A 60-year-old man presents to the emergency department with a 5-day history of fever, productive cough with purulent sputum, and right-sided chest pain worsened by breathing. He denies recent travel or sick contacts. Past medical history is significant for type 2 diabetes which is treated with metformin. Vital signs are temperature 39.0°C (102.2°F), pulse 110 beats/min, respirations 24/min and blood pressure 104/62 mm Hg. On physical examination, there are crackles in the right lower lobe. Chest X-ray reveals a right lower lobe consolidation with a cavity and air-fluid level. Sputum culture grows gram-positive cocci in chains which are alpha-hemolytic on blood agar and optochin-sensitive.
Which of the following is the most likely causative organism?
The correct answer is:
D) Streptococcus pneumoniae
This patient has symptoms, signs, and radiologic findings consistent with pneumonia. The sputum culture growing gram-positive cocci in chains which are alpha-hemolytic and optochin-sensitive indicate Streptococcus pneumoniae pneumonia. This organism is a common cause of community-acquired pneumonia, occasionally forming cavitary lesions in high-risk patients like those with diabetes.
Answer choice A: Klebsiella pneumoniae, is incorrect. Klebsiella pneumoniae is a gram-negative bacillus which causes necrotizing lobar pneumonia, often with “currant jelly” sputum. It is not a gram-positive coccus or optochin sensitive.
Answer choice B: Mycobacterium tuberculosis, is incorrect. Mycobacterium tuberculosis causes chronic cavitary lung disease, often with weight loss and night sweats, and is identified by acid-fast bacilli, not gram-positive cocci or optochin sensitivity.
Answer choice C: Staphylococcus aureus, is incorrect. Staphylococcus aureus causes necrotizing pneumonia and is often post-viral. Sputum culture grows gram-positive cocci in clusters (not chains) with beta-hemolysis. It is not optochin-sensitive.
Answer choice E: Streptococcus pyogenes, is incorrect. Streptococcus pyogenes (group A Streptococcus) is beta-hemolytic, not alpha-hemolytic, and typically causes pharyngitis or skin infections, only rarely causing cavitary pneumonia.
Key Learning Point
Streptococcus pneumoniae is a leading cause of community-acquired pneumonia, identified by gram-positive cocci in chains, alpha-hemolysis, and optochin sensitivity. Cavitation is rare but occurs in high-risk patients.
