This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteria, Subject name: Microbiology
[{'content': 'If a quellung test was done on the following bacterial isolates, which one would you expect to be positive? \nSelect and explain. \nA. S. pneumoniae, B. Enterobacter, C. Haemophilus parainfluenzae, D. C. diphtheriae', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The quellung test determines the presence of bacterial capsules. Specific antibody is mixed with the bacterial suspension or with clinical material. The polysaccharide capsule-antibody complex is visible microscopically. The test is also termed capsular swelling. The capsules of S. pneumoniae as well as N. meningitidis, H. influenzae, and K. pneumoniae play a role in the pathogenicity of the organisms. These surface structures inhibit phagocytosis, perhaps by preventing attachment of the leukocyte pseudopod. C. diphtheriae, Enterobacter, and H. parainfluenzae are nonencapsulated.', 'role': 'assistant'}]