A 34-year-old man presents with fever, productive cough, and lobar consolidation on chest X-ray. Which organism should be considered as a common cause in older adults?

• A. Streptococcus pneumoniae
• B. Mycobacterium tuberculosis
• C. Staphylococcus aureus
• D. Klebsiella pneumoniae

Answer: (A)

Streptococcus pneumoniae is the most common cause of community-acquired bacterial pneumonia in adults, including older adults, and it classically presents with acute fever, productive cough, and lobar consolidation on chest imaging. This pattern reflects how pneumococcus tends to fill and inflame an entire lobe, producing the neat, segmental consolidation seen on X-ray. While other organisms can cause pneumonia, they often present with different clinical courses or radiographic features: Mycobacterium tuberculosis typically causes a longer, subacute illness with weight loss and night sweats and often apical disease with possible cavitation; Staphylococcus aureus pneumonia is more likely after influenza or in hospitalized patients and often shows multilobar involvement or abscesses; Klebsiella pneumoniae is linked to risk factors like heavy alcohol use and tends to produce upper-lobe involvement with thick, gelatinous sputum and sometimes cavities. Thus, the organism to consider most commonly in this context is Streptococcus pneumoniae.