A 63-year-old man presents to the emergency department with a three-day history of worsening fever, chills, and productive cough. He reports yellow-green sputum and mild pleuritic chest pain. His past medical history includes chronic obstructive pulmonary disease (COPD) and hypertension, for which he takes tiotropium and lisinopril. He has a 45-pack-year smoking history and continues to smoke a half-pack of cigarettes daily. He denies alcohol or drug use. Vital signs are temperature 101.9°F (38.8°C), blood pressure 118.64 mm Hg, heart rate 108 beats/min, and respirations 22/min. On physical examination, lung auscultation reveals decreased breath sounds and rales over the right lower lung field. Chest X-ray demonstrates consolidation in the right lower lobe.
Which of the following is the most likely causative organism?
The correct answer is:
E) Streptococcus pneumoniae
Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, particularly in older patients and those with COPD or smoking history. The presence of fever, cough with sputum production, and lobar consolidation is characteristic of pneumococcal pneumonia.
Answer choice A: Klebsiella pneumoniae, is incorrect. Klebsiella pneumonia is more common in patients with alcohol use disorder as well as those with impaired host defenses such as diabetes mellitus, chronic obstructive pulmonary disease, and malignancy. It classically presents with currant-jelly sputum (thick, mucoid, blood-tinged sputum).
Answer choice B: Legionella pneumophila, is incorrect. Legionella pneumonia is associated with exposure to contaminated water sources, with symptoms typically arising 2-10 days after exposure. Fever and fatigue often precede the onset of cough. Features that should increase the index of suspicion for Legionella infection include gastrointestinal symptoms, hyponatremia, and elevated transaminases.
Answer choice C: Mycoplasma pneumoniae, is incorrect. Mycoplasma infection can range from asymptomatic to upper respiratory tract infection to pneumonia, though pneumonia is a less common manifestation.
Answer choice D: Pseudomonas aeruginosa, is incorrect. Pseudomonas is a common cause of gram-negative hospital-acquired pneumonia. Community-acquired P. aeruginosa pneumonia is relatively rare. Risk factors include advanced age, smoking, alcohol use, and exposure to contaminated liquids. It also occurs in individuals who are immunocompromised, have recent prior antibiotic use, or have structural lung abnormalities such as bronchiectasis or cystic fibrosis.
Key Learning Point
Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in adults and typically presents with fever, productive cough, and lobar consolidation.
