Rusty sputum pneumonia is most commonly caused by which organism?

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

Answer: (B)

Rusty sputum is a classic clue pointing to pneumococcal pneumonia. The organism responsible, Streptococcus pneumoniae, is the most common cause of community-acquired pneumonia overall. The rusty color comes from blood-tinged sputum produced during the inflammatory exudate and hemoptysis that can accompany this infection, especially with lobar consolidation. This pattern—sudden onset fever, pleuritic chest pain, and rust-colored sputum—fits pneumococcal pneumonia well.

Haemophilus influenzae is a common cause in people with COPD or older adults, but it’s less frequent overall and not as characteristically tied to rusty sputum. Staphylococcus aureus can cause pneumonia, particularly after influenza or in hospital settings, and may have purulent sputum with possible bleeding, but it isn’t the leading cause. Mycoplasma pneumoniae typically presents with a milder, atypical pneumonia featuring a dry or minimally productive cough rather than rusty sputum.