Rust-colored sputum in lobar pneumonia is most characteristic of infection with which organism?

• A. Corynebacterium jeikeium
• B. Staphylococcus aureus
• C. Streptococcus pneumoniae
• D. Streptococcus pyogenes

Answer: (C)

Rust-colored sputum is a classic sign of pneumococcal pneumonia, caused by Streptococcus pneumoniae. This color comes from blood-tinged purulent material produced when the alveolar spaces are filled with inflammatory exudate rich in neutrophils and red blood cells during lobar consolidation. Streptococcus pneumoniae is the most common cause of community-acquired lobar pneumonia, making this presentation highly characteristic.

Other organisms can cause pneumonia, but the distinctive rust-colored sputum is much less typical for them. For example, Corynebacterium jeikeium is mainly a concern in immunocompromised or hospital settings, often with skin/soft-tissue involvement rather than classic lobar pneumonia. Staphylococcus aureus can cause pneumonia, especially after influenza or as a bronchopneumonia pattern, but the sputum color isn’t as classically rust-colored. Streptococcus pyogenes can cause pneumonia too, but it’s not the hallmark cause of rust-colored sputum in lobar pneumonia.