This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
In a patient of orbital cellulitis, microorganism on culture shows greenish colonies and optochin sensitivity. The most likely organism is 
Select and explain. 
A. Streptococcus viridans, B. Staphylococcus, C. Pseudomonas, D. Pneumococcus
D: Pneumococcus
Answer: D: Pneumococcus 
Explanation: Pneumococci or Streptococcus pneumoniae on blood agar, after incubation for 18 hours, colonies are small, dome-shaped, glistening with an area of green discoloration ( alpha hemolysis due to paial hemolysis). Fuher incubation gives flat colonies with raised edges and central combination ( draughtsman or carrom coin appearance). Optochin sensitivity: differentiating features of pneumococci from viridans streptococci. When a disc impregnated with optochin is applied on a plate of blood agar inoculated with pneumococci, a wide zone of inhibition appears on incubation. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 225