A patient with pneumonia and hyponatremia is classically associated with infection by which organism?

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

Answer: (A)

Hyponatremia with pneumonia is classically linked to Legionella pneumophila. This organism causes Legionnaires’ disease, an atypical pneumonia often accompanied by high fever, GI symptoms (like diarrhea), confusion, and electrolyte disturbances, with hyponatremia arising from SIADH. That combination—pneumonia plus low sodium—is a classic clue pointing to Legionella.

In contrast, Mycoplasma pneumoniae can cause an atypical pneumonia, especially in younger patients, but hyponatremia is not a characteristic feature. Streptococcus pneumoniae is the typical cause of lobar, more fulminant pneumonia and is not usually associated with hyponatremia. Chlamydophila pneumoniae also causes atypical pneumonia, but hyponatremia is not a defining association.