This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System, Subject name: Pediatrics
A 4 year old child presents with fever, and increased work of breathing manifested by tachypnea and retractions. A chest X ray confirms lobar pneumonia. What is the most likely causative organism? 
Select and explain. 
A. Mycoplasma pneumoniae, B. Streptococcus pyogenes, C. Chlamydiae pneumoniae, D. Streptococcus pneumoniae
D: Streptococcus pneumoniae
Answer: D: Streptococcus pneumoniae 
Explanation: Ans. D. Streptococcus pneumoniaeCausative organisms may differ in different age groups.< 2 months: Gram negative bacteria such as Klebsiella, E. coliBetween 3 months- 3 years: S pneumonia, H influenza, StaphylococciAfter 3 years of age: S pneumonia and staphylococcus