Which organism is alpha-hemolytic, optochin sensitive, lancet-shaped diplococci, encapsulated, and causes rust-colored sputum in community-acquired pneumonia?

• A. Staphylococcus aureus
• B. Streptococcus pyogenes
• C. Streptococcus viridans
• D. Streptococcus pneumoniae

Answer: (D)

This question tests how to recognize Streptococcus pneumoniae from laboratory traits and the classic clinical presentation of its pneumonia. S. pneumoniae is alpha-hemolytic, so it shows partial hemolysis on blood agar with a greenish hue. It is optochin sensitive, meaning its growth is inhibited by optochin—this helps distinguish it from optochin-resistant viridans streptococci. Morphologically, it appears as lancet-shaped diplococci on Gram stain. The capsule is a key virulence factor and is a common target of vaccines, explaining its encapsulated nature. Clinically, it characteristically causes community-acquired pneumonia with rust-colored sputum due to blood-tinged alveolar exudate.

Other organisms either do not show alpha-hemolysis, or are optochin resistant, or lack the distinctive encapsulated lancet-shaped diplococci appearance, and they don’t classically produce rust-colored sputum in this setting.