WBR0055

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Musculoskeletal/Rheumatology, SubCategory::General Principles, SubCategory::Infectious Disease
Prompt	[[Prompt::A soldier suffers an open leg wound after an explosion. He is treated upon returning to base, but he soon develops widespread infection of the wound site with blisters and large bubbles arising in nearby muscle and subcutaneous tissue. What is the mechanism of the toxin in the most likely causal organism?]]
Answer A	AnswerA::Bring MHCII and TCR together to stimulate leukocytes.
Answer A Explanation	[[AnswerAExp::This is the mechanism of the Staphylococcus aureus' Toxic Shock Syndrome toxin and Streptococcus pyogenes' Endotoxin A. Because of their unique ability to stimulate leukocytes without co-stimulatory factors, these toxins are called "superantigens".]]
Answer B	AnswerB::Phospholipase
Answer B Explanation	AnswerBExp::Clostridium perfringens causes gas gangrene and produces alpha toxin, a phospholipase that degrades tissues.
Answer C	AnswerC::Cleaves SNARE protein
Answer C Explanation	AnswerCExp::Both Botulinum and Tetanus toxin cleave SNARE protein at synapses, thereby disabling neurotransmitter release at the synaptic cleft.
Answer D	AnswerD::Inactivate 60S ribsosome by cleaving rRNA
Answer D Explanation	AnswerDExp::The toxins produced by Shigella and EHEC inactivate the 60S ribosome by cleaving rRNA.
Answer E	AnswerE::Increases cAMP by intrinsic enzymatic activity.
Answer E Explanation	[[AnswerEExp::Edema factor toxin produced by Bacillus anthracis increases cAMP by increasing enzymatic activity.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::The patient is most likely suffering from gas gangrene (clostridial myonecrosis), caused by Clostridium perfringens. C. perfringens is found in soil and the environment as spores, which can enter open wounds. The organism produces alpha toxin, a phospholipase which degrades cell membranes, thereby producing tissue destruction. Infections can progress rapidly, leading to myonecrosis and the formation of gas as tissue is destroyed. Potentially lethal sepsis can ensue within hours without proper treatment. The treatment for gas gangrene is wound debridement and often amputation of the affected area with adjuvant penicillin. Educational Objective: Clostridium perfringens produces gas gangrene and alpha toxin, a phospholipase that degrades tissues. References: First Aid 2014 page 127]]
Approved	Approved::Yes
Keyword	WBRKeyword::Microbiology, WBRKeyword::Bacteria, WBRKeyword::Toxin, WBRKeyword::Toxins, WBRKeyword::Mechanism, WBRKeyword::Gangrene
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