An infant had a high-grade fever and respiratory distress at the time of presentation to the emergency room. The sputum sample showed gram-positive cocci having an a hemolytic activity. The suspected pathogen is susceptible to which of the following agent:

Sputum showing gram-positive cocci with alpha-hemolytic activity in a patient with high-grade fever and respiratory distress is suggestive of pneumococcal pneumonia which shows optochin susceptibility. Both pneumococcus and Streptococcus viridans produce alpha-hemolysis on blood agar. Optochin sensitivity test is done to differentiate Pneumococcus from commensal like Streptococcus viridans. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition