diversity and specificity of antibodies.