WBR0049

Author	[[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Pulmonology, SubCategory::Infectious Disease
Prompt	[[Prompt::A 57-year-old man is brought to the emergency department by his wife for a 3-day history of increasing fever, nausea, vomiting, cough and malaise. The wife explains they recently returned from a cruise trip at a nearby lake. On admission, his temperature is 103°F (39.5°C), his heart rate is 86/min, and his respiratory rate is 25/min. Initial work-up reveals the following set of electrolytes in this patient: Na=129 mEq/L, K=4.5 mEq/L, Cl=108 mEq/L, and HCO3=21 mEq/L. Which of the following media could be used to isolate the causal organism?]]
Answer A	AnswerA::Eaton’s Agar
Answer A Explanation	[[AnswerAExp::Eaton’s agar is used to culture ''Mycoplasma pneumoniae''. It causes a mild gradually worsening atypical pneumonia, not the acute-onset severe pulmonary infection seen in this patient.]]
Answer B	AnswerB::Charcoal yeast extract
Answer B Explanation	[[AnswerBExp::Charcoal yeast extract is used to culture ''Legionella pneumophila'', the organism responsible for Legionnaires' disease.]]
Answer C	AnswerC::Tellurite Agar
Answer C Explanation	[[AnswerCExp::Tellurite agar is used to culture ''Corynebacterium diphtheriae''. C. diptheriae is largely eliminated from developed countries and would therefore be unlikely in this patient. C. diptheria infection causes severe pharyngitis accompanied by the development of pseudomembranes that coat the throat and tonsils. The pulmonary symptoms and fever observed in this patient are inconsistent with C. diptheria infection.]]
Answer D	AnswerD::Maconkey’s Agar
Answer D Explanation	[[AnswerDExp::Maconkey’s agar is used to culture a variety of organisms. Lactose-fermenting enterics (eg ''E. coli'') will appear as pink colonies.]]
Answer E	AnswerE::Sabouraud’s Agar
Answer E Explanation	[[AnswerEExp::Sabouraud’s agar is used to culture fungi.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::The patient is most likely suffering from Legionnaires' disease, a severe pulmonary infection caused by ''Legionella pneumophila'', a gram-negative rod. Legionnaires' disease typically affects smokers, older individuals, or people who are immunocompromised. Healthy individuals often have a less severe flu-like Legionalla infection called Pontiac fever. Legionnaires' disease requires treatment with antibiotics and aggressive pulmonary management to resolve. Legionella should be suspected in patients who were recently exposed to water sources, such as rivers, lakes, or even airconditioning systems. It classically causes a fever-heart rate dissociation, whereby the patient is febrile but not as tachycardic as expected in a febrile patient. Additionally, hyponatremia is characteristically observed in patients with Legionella infection. A mild but clinically insignificant decrease in bicarbonate may be evident in this patient as a compensatory mechanism of the patient's hyperventilation. It grows on charcoal yeast extract using iron and cysteine. Because gram stains poorly in cultures of Legionella, silver stain is used instead. More rapid urinary antigen for Legionella is also available. Treatment is by antibiotics, such as macrolides (erythromycin). Educational Objective: ''Legionella pneumophila'' causes acute respiratory disease called Legionnaires' disease, classically associated with fever-heart rate dissociation and hyponatremia. Itcan 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	Approved::Yes
Keyword	WBRKeyword::Legionnaires', WBRKeyword::legionnaire, WBRKeyword::legionnaire's, WBRKeyword::microbiology, WBRKeyword::Bacteria, WBRKeyword::Pneumonia, WBRKeyword::Laboratory, WBRKeyword::gram, WBRKeyword::negative, WBRKeyword::rod, WBRKeyword::charcoal, WBRKeyword::yeast, WBRKeyword::extract
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