A patient with pneumonia symptoms and bilateral interstitial infiltrates is most likely infected with which pathogen?

• A. Streptococcus pneumoniae
• B. Mycoplasma pneumoniae
• C. Chlamydophila pneumoniae
• D. Legionella pneumophila

Answer: (B)

When pneumonia shows a diffuse, bilateral interstitial pattern on imaging, consider atypical pathogens that cause inflammation in the interstitium rather than a single lobar consolidation. Mycoplasma pneumoniae is the classic cause of this presentation, often producing a milder, walking-pneumonia picture with gradual onset, dry cough, and minimal systemic symptoms. Radiographs typically reveal diffuse or patchy interstitial infiltrates rather than a focal air-space consolidation.

Mycoplasma also fits clinically because it commonly affects younger patients and lacks a cell wall, which is why beta-lactam antibiotics are ineffective and agents like macrolides, tetracyclines, or fluoroquinolones are preferred. In contrast, Streptococcus pneumoniae usually causes abrupt onset with high fever and a single-lobe (lobar) consolidation. Legionella can present with high fever and GI symptoms and often has patchy or multilobar involvement but is less classically described as diffuse bilateral interstitial infiltrates. Chlamydophila pneumoniae can cause atypical pneumonia with interstitial changes as well, but the textbook association with diffuse bilateral interstitial patterns in a milder clinical course points most strongly to Mycoplasma.