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 The patient is admitted and blood cultures are withdrawn.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::''Staphylococcus saprophyticus''
Answer B Explanation	[[AnswerBExp::S. saprophyticus is a common cause of urinary tract infections in sexually active women. It is not a common cause of infections of implanted prosthetic material. All staphylococcal organisms are non-motile. To distinguish, S. saprophyticus is novobiocin-resistant, while S. epidermidis is novobiocin-sensitive.]]
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. Of note, S. epidermidis is part of the normal skin and mucosal microflora and often contaminates blood or urine cultures. 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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