This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
If a quellungs test was done on a bacterial isolate, which one would you expect to be positive? 
Select and explain. 
A. S.pneumoniae, B. Enterobacter, C. Haemophilus parainfluenza, D. C.diphtheriae
A: S.pneumoniae
Answer: A: S.pneumoniae 
Explanation: 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. Also Know: Pneumococci are gram-positive lancet-shaped cocci arranged in pairs (diplococci) or sho chains . On blood agar they produce alpha-hemolysis. In contrast to viridans streptococci, they are lysed by bile or deoxycholate, and their growth is inhibited by optochin. Ref: Levinson W. (2012). Chapter 15. Gram-Positive Cocci. In W. Levinson (Ed),Review of Medical Microbiology & Immunology, 12e.