In adults with community-acquired pneumonia, which organism is the second most common cause and is associated with being a gram-negative rod and often causes green sputum?

• A. Streptococcus pneumoniae
• B. Haemophilus influenzae
• C. Moraxella catarrhalis
• D. Staphylococcus aureus

Answer: (B)

In adults with community-acquired pneumonia, the second most common organism is Haemophilus influenzae, a small gram-negative rod that often colonizes the upper airway and can invade the lower respiratory tract, especially in those with underlying lung disease like COPD. Infections with this organism can produce purulent sputum that is often described as green due to the inflammatory response and bacterial components. Haemophilus influenzae is fastidious, requiring X and V factors for growth, and it commonly appears in COPD or older adults with CAP. This fits the description of a gram-negative rod and the typical clinical clue of green sputum. By comparison, Streptococcus pneumoniae is the most common CAP cause and is a gram-positive diplococcus, Moraxella catarrhalis is a gram-negative diplococcus, and Staphylococcus aureus is a gram-positive cocci, so they do not match the specific gram-negative rod pattern.