Breathe Easy in 2026: Ace the Respiratory CAS Test with Confidence!

Legionella infection is classically linked to hyponatremia and gastrointestinal symptoms such as diarrhea, nausea, or abdominal pain. This combination is a well-recognized clue in patients with pneumonia, because Legionella pneumophila often drives a systemic response that includes hyponatremia (sometimes due to a SIADH-like effect and other mechanisms) and prominent GI symptoms. In practice, when you see pneumonia presenting with low sodium and GI upset, Legionella rises to the top of the differential.

The other organisms listed don’t typically present with this same pattern. For example, E. coli mainly causes gastroenteritis without the hyponatremia seen with Legionella; Staphylococcus aureus food poisoning causes rapid vomiting but not the hyponatremia-to-GI-symptom cluster in pneumonia; Mycoplasma pneumoniae can cause atypical pneumonia but hyponatremia and GI symptoms are less characteristic.