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
Which of the following is the most common pathogens responsible for nosocomial pneumonias in the ICU? 
Select and explain. 
A. Gram positive bacteria, B. Gram negative bacteria, C. Mycoplasma, D. Virus infections
B: Gram negative bacteria
Answer: B: Gram negative bacteria 
Explanation: Most common organisms causing nosocomial pneumonia are gram negative bacteria in 50-70% cases, which reach the lower airways by aspiration of gastric contents or by microaspiration of upper airway secretions. Most common organism is pseudomonas aeruginosa followed by enterobacteriacea. Staphylococci aureus is second to gram negative bacteria and accounts for 15 -30%. Anaerobic bacteria is found in 10-30% cases. Ref: Infectious Diseases By Sherwood L. Gorbach, 3rd Edition, Page 477