WBR295
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| Author | [[PageAuthor::Gerald Chi (Reviewed by Yazan Daaboul, M.D.)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Microbiology |
| Sub Category | Gastrointestinal |
| Prompt | A 29-year-old man presents to the clinic with symptoms of abdominal pain, malaise, and watery diarrhea that begin a day after coming back from a travel to Bangladesh. He complains of orthostatic dizziness and has dry mucosa and decreased skin turgor. Few leukocytes are found in his fecal specimen. Growth of purple colonies on Eosin methylene blue agar is noted. Presence of the heat-labile enterotoxin is confirmed by enzyme-linked immunosorbent assay (ELISA). Which of the following events best describes the mechanism of action of this toxin? |
| Answer A | Increase of intracellular cyclic GMP |
| Answer A Explanation | Increase of intracellular cyclic GMP is mediated by heat-stable enterotoxin (ST). |
| Answer B | Phosphorylation of CFTR by protein kinase A |
| Answer B Explanation | Phosphorylation of CFTR by protein kinase A is mediated by LT. |
| Answer C | Cytotoxicity to intestinal epithelium |
| Answer C Explanation | Cytotoxicity to intestinal epithelium is mediated by EAEC toxins such as PET. |
| Answer D | Adherence to intestinal epithelium |
| Answer D Explanation | Adherence to intestinal epithelium may be mediated by colonizing factor antigen (CFA) of ETEC, Tir (translocated intimin receptor) of EPEC, or surface adhesins. |
| Answer E | ADP ribosylation of Gi protein |
| Answer E Explanation | ADP-ribosylation of Gi protein is mediated by pertussis toxin. |
| Right Answer | B |
| Explanation | Enterotoxigenic Escherichia coli (ETEC) is a type of Escherichia coli and the leading bacterial cause of diarrhea in the developing world, as well as the most common cause of travelers' diarrhea. Enterotoxins produced by ETEC include heat-labile enterotoxin (LT) and heat-stable enterotoxin (ST). LT activates adenylyl cyclase and increases the amount of cAMP, and results in profound chloride efflux secondary to phosphorylation of CFTR by persistently activated protein kinase A. ST activates guanylyl cyclase in the signal pathway of the enteric epithelial cells and triggers fluid secretion. Educational Objective: Enterotoxins produced by ETEC include heat-labile enterotoxin (LT) and heat-stable enterotoxin (ST). LT activates adenylyl cyclase and increases the amount of cAMP, and results in profound chloride efflux secondary to phosphorylation of CFTR by persistently activated protein kinase A. ST activates guanylyl cyclase in the signal pathway of the enteric epithelial cells and triggers fluid secretion. |
| Approved | Yes |
| Keyword | Escherichia coli, Gastroenteritis, Enterotoxins, Heat-labile enterotoxin, Heat-stable enterotoxin |
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