This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
[{'content': 'Reactive cold agglutinins suggests which one of the following diseases? \nSelect and explain. \nA. Chronic infectious mononucleosis, B. Primary syphilis, C. Scarlet fever, D. Primary atypical pneumonia', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Mycoplasma pneumoniae causes primary atypical pneumonia. Although physicians rely on the presence of cold agglutinins (CA) for diagnosis, CA may be negative in up to 50% of cases. With symptoms of pneumonia, a positive CA is reliable. A complement-fixation (CF) test for M. pneumoniae is indicated when CA is negative. Isolation of M. pneumoniae is time-consuming (1 to 3 weeks) and not practical in most circumstances.', 'role': 'assistant'}]