input: The following are multiple choice questions (with answers) about medical knowledge.   **Question:** A 67-year-old man with peripheral neuropathy comes to the physician for a follow-up examination after the results of serum protein electrophoresis showed monoclonal gammopathy. A complete blood count, serum creatinine, and serum electrolyte concentrations are within the reference ranges. A bone marrow biopsy shows 6% monoclonal plasma cells. Further analysis shows that class I major histocompatibility molecules are downregulated in these monoclonal plasma cells. The proliferation of these monoclonal plasma cells is normally prevented by a class of immune cells that lyse abnormal cells without the need for opsonization, priming, or prior activation. Which of the following best describes this class of immune cells?   (A) Bone marrow-derived macrophages   (B) CD4+ T lymphocytes   (C) Natural killer cells   (D) Band neutrophils
output: ['(C) Natural killer cells <> answer', 'The following are multiple choice questions (with answers) about medical knowledge. <> instruction', '(A) Bone marrow-derived macrophages  (B) CD4+ T lymphocytes  (C) Natural killer cells  (D) Band neutrophils <> possible answers', '  A 67-year-old man with peripheral neuropathy comes to the physician for a follow-up examination after the results of serum protein electrophoresis showed monoclonal gammopathy. A complete blood count, serum creatinine, and serum electrolyte concentrations are within the reference ranges. A bone marrow biopsy shows 6% monoclonal plasma cells. Further analysis shows that class I major histocompatibility molecules are downregulated in these monoclonal plasma cells. The proliferation of these monoclonal plasma cells is normally prevented by a class of immune cells that lyse abnormal cells without the need for opsonization, priming, or prior activation. Which of the following best describes this class of immune cells? <> context']