An older adult has fallen and sprained his ankle in a local park. Which action should the responder perform first?

Correct Answer: A

Rationale: The correct action is to elevate the foot first. Elevating the foot helps reduce swelling and pain by promoting better circulation. This step is crucial in the initial management of a sprained ankle. Applying ice and administering aspirin can come after elevating the foot. Assisting with ambulation should only be considered after the initial measures are taken to address the sprained ankle.

Correct Answer: D

Rationale: The correct answer is D because CD3 is indeed necessary for TCR signaling in cytotoxic T cells. CD3 is a crucial component of the T cell receptor (TCR) complex, which is responsible for initiating intracellular signaling upon recognition of antigen-MHC complexes. Without CD3, TCR signaling cannot occur, leading to impaired T cell activation and cytotoxic function. Choices A, B, and C are incorrect because they accurately describe the functions and interactions of T cells expressing CD4 and CD8 with antigen presenting cells, as well as the role of CD4 T cells in helping B cells through antigen presentation.

Correct Answer: a

Rationale: The correct answer is A: Mediated by IgE. Leishmania infection is not primarily mediated by IgE antibodies. The immune response against Leishmania involves T cells, macrophages, and cytokines. IgE is not a major player in the defense against Leishmania. Choices B, C, and D are incorrect because Leishmania is indeed a unicellular parasite, activation of macrophages is essential for killing the parasite, and Leishmania can hide inside neutrophils to escape the host immune attack.

Correct Answer: A

Rationale: Rationale: A. T regulatory cells can produce TGF-β, which is a key immunosuppressive cytokine regulating immune responses. B. T regulatory cells do not express AIRE, a transcription factor mainly involved in central tolerance. C. T regulatory cells can control both TH1 and TH2 responses, maintaining immune homeostasis. D. T regulatory cells do not act by killing other T cell subsets, but rather through suppressive cytokines and cell-cell interactions.

Correct Answer: A

Rationale: The correct answer is A. Integration is a stage of HIV life cycle that involves the insertion of viral DNA into the host cell's genome, facilitated by the enzyme integrase. To explain why A is incorrect, integrase is a viral enzyme, not a host enzyme. B is correct as some anti-HIV drugs target the integration stage to prevent viral replication. C is correct as integrated proviral DNA allows for duplication during cell division. D is correct as integration requires the formation of double-stranded viral DNA.