A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 52-year-old man with a history of chronic obstructive pulmonary disease presents with sudden onset fever, productive cough, and diarrhea. He recently returned from a business trip where he stayed at a hotel. His temperature is 39.2°C (102.6°F) and blood pressure is 100/60 mmHg. Laboratory tests reveal hyponatremia and elevated liver enzymes. A urine antigen test is positive. Which of the following organisms is most likely responsible for his condition?
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A
Haemophilus influenzaeIncorrect. Haemophilus influenzae causes pneumonia in COPD but lacks the hyponatremia, diarrhea, and urine antigen pattern of Legionella.
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B
Legionella pneumophilaCorrect. Legionella pneumophila causes high fever, GI symptoms, hyponatremia, and hepatic dysfunction; urine antigen confirms diagnosis after hotel exposure.
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C
Streptococcus pneumoniaeIncorrect. Streptococcus pneumoniae causes lobar pneumonia with rusty sputum but does not classically cause hyponatremia or positive urine antigen for Legionella.
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D
Mycoplasma pneumoniaeIncorrect. Mycoplasma causes walking pneumonia in young adults with dry cough; hyponatremia and positive urine antigen are atypical.
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E
Chlamydia pneumoniaeIncorrect. Chlamydia pneumoniae causes mild atypical pneumonia without diarrhea, hyponatremia, or positive urine antigen testing.
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Answer: B. The presentation of fever, productive cough, diarrhea, and hyponatremia in conjunction with a positive urine antigen test after a hotel stay is highly indicative of Legionella pneumophila. The organism is known for causing Legionnaires' disease, associated with contaminated water sources. Mycoplasma pneumoniae and Chlamydia pneumoniae typically cause milder, non-productive coughs and do not present with hyponatremia or positive urine antigen tests. Streptococcus pneumoniae and Haemophilus influenzae are less likely given the negative culture results and atypical presentation.