WBR0381

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Genitourinary
Prompt	[[Prompt::A 45 year old Caucasian female patient presents to the emergency department with fever and flank pain that started on the same day. Upon further questioning, the patient admits to having recent onset dysuria and urinary frequency for which she did not seek medical care. Her temperature is 38.9 degrees C (102 degrees F), her heart rate is 96 beats per minute, and her blood pressure measures 116/88 mmHg. Physical examination reveals positive Murphy’s punch sign (costovertebral angle tenderness). The diagnosis of acute pyelonephritis is made. Urine cultures are taken and appropriate antibiotic therapy is initiated. Eosin Methylene Blue (EMB) agar shows the growth of a specific bacterial strain as seen in the image below. Which of the following virulence factors is implicated in this patient’s condition? ]]
Answer A	AnswerA::Capsule
Answer A Explanation	AnswerAExp::Capsule is also a virulence factor of E. coli that causes pneumonia and meningitis in neonates. It has no significant role in acute pyelonephritis.
Answer B	AnswerB::Fimbriae
Answer B Explanation	AnswerBExp::E. coli is a a lactose-fermenting gram-negative, catalase-positive rod that grows on EMB agar and appears as green metallic sheen.
Answer C	AnswerC::Lipopolysaccaride (LPS) endotoxin
Answer C Explanation	AnswerCExp::LPS endotoxin is a virulence factor of E. coli that causes septic shock.
Answer D	AnswerD::Streptolysin O
Answer D Explanation	AnswerDExp::Streptolysin O is a the hemolytic toxin of group A streptococci; it is not usually found in E. coli.
Answer E	AnswerE::Erythrogenic toxin
Answer E Explanation	AnswerEExp::Erythrogenic toxin is produced by streptococcus pyogenes and is implicated in scarlet fever. The toxin itself is encoded in a lysogenic phage by specialized transduction.
Right Answer	RightAnswer::B
Explanation	[[Explanation::E. coli is the most common causative agent of acute pyelonephritis. It is a lactose-fermenting gram-negative, catalase-positive rod that grows on the selective EMB agar. As depicted in the image above, metallic green sheen is characteristic of E. coli strains on EMB. E. coli contain several virulence factors. Fimbriae are adhesions that are implicated in cystitis and acute pyelonephritis. Fimbriae are present in nearly all strains of E. coli that cause pyelonephritis. Educational Objective: E. coli is a lactose-fermenting gram-negative, catalase-positive rod that grows on EMB agar and appears as green metallic sheen. The virulence factor responsible for pyelonephritis in E. coli is its fimbriae. Reference: Stapleton A. Novel mechanism of P-fimbriated Escherichia coli virulence in pyelonephritis. J Am Soc Nephrol. 2005;16(12):3458-3460. Educational Objective: References: ]]
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