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Author [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]]
Exam Type USMLE Step 1
Main Category Microbiology
Sub Category Pulmonology
Prompt A 69-year-old man is brought to the emergency department by his wife for worsening fever, vomiting, and cough for the past 3 days. The patient's wife explains that they recently returned from a cruise trip on a nearby lake. His past medical history is significant for well-controlled essential hypertension managed by lisinopril. He has smoked one pack of cigarettes daily since the past 40 years. He denies drinking alcohol or any use of illicit drugs. On admission, his blood pressure is 134/88 mmHg, heart rate is 86/min, temperature is 103 °F (39.5 °C), and respiratory rate is 25/min. Initial work-up reveals the following set of electrolytes: Na=129 mEq/L, K=4.5 mEq/L, Cl=108 mEq/L, and HCO3=21 mEq/L. Which of the following media may be used to grow the infectious agent responsible for this patient's condition?
Answer A Eaton’s agar
Answer A Explanation [[AnswerAExp::Eaton’s agar is used to culture Mycoplasma pneumoniae, a common cause of atypical pneumonia.]]
Answer B Charcoal yeast extract
Answer B Explanation [[AnswerBExp::Charcoal yeast extract is used to culture Legionella pneumophila, the organism responsible for Legionnaires' disease.]]
Answer C Tellurite Agar
Answer C Explanation [[AnswerCExp::Tellurite agar is used to culture Corynebacterium diphtheriae. C. diptheriae is largely eliminated from the developed world and therefore would be unlikely to be present in this patient. C. diptheriae infection causes severe pharyngitis accompanied by the development of greyish pseudomembranes that coat the throat and tonsils. The pulmonary symptoms and fever observed in this patient are inconsistent with C. diptheriae infection.]]
Answer D MacConkey’s agar
Answer D Explanation [[AnswerDExp::MacConkey's agar (MAC) is used to culture a variety of gram-negative organisms. Lactose-fermenting enterics (eg E. coli) will appear as pink colonies on MacConkey's agar.]]
Answer E Sabouraud’s agar
Answer E Explanation [[AnswerEExp::Sabouraud’s agar is used to culture fungi.]]
Right Answer B
Explanation [[Explanation::Legionnaires' disease is a severe pulmonary infection caused by Legionella pneumophila, a gram-negative rod. Legionnaires' disease typically affects smokers, elderly, or patients who are immunocompromised. Healthy individuals often have a less severe flu-like Legionella infection called Pontiac fever.

Legionella pneumophila should be suspected in patients who were recently exposed to water sources, such as rivers, lakes, or even air conditioners. It classically causes a fever-heart rate dissociation, whereby the patient is febrile but not as tachycardic as it would be expected in a febrile patient. Additionally, hyponatremia is characteristically observed in patients with Legionella infection. The mechanism of hyponatremia in Legionella infections was once thought to be due to SIADH, but more recent data suggests ADH does not play a major role and other mechanisms are yet to be elucidated. Finally, a mild but clinically insignificant decrease in bicarbonate may be evident in this patient as a compensatory mechanism of the patient's hyperventilation.

Legionella grows on charcoal yeast extract using iron and cysteine. Because gram stains poorly in cultures of Legionella, silver stain is used instead. A more rapid urinary antigen for Legionella is also available. Treatment is by antibiotics with atypical coverage, such as macrolides.
Educational Objective: Legionella pneumophila causes an acute respiratory disease called Legionnaires' disease, classically associated with fever-heart rate dissociation, and hyponatremia. It can be cultured using charcoal yeast agar.
References: Schuetz P, Haubitz S, Christ-Crain M, et al. Hyponatremia and anti-diuretic hormone in Legionnaires' disease. BMC Infect Dis. 2013;13:585 First Aid 2014 page 138]]

Approved Yes
Keyword Legionnaires', legionnaire, legionnaire's, microbiology, Bacteria, Pneumonia, Laboratory, gram, negative, rod, charcoal, yeast, extract
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