<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 50-year-old man presents to a clinic with a skin lesion on his forearm, which recently changed in color from light brown to brown-black. The lesion was initially pinhead in size but now has grown to the size of a penny. On examination, there is a 3.5 x 2.5 cm irregularly-shaped lesion on his left forearm. A photograph of the lesion is presented. A biopsy of the lesion shows nests of pigmented melanocytes, with few areas showing invasion into the superficial dermis. The physician talks to the patient about a new drug, a monoclonal antibody against the cytotoxic T lymphocyte-associated antigen 4 CTLA-4 molecule, that has recently been launched in the market for his condition. The CTLA-4 molecule usually interacts with which of the following on the antigen-presenting cells?? A: MHC class II, B: MHC class I, C: T cell receptor, D: CD 28, E: B7,  <response>: This is a clinical vignette describing a patient with a pigmented skin lesion on the forearm that has recently changed in color and grown larger. The biopsy results indicate it is a melanoma. The question asks about the mechanism of action of ipilimumab, a monoclonal antibody that blocks CTLA-4. CTLA-4 is an inhibitory receptor expressed on T cells that binds to B7 on antigen presenting cells. This interaction downregulates T cell activation. By blocking CTLA-4, ipilimumab enhances T cell activation against melanoma cells.So the correct answer is E, as B7 is the ligand on antigen presenting cells that binds to CTLA-4.E: B7 