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Legionella Pneumonia — RACP Adult Medicine MCQ

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ModerateInfectious DiseasesLegionella PneumoniaRACP Adult Medicine

A 55-year-old man staying at a hotel presents with high fever, confusion, diarrhoea, and hyponatraemia (Na⁺ 125 mmol/L). CXR shows a rapidly progressive consolidation. Urinary Legionella antigen (serogroup 1) is positive. What is the most likely pathogen?

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Correct answer: E — Legionella pneumophila

Severe CAP with confusion, diarrhoea, hyponatraemia, rapidly progressive consolidation, and positive Legionella urinary antigen (detects serogroup 1 only, ~70% of cases) is Legionella pneumonia. Linked to contaminated water systems (hotels, hospitals, cooling towers). Treatment: azithromycin or a fluoroquinolone. Legionella urinary antigen has ~70% sensitivity; NAAT on sputum/BAL is more sensitive.

Reference: eTG Antibiotic – 2025 – Legionella; CDNA 2024