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
A child had pustular lesion on leg. On gram staining gram positive cocci are seen. To establish the diagnosis of Group A streptococcal erythroderma, the test used is? 
Select and explain. 
A. Bile solubility test, B. Novobiocin sensitivity, C. Optochin sensitivity, D. Bacitracin sensitivity
D: Bacitracin sensitivity
Answer: D: Bacitracin sensitivity 
Explanation: Streptococcal erythroderma is caused by streptococcus pyogenes. Sensitivity to bacitracin is employed as a convenient method for differentiating str.pyogenes from other hemolytic streptococci. Pustule: A vesicle filled with leukocyte. Most common cause is virus. Most common bacterial cause is str.pyogenes. Str.pyogenes is the etiological agent in most of streptococcal skin infection. Vancomycin is the drug of choice. Ref: Ananthanarayan 7th Ed Page 205