WBR0054

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Head and Neck, SubCategory::Neurology, SubCategory::General Principles, SubCategory::Infectious Disease
Prompt	[[Prompt::A 5-year-old boy from rural Africa has recently immigrated to the United States with his family. His father brings him to the emergency room, where the patient is found to have a fever of 100.7 F (38.2 C), fatigue, and severe pharyngitis. Physical exam reveals marked cervical lymphadenopathy, and pseudomembranes in the throat. A throat swab is performed which reveals gram positive bacilli. What is the mechanism of action of the toxin produced by the causative organism?]]
Answer A	AnswerA::Inactivate 60S ribsosome by cleaving rRNA
Answer A Explanation	AnswerAExp::The toxins produced by Shigella and EHEC inactivate the 60S ribosome by cleaving rRNA.
Answer B	AnswerB::Activates adenylate cyclase by stimulating Gs
Answer B Explanation	AnswerBExp::The toxin produced by Vibrio cholera activated adenylate cyclase by stimulating Gs. This activation of the cAMP pathway in enterocytes leads to a secretory diarrhea. A hallmark of Cholera is the development of "rice-water" stools.
Answer C	AnswerC::Activates adenylate cyclase by disabling Gi
Answer C Explanation	AnswerCExp::The toxin produced by Bordetella pertussis activates adenylate cyclase by disabling Gi. Pertussis infection represents a highly contagious bacterial disease that manifests as "whooping cough".
Answer D	AnswerD::Cleaves SNARE protein
Answer D Explanation	AnswerDExp::Both Botulinum and Tetanus toxin cleave SNARE protein at synapses, thereby disabling neurotransmitter release at the synaptic cleft.
Answer E	AnswerE::Inactivate EF-2
Answer E Explanation	AnswerEExp::Diptheria toxin causes disease by inactivating Elongation Factor 2 (EF-2). Diptheria infection can be diagnosed by the presence of pseudomembranes in the throat.
Right Answer	RightAnswer::E
Explanation	[[Explanation::This patient is suffering from an infection caused by Corynebacterium diptheriae, a gram positive rod that has mostly been eradicated in the developed world through vaccination. Diptheria causes upper respiratory tract infections with sore throat and low-grade fever. The hallmark of this infection is an adherent true membrane on the tonsils referred to as pseudomembranes. Educational Objective: Diptheria toxin causes disease by inactivating Elongation Factor 2 (EF-2). Diptheria infection can be diagnosed by the presence of pseudomembranes in the throat. References: First Aid 2014 page 132 (diptheria) First Aid 2014 page 126 (toxins) First Aid 2012 page 152]]
Approved	Approved::Yes
Keyword	WBRKeyword::Microbiology, WBRKeyword::Diptheria, WBRKeyword::Toxin, WBRKeyword::Bacteria, WBRKeyword::Mechanism, WBRKeyword::Toxins
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