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
An infant had high grade fever and respiratory distress at the time of presentation to the emergency room. The sample collected for blood culture was subsequently  positive  showing  growth  of  α- haemolytic colonies. On Gram staining these were gram positive cocci. In the screening test for identification, the suspected pathogen is likely to be susceptible to the following agent : 
Select and explain. 
A. Bacitracin, B. Novobiocin, C. Optochin, D. Cloxacillin
C: Optochin
Answer: C: Optochin 
Explanation: “Sensitivity to bacitracin is employed as a convenient method for differentiating Str. pyogenes from other hemolytic streptococci.”