Practice 80 ASCP MLS microbiology questions on Gastrointestinal Infections. Strengthen your exam preparation with clinically relevant scenarios on bacterial, viral, and parasitic pathogens.
This section of our ASCP MLS exam practice series provides 80 Microbiology MCQs on Gastrointestinal Infections, focusing on bacterial, viral, and parasitic causes of diarrhea and foodborne illness. Topics include Salmonella, Shigella, E. coli O157:H7, Vibrio cholerae, Campylobacter jejuni, Helicobacter pylori, Clostridioides difficile, and intestinal parasites like Giardia and Entamoeba. These practice questions are designed to help medical laboratory students and professionals test their knowledge of pathogen identification, biochemical tests, serological markers, and clinical correlations.
80 MCQs (2441-2520):
- Which of the following organisms should a routine stool culture for a 3-month-old infant be designed to detect?
a) Clostridium botulinum
b) Entamoeba hartmanni
c) Enterotoxigenic Escherichia coli
d) Campylobacter species - An isolate from a stool culture gives the following reactions: colorless colonies on MacConkey agar, yellow-orange colonies on Hektoen agar, acid slant/acid butt on TSI with no gas or H₂S, and is urease positive. Which enteric pathogen is this consistent with?
a) Shigella sonnei
b) Vibrio parahaemolyticus
c) Yersinia enterocolitica
d) Campylobacter jejuni - MacConkey media used to screen for hemorrhagic E. coli O157:H7 must contain which of the following sugars?
a) Lactose
b) Mannitol
c) Sorbitol
d) Arabinose - Which organism can cause diarrhea in children, traveler’s diarrhea, or a severe cholera-like syndrome through the production of enterotoxins?
a) Yersinia enterocolitica
b) Shigella dysenteriae
c) Salmonella typhi
d) Escherichia coli - Shigella species are characteristically:
a) Urease-positive
b) Oxidase-positive
c) Nonmotile
d) Lactose fermenters - If a biochemical profile is consistent with Shigella but the organism does not agglutinate in Shigella antisera, what should be done next?
a) Test the organism with a new lot of antisera
b) Test with VI antigen
c) Repeat the biochemical tests
d) Boil the organism and retest with the antisera - A biochemical suspension of an organism consistent with Shigella only agglutinates in group-D antisera after boiling. The Shigella species is:
a) dysenteriae
b) flexneri
c) boydii
d) sonnei - An 8-year-old girl is admitted with fever, abdominal pain, diarrhea, and vomiting. A stool culture grows many lactose-negative colonies that are urease and ornithine decarboxylase positive, and motile at 25°C. The most probable organism is:
a) Escherichia coli
b) Providencia stuartii
c) Yersinia enterocolitica
d) Edwardsiella tarda - A fecal specimen produces colonies with black centers on XLD and HE agar. The isolate is H₂S and lysine decarboxylase positive, urease and ONPG negative, and indole positive. It does not agglutinate in any Salmonella antisera. The most probable identification is:
a) Salmonella enterica
b) Edwardsiella tarda
c) Proteus mirabilis
d) Shigella sonnei - A 10-year-old boy presents with right lower quadrant pain mimicking appendicitis. During surgery, the appendix is normal but an enlarged node is removed. Small gram-negative bacilli are best isolated on a room temperature plate. The organism most likely is:
a) Prevotella melaninogenica
b) Shigella sonnei
c) Listeria monocytogenes
d) Yersinia enterocolitica - If biochemical reactions are consistent with Salmonella and a suspension agglutinates in Vi antiserum only, what is the next step?
a) Report “no Salmonella isolated”
b) Boil the suspension to inactivate the Vi antigen
c) Test organism with individual antisera
d) Repeat biochemical identification - The optimal incubator temperature for isolation of the Campylobacter jejuni/coli group is:
a) 4°C
b) 20°C
c) 25°C
d) 42°C - Which test can be used to diagnose infection and confirm eradication of Helicobacter pylori?
a) DNase test
b) Hippurate hydrolysis
c) String test
d) Urea breath test - A Gram stain of a touch prep from a gastric biopsy shows slender, curved gram-negative bacilli. The most likely pathogen is:
a) Burkholderia cepacia
b) Corynebacterium urealyticum
c) Helicobacter pylori
d) Pasteurella multocida - A very bloody stool is received, and a pathogenic strain of E. coli is isolated. Which sugar should this isolate be tested against to begin the identification process for O157:H7?
a) Mannitol
b) Sorbitol
c) Lactose
d) Arabinose - Optimum growth of Campylobacter jejuni is obtained in an atmosphere containing:
a) 6% O₂, 10-15% CO₂, 85-90% nitrogen
b) 10% H₂, 5% CO₂, 85% nitrogen
c) 10% H₂, 10% CO₂, 80% nitrogen
d) 25% O₂, 5% CO₂, 70% nitrogen - Which of the following tests would be most appropriate to biochemically differentiate Shigella and E. coli?
a) Hydrogen sulfide, ONPG, motility, urease
b) Lactose, indole, ONPG, motility
c) Urease, citrate, VP, hydrogen sulfide
d) Gas, MR, urease, citrate - Which organism commonly causes food poisoning by consumption of foods containing excessive populations of organisms and/or preformed enterotoxin?
a) Salmonella enteritidis
b) Shigella sonnei
c) Bacillus cereus
d) Aeromonas hydrophila - A rapid and sensitive “stand alone” method that detects Clostridioides difficile toxins A and B is:
a) Cell culture cytotoxin assay
b) Latex agglutination
c) Lecithinase production
d) NAAT (Nucleic Acid Amplification Test) - Clostridioides difficile can be presumptively detected by testing for the presence of:
a) Fluorescent staining
b) Glutamate dehydrogenase
c) Growth on LKV media
d) High pressure liquid chromatography - If a stool sample is sent to rule out Clostridioides difficile, what is the appearance of this organism on cycloserine-cefoxitin-fructose agar (CCFA)?
a) Colonies turn black
b) Red pigmented colonies
c) Yellow, ground glass colonies
d) Double zone hemolytic colonies - The enterotoxin produced by certain strains of hemolytic, coagulase-positive Staphylococcus aureus:
a) Is the primary cause of subacute endocarditis
b) Creates a biofilm on indwelling catheters
c) Causes a rapidly occurring (2-6 hours after ingestion) food poisoning
d) Is of extremely low virulence - A stool culture isolate shows the following: TSI reaction is acid butt, alkaline slant, no gas, no H₂S; phenylalanine deaminase negative; nonmotile; and serologically types as Shigella flexneri. The best course of action is to:
a) Report the organism as Shigella flexneri without further testing
b) Verify reactivity of the motility medium
c) Verify reactivity of the TSI slants for H₂S production
d) Verify reactivity of phenylalanine deaminase - Which of the following is a characteristic of Plesiomonas shigelloides that differentiates it from other Enterobacteriaceae?
a) Positive oxidase
b) Glucose fermentation
c) Reduction of nitrates to nitrites
d) Growth on MacConkey agar - The primary cause of pediatric viral gastroenteritis is:
a) Adenovirus, serotypes 40 and 41
b) Norwalk virus
c) Coronavirus
d) Rotavirus - The most common cause of viral gastroenteritis outbreaks on cruise ships is:
a) Rotavirus
b) Norwalk virus
c) Coronavirus
d) Enteric adenovirus - A patient with diarrhea has a stool specimen that reveals trophozoites measuring 25 µm with progressive, unidirectional motility, evenly distributed peripheral chromatin, and finely granular cytoplasm. This indicates:
a) Entamoeba coli
b) Entamoeba histolytica
c) Endolimax nana
d) Iodamoeba bütschlii - Small protozoan cysts are found in a stool concentrate. Each cyst has 4 nuclei that lack peripheral chromatin, and each nucleus has a large karyosome. These oval cysts are most likely:
a) Endolimax nana
b) Chilomastix mesnili
c) Entamoeba histolytica
d) Entamoeba hartmanni - The term “internal autoinfection” is generally used in referring to infections with:
a) Ascaris lumbricoides
b) Necator americanus
c) Trichuris trichiura
d) Strongyloides stercoralis - The causative agent of cysticercosis is:
a) Taenia solium
b) Taenia saginata
c) Ascaris lumbricoides
d) Trichuris trichiura - Which pathogen is most commonly associated with “rice-water stools” in severe diarrheal disease?
a) Shigella sonnei
b) Vibrio cholerae
c) Salmonella enteritidis
d) Campylobacter jejuni - Which organism is most likely to cause gastroenteritis from undercooked poultry?
a) Escherichia coli
b) Shigella flexneri
c) Campylobacter jejuni
d) Vibrio vulnificus - Which test is used for rapid detection of Shiga toxin-producing E. coli (STEC) in stool?
a) Coagulase test
b) Latex agglutination
c) Urease test
d) Indole test - Which enteric pathogen is associated with hemolytic uremic syndrome (HUS)?
a) Salmonella typhi
b) Escherichia coli O157:H7
c) Yersinia pestis
d) Clostridioides difficile - A patient develops diarrhea after eating raw oysters. The most likely pathogen is:
a) Vibrio vulnificus
b) Salmonella typhi
c) Campylobacter coli
d) Clostridium perfringens - The characteristic virulence factor of Vibrio cholerae is:
a) Heat-labile enterotoxin
b) Shiga toxin
c) Cholera toxin
d) Lipid A endotoxin - Which organism is transmitted through ingestion of cysts in contaminated water?
a) Giardia lamblia
b) Strongyloides stercoralis
c) Entamoeba coli
d) Enterobius vermicularis - Which intestinal parasite is identified by finding acid-fast oocysts in stool?
a) Giardia lamblia
b) Cryptosporidium parvum
c) Entamoeba histolytica
d) Ascaris lumbricoides - A stool specimen shows trophozoites with a “falling leaf” motility. The organism is:
a) Giardia lamblia
b) Entamoeba histolytica
c) Chilomastix mesnili
d) Dientamoeba fragilis - Which bacterium produces spores and causes food poisoning from reheated rice?
a) Clostridium botulinum
b) Bacillus cereus
c) Clostridium difficile
d) Salmonella enteritidis - What is the most reliable method for diagnosing Clostridioides difficile infection?
a) Blood culture
b) Stool culture only
c) Detection of toxin genes by NAAT
d) Indole test - The “triple sugar iron agar” (TSI) test is useful in differentiating which group of pathogens?
a) Staphylococci
b) Enterobacteriaceae
c) Mycobacteria
d) Vibrios - Which parasite is commonly known as the “whipworm”?
a) Ascaris lumbricoides
b) Trichuris trichiura
c) Ancylostoma duodenale
d) Taenia saginata - The carrier state of which organism is associated with gallbladder colonization?
a) Salmonella typhi
b) Shigella flexneri
c) Vibrio cholerae
d) Campylobacter jejuni - Which bacterial infection is most likely linked to antibiotic-associated diarrhea?
a) Pseudomonas aeruginosa
b) Clostridioides difficile
c) Helicobacter pylori
d) Vibrio parahaemolyticus - Which of the following is a microaerophilic organism?
a) Escherichia coli
b) Campylobacter jejuni
c) Salmonella enteritidis
d) Proteus mirabilis - A stool exam shows proglottids and eggs with radial striations. The organism is:
a) Taenia saginata
b) Taenia solium
c) Diphyllobothrium latum
d) Hymenolepis nana - Which organism is associated with peptic ulcer disease?
a) Escherichia coli
b) Helicobacter pylori
c) Klebsiella pneumoniae
d) Proteus vulgaris - A stool culture shows “swarming colonies” on agar. The likely organism is:
a) Shigella flexneri
b) Proteus mirabilis
c) Salmonella typhi
d) Vibrio vulnificus - Which helminth infection can be diagnosed using the “cellophane tape test”?
a) Trichuris trichiura
b) Enterobius vermicularis
c) Necator americanus
d) Strongyloides stercoralis - Which pathogen is associated with “pseudoappendicitis” in children?
a) Yersinia enterocolitica
b) Salmonella typhi
c) Campylobacter jejuni
d) Clostridium perfringens - Which enteric organism is urease-positive?
a) Shigella sonnei
b) Helicobacter pylori
c) Salmonella enteritidis
d) Vibrio cholerae - Which organism causes liver abscesses with anchovy-paste pus?
a) Entamoeba histolytica
b) Giardia lamblia
c) Cryptosporidium parvum
d) Balantidium coli - Which bacterial food poisoning has the shortest incubation period (1–6 hours)?
a) Bacillus cereus
b) Salmonella enteritidis
c) Clostridium perfringens
d) Escherichia coli - Which organism is diagnosed by the presence of large trophozoites with a kidney-shaped macronucleus?
a) Balantidium coli
b) Giardia lamblia
c) Entamoeba coli
d) Endolimax nana - Which bacterium is best identified using thiosulfate-citrate-bile salts-sucrose (TCBS) agar?
a) Vibrio cholerae
b) Shigella flexneri
c) Escherichia coli
d) Yersinia pestis - Which virus is the leading cause of acute diarrhea in infants worldwide?
a) Norovirus
b) Rotavirus
c) Adenovirus 41
d) Astrovirus - Which bacterial species produces a “double zone of hemolysis” on blood agar?
a) Bacillus cereus
b) Clostridium perfringens
c) Listeria monocytogenes
d) Escherichia coli - Which intestinal parasite has polar filaments inside its eggs?
a) Trichuris trichiura
b) Hymenolepis nana
c) Enterobius vermicularis
d) Taenia saginata - Which diagnostic method is most useful for detecting norovirus in outbreaks?
a) Culture on viral media
b) PCR-based molecular assay
c) Stool antigen ELISA
d) Serology - 31. A key characteristic that helps differentiate Plesiomonas shigelloides from Vibrio species is:
a) Its ability to grow on TCBS agar
b) Its requirement for 1% NaCl for growth
c) Its positive oxidase reaction
d) Its fermentation of glucose - For which organism is the “string test” used as a rapid presumptive identification method?
a) Vibrio cholerae
b) Campylobacter jejuni
c) Helicobacter pylori
d) Escherichia coli O157:H7 - A stool culture from a patient with seafood-associated diarrhea grows yellow, sucrose-fermenting colonies on TCBS agar. The most likely organism is:
a) Vibrio parahaemolyticus
b) Vibrio vulnificus
c) Vibrio cholerae
d) Aeromonas hydrophila - Which of the following organisms is most associated with Guillain-Barré syndrome as a post-infectious complication?
a) Salmonella Typhi
b) Campylobacter jejuni
c) Shigella sonnei
d) Yersinia enterocolitica - The H antigen used in the serological typing of Salmonella species is associated with:
a) The capsule
b) The flagella
c) The cell wall
d) The O antigen - A patient presents with fever and a rose-spot rash. The most likely causative agent isolated from blood or stool culture would be:
a) Shigella dysenteriae
b) Campylobacter fetus
c) Salmonella Typhi
d) Yersinia pseudotuberculosis - Which E. coli pathotype is characterized by its attachment to the intestinal mucosa, causing effacement of the microvilli (forming a “pedestal”)?
a) ETEC
b) EPEC
c) EHEC
d) EIEC - The primary reservoir for Campylobacter jejuni is:
a) Contaminated water
b) Domestic cats
c) Poultry
d) Humans - A key test to differentiate Proteus mirabilis (indole-negative) from other Proteus species is:
a) Urease production
b) Phenylalanine deaminase production
c) Hydrogen sulfide production
d) Indole test - Which of the following is a non-lactose fermenter that is urease positive and motile at 25 °C but not at 37 °C?
a) Shigella flexneri
b) Edwardsiella tarda
c) Yersinia enterocolitica
d) Providencia stuartii - The K antigen of E. coli and other Enterobacteriaceae is:
a) A flagellar antigen
b) A somatic O antigen
c) A capsular antigen
d) Used to identify Shigella species - Which medium is both selective and differential for Vibrio species?
a) MacConkey agar
b) Hektoen Enteric (HE) agar
c) TCBS agar
d) XLD agar - A positive result in the Sorbitol-MacConkey agar test appears as:
a) Pink colonies
b) Colorless colonies
c) Black colonies
d) Swarming growth - Which of the following best describes the pathogenesis of Vibrio cholerae?
a) Invasion of the colonic epithelium
b) Production of a cytotoxin causing cell death
c) Production of an enterotoxin that stimulates cAMP
d) Attachment and effacement of the intestinal brush border - The specimen of choice for the recovery of Giardia lamblia is:
a) Serum
b) Duodenal aspirate
c) Urine
d) Fresh stool - A patient with a history of antibiotic use presents with watery diarrhea. A stool test is positive for GDH but negative for toxins A and B by EIA. The next best step is to:
a) Report as negative for C. difficile infection
b) Perform a NAAT to detect the toxin gene
c) Request a new stool specimen
d) Report as positive for C. difficile infection - Which parasite is identified by its “red blood cell-like” appearance on a modified acid-fast stain of stool?
a) Cryptosporidium parvum
b) Cyclospora cayetanensis
c) Entamoeba histolytica
d) Giardia lamblia - The most common cause of watery diarrhea in infants in daycare settings is:
a) Salmonella species
b) Rotavirus
c) Cryptosporidium parvum
d) Clostridioides difficile - Which of the following is a characteristic of enteroinvasive E. coli (EIEC)?
a) Produces a heat-stable toxin (ST)
b) Causes a dysentery-like illness similar to Shigella
c) Produces a Shiga-like toxin
d) Causes hemolytic uremic syndrome (HUS) - The recommended method for confirming the presence of Salmonella after screening biochemical tests is:
a) Coagulase test
b) Oxidase test
c) Serological agglutination
d) Bile esculin hydrolysis
Answer Key
Answer Key:
- d) Campylobacter species
- c) Yersinia enterocolitica
- c) Sorbitol
- d) Escherichia coli
- c) Nonmotile
- d) Boil the organism and retest with the antisera
- d) sonnei
- c) Yersinia enterocolitica
- b) Edwardsiella tarda
- d) Yersinia enterocolitica
- b) Boil the suspension to inactivate the Vi antigen
- d) 42°C
- d) Urea breath test
- c) Helicobacter pylori
- b) Sorbitol
- a) 6% O₂, 10-15% CO₂, 85-90% nitrogen
- b) Lactose, indole, ONPG, motility
- c) Bacillus cereus
- d) NAAT (Nucleic Acid Amplification Test)
- b) Glutamate dehydrogenase
- c) Yellow, ground glass colonies
- c) Causes a rapidly occurring (2-6 hours after ingestion) food poisoning
- a) Report the organism as Shigella flexneri without further testing
- a) Positive oxidase
- d) Rotavirus
- b) Norwalk virus
- b) Entamoeba histolytica
- a) Endolimax nana
- d) Strongyloides stercoralis
- a) Taenia solium
- b) Vibrio cholerae
- c) Campylobacter jejuni
- b) Latex agglutination
- b) Escherichia coli O157:H7
- a) Vibrio vulnificus
- c) Cholera toxin
- a) Giardia lamblia
- b) Cryptosporidium parvum
- a) Giardia lamblia
- b) Bacillus cereus
- c) Detection of toxin genes by NAAT
- b) Enterobacteriaceae
- b) Trichuris trichiura
- a) Salmonella typhi
- b) Clostridioides difficile
- b) Campylobacter jejuni
- b) Taenia solium
- b) Helicobacter pylori
- b) Proteus mirabilis
- b) Enterobius vermicularis
- a) Yersinia enterocolitica
- b) Helicobacter pylori
- a) Entamoeba histolytica
- a) Bacillus cereus
- a) Balantidium coli
- a) Vibrio cholerae
- b) Rotavirus
- b) Clostridium perfringens
- b) Hymenolepis nana
- b) PCR-based molecular assay
- b) Its requirement for 1% NaCl for growth
- a) Vibrio cholerae
- c) Vibrio cholerae
- b) Campylobacter jejuni
- b) The flagella
- c) Salmonella Typhi
- b) EPEC
- c) Poultry
- d) Indole test
- c) Yersinia enterocolitica
- c) A capsular antigen
- c) TCBS agar
- b) Colorless colonies
- c) Production of an enterotoxin that stimulates cAMP
- d) Fresh stool
- b) Perform a NAAT to detect the toxin gene
- b) Cyclospora cayetanensis
- b) Rotavirus
- b) Causes a dysentery-like illness similar to Shigella
- c) Serological agglutination
Top 8 Medical Laboratory Scientist (MLS) Exams:
Top 8 Medical Laboratory Scientist (MLS) Exams that are recognized globally and can help professionals validate their credentials and enhance their career opportunities:
1. ASCP – American Society for Clinical Pathology (USA)
- Exam Name: MLS(ASCP)
- Eligibility: Bachelor’s degree with clinical laboratory experience.
- Global Recognition: High
- Purpose: Certifies Medical Laboratory Scientists in the United States and internationally.
2. AMT – American Medical Technologists (USA)
- Exam Name: MLT(AMT) or MT(AMT)
- Eligibility: Academic and/or work experience in medical laboratory technology.
- Global Recognition: Moderate
- Purpose: Credentialing for medical technologists and technicians.
3. AIMS – Australian Institute of Medical and Clinical Scientists
- Exam Name: AIMS Certification Exam
- Eligibility: Assessment of qualifications and work experience.
- Recognition: Required for practice in Australia.
- Purpose: Certification and registration in Australia.
4. CSMLS – Canadian Society for Medical Laboratory Science
- Exam Name: CSMLS General or Subject-specific Exams
- Eligibility: Graduation from a CSMLS-accredited program or equivalent.
- Recognition: Canada
- Purpose: Entry-to-practice certification in Canada.
5. IBMS – Institute of Biomedical Science (UK)
- Exam Name: Registration and Specialist Portfolio Assessment
- Eligibility: Accredited degree and lab experience.
- Recognition: UK and some Commonwealth countries.
- Purpose: Biomedical Scientist registration with the HCPC (UK).
6. HAAD / DOH – Department of Health, Abu Dhabi (UAE)
- Exam Name: DOH/HAAD License Exam
- Eligibility: Degree in medical laboratory science and experience.
- Recognition: UAE (Abu Dhabi)
- Purpose: Licensure for medical laboratory practice in Abu Dhabi.
7. DHA – Dubai Health Authority (UAE)
- Exam Name: DHA License Exam for Medical Laboratory Technologists
- Eligibility: Relevant degree and experience.
- Recognition: Dubai, UAE
- Purpose: Professional license for clinical laboratory practice in Dubai.
8. MOH – Ministry of Health (Gulf Countries like UAE, Saudi Arabia, Kuwait)
- Exam Name: MOH License Exam
- Eligibility: BSc/Diploma in Medical Laboratory + experience.
- Recognition: Varies by country.
- Purpose: Required for practicing in public and private sector labs.
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