A 35-year-old woman who returned from trekking in Nepal 2 months ago has mild bloating but no diarrhoea or fever. A formalin-ethyl acetate concentrate of her stool shows many spherical cysts that measure 6-8 µm with a calibrated ocular micrometer. On a trichrome-stained smear, mature cysts have up to four nuclei, each with fine, even peripheral chromatin and a small central karyosome, and a few cysts contain chromatoid bodies with rounded ends. No trophozoites are seen. How should the finding be reported?

Entamoeba hartmanni, a nonpathogen

Entamoeba histolytica/E. dispar group

A 4-year-old at a day-care centre has had bloating and pale, greasy, foul-smelling stools for 3 weeks. One O&P examination was negative. Which test is most likely to make the diagnosis?

Giardia antigen test (EIA or DFA) on stool

Serum antibody to Entamoeba histolytica

Stool culture for Salmonella and Shigella

Modified acid-fast stain for coccidia

Cyclospora cayetanensis outbreaks are usually linked to fresh produce, not to direct person-to-person spread. Why?

Oocysts must sporulate outside the body

Its oocysts are killed quickly by stomach acid

It infects only people with weak immunity

It needs an insect vector to reach people

A man with HIV (CD4 count 0.05 x 10^9/L [50/µL]) has had profuse watery diarrhea for a month. Three O&P examinations with concentration and trichrome stain are negative. Which parasitology test should be added?

Modified acid-fast stain for coccidia

A fourth routine O&P examination with trichrome

Serum antibody to Entamoeba histolytica

Thick and thin blood films for parasites

A 34-year-old man who returned from South Asia 3 months ago has fever, right upper quadrant pain and a single liver abscess on ultrasound. Three stool examinations are negative. Which test best supports amebic liver abscess?

Serum antibody to Entamoeba histolytica

Bacterial culture of the abscess fluid

Modified acid-fast stain of stool

Intestinal protozoa

13 DOH Abu Dhabi (HAAD) lab licensing practice questions on Intestinal protozoa, part of Microbiology. The first 13 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 35-year-old woman who returned from trekking in Nepal 2 months ago has mild bloating but no diarrhoea or fever. A formalin-ethyl acetate concentrate of her stool shows many spherical cysts that measure 6-8 µm with a calibrated ocular micrometer. On a trichrome-stained smear, mature cysts have up to four nuclei, each with fine, even peripheral chromatin and a small central karyosome, and a few cysts contain chromatoid bodies with rounded ends. No trophozoites are seen. How should the finding be reported?
1. AEntamoeba hartmanni, a nonpathogen
2. BEndolimax nana, a nonpathogen
3. CEntamoeba coli, a nonpathogen
4. DEntamoeba histolytica/E. dispar group
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Answer: A · Entamoeba hartmanni, a nonpathogen
Cysts with up to four nuclei, each with even peripheral chromatin and a small central karyosome, and with rounded chromatoid bodies, have the morphology of the E. histolytica type. Size then decides: E. histolytica and E. dispar cysts measure about 10-20 µm, while E. hartmanni cysts are under 10 µm. These cysts measure 6-8 µm with a calibrated micrometer, so they are E. hartmanni, a nonpathogenic commensal that needs no treatment or further testing. Reporting them as E. histolytica/E. dispar would lead to needless antigen testing or treatment. Its presence still shows exposure to faecally contaminated food or water, so other pathogens should not be overlooked.
- B: E. nana nuclei have large blot-like karyosomes and no peripheral chromatin.
- C: E. coli cysts are larger, with up to eight nuclei and eccentric karyosomes.
- D: E. histolytica/E. dispar cysts measure more than 10 µm.
Question 2 · Hard
A 4-year-old at a day-care centre has had bloating and pale, greasy, foul-smelling stools for 3 weeks. One O&P examination was negative. Which test is most likely to make the diagnosis?
1. ASerum antibody to Entamoeba histolytica
2. BStool culture for Salmonella and Shigella
3. CModified acid-fast stain for coccidia
4. DGiardia antigen test (EIA or DFA) on stool
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Answer: D · Giardia antigen test (EIA or DFA) on stool
Pale, greasy, foul stools with bloating in a day-care child point to giardiasis, which causes malabsorption. Giardia cysts are shed intermittently, so a single O&P is often negative. A stool antigen test by EIA or direct fluorescent antibody is more sensitive than one microscopic examination and detects Giardia even when few cysts are present. Many kits detect Giardia and Cryptosporidium together.
- A: Amebic serology is for invasive amoebiasis such as liver abscess.
- B: Salmonella and Shigella cause acute diarrhea, often with fever or blood, not weeks of greasy stools.
- C: Cryptosporidium causes watery diarrhea rather than greasy, malabsorptive stools.
Question 3 · Medium
Cyclospora cayetanensis outbreaks are usually linked to fresh produce, not to direct person-to-person spread. Why?
1. AIts oocysts are killed quickly by stomach acid
2. BIt infects only people with weak immunity
3. COocysts must sporulate outside the body
4. DIt needs an insect vector to reach people
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Answer: C · Oocysts must sporulate outside the body
Cyclospora oocysts are passed unsporulated and are not infectious at first. They need days to weeks in the environment, in warm conditions, to sporulate and become infective. Contaminated water used on berries, lettuce and herbs is the usual route, so outbreaks follow food rather than household contact. Cryptosporidium oocysts, by contrast, are infectious when passed and spread directly.
- A: Oocysts survive stomach acid; infection begins when sporulated oocysts are swallowed.
- B: Healthy people are infected too, often with prolonged watery diarrhea.
- D: No insect vector is involved in Cyclospora transmission.
Question 4 · Hard
A man with HIV (CD4 count 0.05 x 10^9/L [50/µL]) has had profuse watery diarrhea for a month. Three O&P examinations with concentration and trichrome stain are negative. Which parasitology test should be added?
1. AModified acid-fast stain for coccidia
2. BA fourth routine O&P examination with trichrome
3. CSerum antibody to Entamoeba histolytica
4. DThick and thin blood films for parasites
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Answer: A · Modified acid-fast stain for coccidia
Cryptosporidium is a leading cause of chronic watery diarrhea in advanced HIV and is easily missed by a routine O&P, because trichrome stains the oocysts poorly. It must be looked for specifically with a modified acid-fast stain or, more sensitively, an antigen or fluorescent antibody test. Repeating the same routine test will not find it. Cyclospora and Cystoisospora are also shown by the modified acid-fast stain.
- B: Repeating a routine O&P adds little, because trichrome misses coccidian oocysts.
- C: Amebic serology is for invasive amoebiasis, such as dysentery or liver abscess.
- D: Blood films look for blood parasites and do not explain watery diarrhea.
Question 5 · Hard
A 34-year-old man who returned from South Asia 3 months ago has fever, right upper quadrant pain and a single liver abscess on ultrasound. Three stool examinations are negative. Which test best supports amebic liver abscess?
1. ASerum antibody to Entamoeba histolytica
2. BThick and thin blood films
3. CBacterial culture of the abscess fluid
4. DModified acid-fast stain of stool
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Answer: A · Serum antibody to Entamoeba histolytica
Most patients with amebic liver abscess have no amoebae in their stool by the time the abscess appears. Serum antibody to E. histolytica is positive in more than 90% of them and, with imaging, is the usual supporting test. In endemic areas a positive result can reflect past infection, so it is read with the clinical picture. Aspirated pus is often sterile and rarely shows trophozoites, which live in the abscess wall.
- B: Blood films detect malaria and other blood parasites, not E. histolytica.
- C: A sterile bacterial culture helps exclude a pyogenic abscess but does not confirm amoebiasis.
- D: Modified acid-fast stain detects coccidia such as Cryptosporidium, not amoebae.
Question 6 · Medium
Eleven children develop watery diarrhea about a week after swimming in a hotel pool that was correctly chlorinated. Which parasite is most likely?
1. ACyclospora cayetanensis
2. BCryptosporidium
3. CGiardia duodenalis
4. DEntamoeba histolytica
Answer and explanation in LearnFree account
Question 7 · Medium
Quadrinucleate cysts reported as Entamoeba histolytica/E. dispar are found in the stool of a food handler with no symptoms. Which test can tell the two species apart?
1. ACounting the chromatoid bodies in each cyst
2. BA modified acid-fast stain of the stool
3. CE. histolytica-specific antigen test or PCR
4. DMeasuring the cysts with an ocular micrometer
Answer and explanation in LearnFree account
Question 8 · Hard
With the 40x objective, the ocular micrometer is calibrated at 2.5 µm per division. Round, acid-fast-variable oocysts in a modified acid-fast stain each span 3.5 divisions. They are most likely:
1. ACyclospora cayetanensis
2. BCryptosporidium species
3. CCystoisospora belli
4. DBlastocystis species
Answer and explanation in LearnFree account
Question 9 · Medium
A modified acid-fast stain of stool shows round, red oocysts measuring 4-6 µm. The most likely organism is:
1. ACystoisospora belli
2. BCyclospora cayetanensis
3. CEnterocytozoon bieneusi
4. DCryptosporidium species
Answer and explanation in LearnFree account
Question 10 · Easy
Cryptosporidium oocysts in a stool smear are routinely shown with:
1. AA Lugol's iodine wet mount
2. BA Gram stain
3. CA trichrome permanent stain
4. DA modified acid-fast stain
Answer and explanation in LearnFree account
Question 11 · Medium
A formalin-ethyl acetate concentrate shows oval cysts about 11 µm long. Each holds four nuclei, lengthwise fibres and curved median bodies. They are:
1. AGiardia duodenalis cysts
2. BEntamoeba histolytica/E. dispar cysts
3. CEndolimax nana cysts
4. DChilomastix mesnili cysts
Answer and explanation in LearnFree account
Question 12 · Hard
A patient with dysentery and a liver abscess has amoebae in a fresh stool. Which wet-mount finding separates the pathogenic Entamoeba histolytica from the look-alike Entamoeba dispar?
1. AChromatoid bars with rounded ends in the cyst
2. BFour nuclei in the mature cyst
3. CFine, even peripheral chromatin on the nucleus
4. DRed blood cells ingested by the trophozoites
Answer and explanation in LearnFree account
Question 13 · Easy
Which description fits a Giardia duodenalis (lamblia) trophozoite?
1. AA large ciliated cell with a macronucleus
2. BA banana-shaped crescent in red cells
3. CAn amoeba containing ingested red cells
4. DPear-shaped, two nuclei, falling-leaf motility
Answer and explanation in LearnFree account

Intestinal protozoa

A 4-year-old at a day-care centre has had bloating and pale, greasy, foul-smelling stools for 3 weeks. One O&P examination was negative. Which test is most likely to make the diagnosis?

Cyclospora cayetanensis outbreaks are usually linked to fresh produce, not to direct person-to-person spread. Why?

A man with HIV (CD4 count 0.05 x 10^9/L [50/µL]) has had profuse watery diarrhea for a month. Three O&P examinations with concentration and trichrome stain are negative. Which parasitology test should be added?

A 34-year-old man who returned from South Asia 3 months ago has fever, right upper quadrant pain and a single liver abscess on ultrasound. Three stool examinations are negative. Which test best supports amebic liver abscess?

Eleven children develop watery diarrhea about a week after swimming in a hotel pool that was correctly chlorinated. Which parasite is most likely?

Quadrinucleate cysts reported as Entamoeba histolytica/E. dispar are found in the stool of a food handler with no symptoms. Which test can tell the two species apart?

With the 40x objective, the ocular micrometer is calibrated at 2.5 µm per division. Round, acid-fast-variable oocysts in a modified acid-fast stain each span 3.5 divisions. They are most likely:

A modified acid-fast stain of stool shows round, red oocysts measuring 4-6 µm. The most likely organism is:

Cryptosporidium oocysts in a stool smear are routinely shown with:

A formalin-ethyl acetate concentrate shows oval cysts about 11 µm long. Each holds four nuclei, lengthwise fibres and curved median bodies. They are:

A patient with dysentery and a liver abscess has amoebae in a fresh stool. Which wet-mount finding separates the pathogenic Entamoeba histolytica from the look-alike Entamoeba dispar?

Which description fits a Giardia duodenalis (lamblia) trophozoite?