WBR0464

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	USMLE Step 1
Main Category	Microbiology
Sub Category	Infectious Disease
Prompt	A 32-year-old woman from southwestern USA presents to the emergency department (ED) with a 2-day history of fever and a painful swelling in her left axilla. She denies any pulmonary symptoms. In the ED, the patient is appropriately isolated and blood cultures are withdrawn. The next day, the diagnosis of plague is made. When the physician explains to the patient about her disease, he informs her that the plague is caused by an organism that cycles between flea, wild rodents, squirrels, prairie dogs, and cats. Which of the following agents is responsible for the patient's diagnosis?
Answer A	Francisella tularensis
Answer A Explanation	[[AnswerAExp::Francisella tularensis is a zoonotic bacterium that causes tularemia. It is usually transmitted by rabbits, deer fly, and ticks.]]
Answer B	Ehrlichia chaffeensis
Answer B Explanation	[[AnswerBExp::Ehrlichia chaffeensis is a zoonotic bacterium that causes ehrlichiosis. It is usually transmitted by lone star ticks.]]
Answer C	Yersinia pestis
Answer C Explanation	[[AnswerCExp::Yersinia pestis is a zoonotic bacterium that causes the plague.]]
Answer D	Chlamydophila psittaci
Answer D Explanation	[[AnswerDExp::Chlamydophila psittaci is a zoonotic bacterium that causes psittacosis. It is usually transmitted by birds.]]
Answer E	Borrelia recurrentis
Answer E Explanation	[[AnswerEExp::Borrelia recurrentis is a zoonotic bacterium that causes relapsing fever. It is usually transmitted by louse.]]
Right Answer	C
Explanation	[[Explanation::The plague is a high-mortality infectious disease caused by a bacterial infection with Yersinia pestis, a gram negative bacterium. In USA, the plaque is extremely rare. Less than 20 cases are reported each year, mostly in southwest USA. The organism's natural reservoir includes flea, wild rodents, squirrels, prairie dogs, and cats. Transmission of the disease requires a bite by any of the natural reservoirs. Although the plague is caused by only one organism, the clinical disease is a spectrum: Bubonic plague: Most common form that manifests are large tender lymphadenopathy (bubos), similar to the presentation in the vignette. Pneumonic plague: Most severe forms that infects the lungs. Septicemic plague: Rarest form that causes bacteremia and disseminated intravascular coagulation (DIC). Educational Objective: The plague is caused by the gram-negative bacterium Yersinia pestis. It is transmitted by flea bite, or bite of wild rodents, squirrels, or other animals like prairie dogs and cats. References: Perry RD, Fetherson JD. Yersinia pestis – etiologic agent of plague. Clin Micriobiol Rev. 1997;10(1):35-66. First Aid 2014 page 142]]
Approved	No
Keyword	Yersinia pestis, Plague, Bubo, Buboes, Fever
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