Practice questions answers 2 Infectious gastroenteritis
Practice questions answers 2
Infectious gastroenteritis
© Jun Wang, MD, PhD
1. F. This case is characterized by profuse watery diarrhea and signs of dehydration. The diarrhea is watery containing whitish material, forming a rice water appearance. These features are consistent with cholera. Diarrhea caused by campylobacter, Shigella and entamoeba histolytica is usually bloody. The watery diarrhea associated with enterotoxigenic e coli, giardia intestinalis or norovirus usually does not have abundant mucus in the stool, nor severe dehydration, as seen in cholera.
2. E. Low blood pressure can be caused by many factors, as those seen in shock. However, the low blood pressure in patients with cholera is associated with loss of circulating volume, resulted from fluid loss through profuse diarrhea.
3. E. While the presentations of high grade fever, frequent, small-volume bloody stools with travel history are highly suggestive of Shigella infection, bloody diarrhea can be caused by other pathogens, such as campylobacter and entamoeba histolytica. Multi-pathogen molecular tests can be used for a quick evaluation of possible pathogens, including virus and bacteria. Colonoscopic examination and biopsy are not commonly used for diagnosing Infectious gastroenteritis, although they can be used for clostridium difficile induced pseudomembranous colitis. Darkfield microscopic examination of stool can be used to identify highly motile comma-shaped bacteria of vibrio cholerae. Molecular tests for enterotoxin LT can be used for quick diagnosing enterotoxigenic e coli infection, a common cause of watery but not bloody diarrhea. Shiga toxin can be produced by enterohemorrhagic e coli and Shigella dysenteriae. However, other strains of Shigella do not produce Shiga toxin. Therefore, a positive Shiga toxin test could not confirm, and a negative Shiga toxin test could not exclude Shigella infection.
4. E. Multiplex molecular panels finding of bacterial pathogens are consistent with bacterial gastroenteritis. Stool culture is commonly used for confirmation of pathogens, especially when molecular tests are positive for bacterial infections. Also see discussion of question 3.
5. D. Stool culture at 37°C finding of reveals Gram-negative, non lactoase-fermenting bacilli that do not produce H2S are consistent with Shigella. Intracellular actin jet trail formation is induced by Shigella and enteroinvasive E. coli to facilitate lateral spreading of bacteria. Host cell apoptosis and membrane damage can be resulted by certain exotoxins. Direct invasion from apical surface of M cells may occur at an early stage of infection of Shigella and salmonella. Formation of fibrin-rich deposits on the surface of epithelial cells is commonly seen in clostridium difficile induced pseudomembranous colitis. Flagellar motility may facilitate initial invasion of campylobacter infection. However, these are not the path for lateral spreading of Shigella and enteroinvasive E. coli.
6. E. Stool culture at 37°C finding of reveals Gram-negative, non lactose-fermenting bacilli that do not produce H2S are consistent with Shigella. Campylobacter need to be cultured in Campylobacter or Skirrow agar at 42.0°C. They usually do not grow at 37°C. Entamoeba histolytica is protozoa. Enterotoxigenic e coli is lactose fermenter and usually cause watery diarrhea. Salmonella produces H2S. Vibrio cholera usually presents with severe diarrhea with rice water appearance and signs of dehydration.
7. E. See discussion of questions 3 and 4.
8. D. Stool culture at 37°C finding of reveals Gram-negative, non lactose-fermenting bacilli that produce H2S. This is consistent with Salmonella. See discussion of question 6.
9. D. Salmonella infection usually starts from Peyer Patches at ileum.
10. B. Salmonella usually cause watery diarrhea by increasing cAMP. Colonic vascular endothelial damage can be caused by Shiga toxin produced by enterohemorrhagic e coli and Shigella dysenteriae. Invasion of jejunal mucosa is commonly seen in campylobacter infection. Malabsorption caused by norovirus and giardia intestinalis can result in diarrhea. Suppression of normal colonic flora is associated with clostridium difficile diarrhea, characterized by green foul smelling watery diarrhea, after usage of antibiotics.
11. A. This case is characterized by foul smelling watery diarrhea following usage of antibiotics. These features are highly suggestive of clostridium difficile diarrhea, and clostridium difficile exotoxin tests would confirm the diagnosis. Colonoscopic examination and biopsy are not usually first line approaches for clostridium difficile pseudomembranous colitis confirmation. Darkfield microscopic examination of stool can be used for cholera. Shiga toxin test may be used to identify the presence of enterohemorrhagic e coli and Shigella dysenteriae. Routine stool culture is not sensitive in patients after antibiotic usage and not recommended for confirmation of clostridium difficile colitis.
12. B. Two toxins can be produced by clostridium difficile. Toxin A causes mucosal injury, fluid loss and inflammation, granulocyte attractant. Toxin B is cytotoxic. While most clostridium difficile can produce both toxins, some produce either A or B. Identification of either toxin should be sufficient to diagnose clostridium difficile colitis. Other pathogens listed do not produce clostridium difficile toxins.
13. B. Clostridium difficile toxin B is cytotoxic and causes colonic mucosal injury. Activation of guanylate cyclase and efflux of Cl– and H2O is seen in enterotoxigenic e coli diarrhea and cholera. Increased cAMP is associated with watery diarrhea caused by vibrio cholerae, enterotoxigenic e coli and salmonella. Invasion of jejunal mucosa is commonly seen in campylobacter infection. Malabsorption caused by norovirus and giardia intestinalis can result in diarrhea.
14. A. Clostridium difficile diarrhea is associated with interruption of normal colonic flora by antibiotics usage, chemotherapy or immunosuppression. In this case, antibiotics usage is the most relevant risk factor. There is no known association between clostridium difficile colitis and other comorbidities of this patient.
15. C. This case is characterized by gradual onset of fever, abdominal pain, diarrhea with visible blood, following a recent travel to a tropical region. These features are suggestive of amebic colitis, caused by the protozoan parasite entamoeba histolytica. An initial regular microscopic exam aiming to find cyst or trophozoite may be used as a quick approach for diagnosis. Also see discussion of questions 2 and 4.
16. B. The image reveal a trophozoite of entamoeba histolytica, characterized by pseudopod projections, small round nucleus with dot-like karyosome and peripheral rim of condensed chromatin. Entamoeba histolytica may cause colonic mucosa damage, forming “flask-shaped” ulcer. See discussion of question 13.
17. E. Entamoeba histolytica infection is usually associated with ingestion of cysts through contaminated water. Beef consumption may be associated with enterohemorrhagic e coli. Poultry consumption is more commonly associated with salmonella. Rice is commonly associated with B. cereus. Sea food is more commonly associated with norovirus and cholera.
18. A. See discussion of question 16. Diarrhea associated with giardia intestinalis is usually fatty foul-smelling watery due to malabsorption. Trophozoite of giardia intestinalis is pear shaped with 2 nuclei. Other pathogens are bacteria that can be confirmed by stool culture.
19. C. This case is characterized by foul-smelling fatty diarrhea following recent travel and usage of untreated water. These features and relatively not so severe clinical presentations are highly suggestive of giardia intestinalis infection. An initial regular microscopic exam aiming to find cyst or trophozoite may be used as a quick approach for diagnosis. Also see discussion of questions 2 and 4.
20. E. The image reveal a trophozoite of giardia intestinalis, characterized by pear shaped protozoa with 2 nuclei. Malabsorption caused by giardia intestinalis and norovirus can result in diarrhea. Also see discussion of question 10.
21. C. See discussion of questions 16 and 20.
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