WBR0473

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Cardiology
Prompt	[[Prompt::A 67-year-old woman presents to the emergency department for unremitting high-grade fever and left leg pain. The patient explains that she had a left total knee replacement 5 months ago. The patient is admitted and blood cultures are withdrawn. The next day, blood cultures grow a non-motile organism. Which of the following pathogens is most likely responsible for the patient's condition?]]
Answer A	AnswerA::''Streptococcus bovis''
Answer A Explanation	AnswerAExp::''S. bovis'' infection is often associated with colon cancer.
Answer B	AnswerB::''S. pyogenes''
Answer B Explanation	AnswerBExp::''S. pyogenes'' commonly cause impetigo and pharyngitis. They are not classically associated with prosthetic infections following total knee replacement.
Answer C	AnswerC::''Neisseria gonorrhea''
Answer C Explanation	AnswerCExp::''N. gonorrhea'' is a sexually transmitted infection that can cause infectious arthritis.
Answer D	AnswerD::''Pseudomonas aeruginosa''
Answer D Explanation	AnswerDExp::''P. aeruginosa'' may be an infectious cause of a complicated total knee replacement. However, ''P. aeroginosa'' is a motile organism.
Answer E	AnswerE::''Staphylococcus epidermidis''
Answer E Explanation	AnswerEExp::''S. epidermidis'' is a gram-positive, catalase-positive, coagulase-negative non-motile organism that is commonly associated with infections following prosthetic implant due to its ability to form antibiotic-resistant biofilms.
Right Answer	RightAnswer::E
Explanation	[[Explanation::Infection of an implanted foreign prosthetic material is a relatively common complication that is often caused by Staphylococcus epidermidis infection. S. epidermidis is a gram-positive coagulase-negative non-motile organism that causes biofilms that grow on surgical implants or catheters. S. epidermidis is a catalase-positive, coagulase-negative, facultative anaerobe. It has a weak positive reaction to nitrate reduction test. Also, S. epidermidis is sensitive to novobiocin. To distinguish S. epidermidis from other Staphylococcal organisms, S. aureus is coagulase-positive and S. saprophyticus is novobiocin-resistant. An important virulence factor of S. epidermidis is its surface proteins that can result in the formation of antibiotic-resistant biofilms on plastic devices. Administration of antibiotics and surgical removal of the implanted device are generally both required for appropriate treatment of prosthetic material infection. Educational Objective: S. epidermidis is a gram-positive, catalase-positive, coagulase-negative non-motile organism that is commonly associated with infections following prosthetic implant due to its ability to form antibiotic-resistant biofilms. References: First Aid 2014 page 130]]
Approved	Approved::Yes
Keyword	WBRKeyword::Staphylococcus, WBRKeyword::Epidermidis, WBRKeyword::Staphylococcus epidermidis, WBRKeyword::Total knee replacement, WBRKeyword::Biofilms, WBRKeyword::Prosthetic device, WBRKeyword::Prosthetic implant, WBRKeyword::Gram, WBRKeyword::Catalase, WBRKeyword::Coagulase, WBRKeyword::Motile, WBRKeyword::Organism, WBRKeyword::Bacteria, WBRKeyword::Bacterial infection, WBRKeyword::Fever, WBRKeyword::Leg pain
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