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 |
| Prompt | [[Prompt::A 69-year-old man is brought to the emergency department by his wife for a 3-day history of increasing fever, vomiting, cough, and malaise. The patient's wife explains that they recently returned from a cruise trip on a nearby lake. His past medical history is significant for a well-controlled essential hypertension managed by daily lisinopril. He smokes one pack of cigarettes daily for the past 40 years. He denies drinking alcohol or use of illicit drugs. On admission, his blood pressure is 134/88 mmH, 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 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, a common cause of atypical pneumonia.]] |
| 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 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 | AnswerD::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 | AnswerE::Sabouraud’s agar |
| Answer E Explanation | [[AnswerEExp::Sabouraud’s agar is used to culture fungi.]] |
| Right Answer | RightAnswer::B |
| Explanation | [[Explanation::Legionnaires' disease is a severe pulmonary infection caused by Legionella pneumophila, a gram-negative rod. Legionnaires' disease typically affects smokers, older individuals, or pepople who are immunocompromised. Healthy individuals often have a less severe flu-like Legionella infection called Pontiac fever. Legionnaires' disease requires treatment with antibiotics that have coverage for atypical organisms.
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. 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 (erythromycin). |
| 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 |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |