A 38-year-old man with advanced HIV (CD4 count 25/µL [0.025 x 10^9/L]) has had watery diarrhea and weight loss for 6 weeks. Stool culture, C. difficile testing and a multiplex gastrointestinal PCR panel are negative. Three O&P examinations, each with a formalin-ethyl acetate concentrate and a trichrome-stained smear, are negative, and a modified acid-fast stain shows no oocysts. On review, the trichrome smears contain many tiny oval bodies about 1-2 µm across, which had been dismissed as yeasts or bacteria. Which test is most likely to identify the cause?

Modified trichrome (Weber) stain

Giardia and Cryptosporidium antigen test

Three more routine O&P examinations

Parasitology

36 ASCP MLS practice questions on Parasitology, part of Microbiology. The first 25 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 42-year-old woman from a sheep-farming area has had right upper quadrant pain, fever and itchy hives for 4 months. She often eats wild watercress. Results: WBC 13.0 x 10^3/µL (13.0 x 10^9/L) with 32% eosinophils; alkaline phosphatase raised; ultrasound shows tracks of small lesions in the liver. A formalin-ethyl acetate concentrate of stool shows a few large, oval, yellow-brown eggs about 140 x 80 µm, with an indistinct operculum and no spine; they are unembryonated. The most likely parasite is:
1. AFasciola hepatica
2. BSchistosoma mansoni
3. CClonorchis sinensis
4. DFasciolopsis buski
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Answer: A · Fasciola hepatica
Large, unembryonated, operculated eggs of about 130-150 µm belong to the large flukes, Fasciola hepatica and Fasciolopsis buski, whose eggs cannot be told apart reliably. The clinical picture decides: sheep farming, watercress, tracks of liver lesions, fever and marked eosinophilia point to F. hepatica, which migrates through the liver to the bile ducts. Fasciolopsis buski is an intestinal fluke acquired from water plants in Asia and does not cause liver lesions. Clonorchis eggs are small, about 30 µm, with shoulders, and Schistosoma mansoni eggs have a lateral spine and no operculum. Because eggs from eaten animal liver can pass through the gut, a repeat stool after a liver-free diet confirms true infection.
- B: S. mansoni eggs have a large lateral spine and no operculum.
- C: Clonorchis eggs are small (about 30 µm), with shoulders around the operculum.
- D: Fasciolopsis eggs look alike, but this intestinal fluke does not damage the liver.
Question 2 · Hard
A 38-year-old man with advanced HIV (CD4 count 25/µL [0.025 x 10^9/L]) has had watery diarrhea and weight loss for 6 weeks. Stool culture, C. difficile testing and a multiplex gastrointestinal PCR panel are negative. Three O&P examinations, each with a formalin-ethyl acetate concentrate and a trichrome-stained smear, are negative, and a modified acid-fast stain shows no oocysts. On review, the trichrome smears contain many tiny oval bodies about 1-2 µm across, which had been dismissed as yeasts or bacteria. Which test is most likely to identify the cause?
1. AGiardia and Cryptosporidium antigen test
2. BModified trichrome (Weber) stain
3. CFungal culture of the stool
4. DThree more routine O&P examinations
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Answer: B · Modified trichrome (Weber) stain
Chronic diarrhea in advanced HIV with negative routine tests and tiny 1-2 µm oval bodies suggests microsporidia, most often Enterocytozoon bieneusi. Their spores are too small to be recognized on a routine trichrome smear and are easily dismissed as yeasts or bacteria, and most multiplex GI panels do not detect them. The modified trichrome stain uses a high concentration of chromotrope 2R, which stains the spores pinkish red, often with a belt-like stripe, against a green or blue background. Yeasts are larger and bud, so fungal culture would not explain the findings. Repeating the same O&P method or testing for Giardia and Cryptosporidium antigens would miss microsporidia again.
- A: These antigen tests do not detect microsporidia.
- C: Microsporidia do not grow in fungal culture, and the bodies are too small for yeasts.
- D: The same methods would miss the spores again.
Question 3 · Hard
A 45-year-old man is admitted with high fever 10 days after a safari in the game parks of northern Tanzania, where large biting flies were common. A painful, indurated sore on his calf appeared before the fever. Platelets are 60 x 10^3/µL (60 x 10^9/L). The malaria antigen test and thick films for malaria are negative, but the Giemsa-stained thin film shows extracellular flagellates 20-30 µm long with an undulating membrane, a free anterior flagellum and a small kinetoplast at the posterior end. Which organism is most likely?
1. ALeishmania donovani
2. BTrypanosoma brucei gambiense
3. CTrypanosoma brucei rhodesiense
4. DTrypanosoma cruzi
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Answer: C · Trypanosoma brucei rhodesiense
Extracellular trypomastigotes with an undulating membrane and a small posterior kinetoplast are African trypanosomes, and the two subspecies of T. brucei look the same on a film. Geography and the course separate them: East African game parks and an acute febrile illness within 2 weeks of a tsetse bite, with a chancre and many parasites in the blood, are typical of T. b. rhodesiense. T. b. gambiense occurs in West and Central Africa and causes a slow illness with few circulating parasites. T. cruzi forms C-shaped trypomastigotes with a large kinetoplast and is found in the Americas, and Leishmania appears as amastigotes inside macrophages. The patient needs urgent treatment, and CSF is examined to stage the disease.
- A: Leishmania appears as amastigotes in macrophages, not as blood trypomastigotes.
- B: T. b. gambiense is a West and Central African, slowly progressive infection.
- D: T. cruzi has a large kinetoplast and a C shape, and occurs in the Americas.
Question 4 · Hard
A 34-year-old woman returned from Nigeria 7 months ago. She took chemoprophylaxis during the trip and for 4 weeks afterwards. She now has fevers every 48 hours. On the thin film, infected red cells are slightly enlarged, many are oval with ragged, fimbriated edges, and they contain Schüffner's (James's) stippling. Trophozoites are compact rather than amoeboid; mature schizonts hold about 8 merozoites around a central pigment clump; round gametocytes fill the cell. Parasitemia is 0.4%. The most likely species is:
1. APlasmodium vivax
2. BPlasmodium ovale
3. CPlasmodium knowlesi
4. DPlasmodium malariae
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Answer: B · Plasmodium ovale
A first attack months after leaving a malarious area, despite prophylaxis that does not kill liver hypnozoites, suggests relapse, which only P. vivax and P. ovale cause. Of the two, oval, fimbriated red cells, compact trophozoites and schizonts of about 8 merozoites fit P. ovale; P. vivax makes larger cells, amoeboid trophozoites and 12-24 merozoites. West Africa also fits, because most people there lack the Duffy antigen that P. vivax mainly uses to enter red cells. P. malariae and P. knowlesi do not enlarge the red cell, and P. knowlesi is acquired in Southeast Asia. G6PD must be tested before primaquine is given to clear the hypnozoites.
- A: P. vivax gives larger, round cells, amoeboid trophozoites and 12-24 merozoites.
- C: P. knowlesi is acquired in Southeast Asia and does not enlarge the red cells.
- D: P. malariae does not enlarge the red cell or show Schüffner's stippling.
Question 5 · Hard
On a thin film, 46 of 2,000 red cells counted contain P. falciparum rings, and 4 of these cells hold two rings each. What is the percent parasitemia?
1. A4.6%
2. B2.3%
3. C2.5%
4. D0.023%
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Answer: B · 2.3%
Percent parasitemia counts infected red cells, not individual parasites: 46 ÷ 2,000 × 100 = 2.3%. A cell with two rings is counted once. Counting every ring gives 50 ÷ 2,000 × 100 = 2.5%, and dividing by 1,000 gives 4.6%. A parasitemia of 5% or more is one sign of severe malaria, and the count is repeated to follow treatment.
- A: Using 1,000 instead of 2,000 red cells as the denominator gives 4.6%.
- C: Counting each ring (50) instead of each infected cell gives 2.5%.
- D: Leaving the fraction (0.023) without multiplying by 100 gives 0.023%.
Question 6 · Hard
A 70-year-old man from coastal Massachusetts, who had a splenectomy, has fever and hemolysis in July. He has not travelled. His thin film shows small rings, some outside red cells, and tetrads, with no pigment or gametocytes. The most likely cause is:
1. AAnaplasma phagocytophilum
2. BPlasmodium malariae
3. CBabesia microti
4. DPlasmodium falciparum
Answer and explanation in LearnFree account
Question 7 · Hard
Two weeks after successful treatment of P. falciparum malaria, a patient's HRP2 rapid antigen test is still positive, but repeated thick and thin films show no parasites. The best explanation is:
1. APoor staining of the thick films
2. BUndetected P. ovale coinfection
3. CHRP2 antigen persisting after cure
4. DRecrudescence of resistant parasites
Answer and explanation in LearnFree account
Question 8 · Hard
A man from the Philippines who eats raw freshwater crabs has chronic cough and hemoptysis. Three AFB smears are negative. Sputum shows golden-brown operculated eggs about 90 µm long, thickened at the end opposite the operculum. The most likely parasite is:
1. AParagonimus westermani
2. BSchistosoma japonicum
3. CClonorchis sinensis
4. DFasciola hepatica
Answer and explanation in LearnFree account
Question 9 · Medium
Rhabditiform larvae in fresh stool have a short buccal canal and a large genital primordium. Which parasite is this?
1. AStrongyloides stercoralis
2. BEnterobius vermicularis
3. CNecator americanus
4. DAncylostoma duodenale
Answer and explanation in LearnFree account
Question 10 · Medium
A Giemsa-stained blood film shows C-shaped trypomastigotes with a large kinetoplast at the posterior end. Which organism is most likely?
1. ATrypanosoma brucei gambiense
2. BTrypanosoma cruzi
3. CTrypanosoma brucei rhodesiense
4. DLeishmania donovani
Answer and explanation in LearnFree account
Question 11 · Easy
Which Plasmodium stage does an infected female Anopheles mosquito inject into a person?
1. AMerozoites
2. BGametocytes
3. CSporozoites
4. DHypnozoites
Answer and explanation in LearnFree account
Question 12 · Hard
A 34-year-old man with advanced HIV has chronic watery diarrhea and blood eosinophilia. A modified acid-fast stain of stool shows red, ellipsoidal oocysts about 25 × 12 µm, each holding a round sporoblast. The most likely organism is:
1. ACystoisospora belli
2. BCyclospora cayetanensis
3. CSarcocystis species
4. DCryptosporidium species
Answer and explanation in LearnFree account
Question 13 · Medium
A cyst 18 µm across has eight nuclei with eccentric karyosomes and uneven peripheral chromatin, plus splinter-shaped chromatoidal bodies. It is:
1. AEntamoeba coli
2. BEndolimax nana
3. CEntamoeba histolytica/dispar
4. DIodamoeba bütschlii
Answer and explanation in LearnFree account
Question 14 · Hard
A man who returned from Ghana 10 days ago has fever, but his first thick and thin films are negative. What should the laboratory advise?
1. AReport that malaria has been excluded
2. BRequest a stool ova and parasite exam
3. CRepeat films every 12-24 hours for 3 sets
4. DRepeat films only during a fever spike
Answer and explanation in LearnFree account
Question 15 · Medium
A thin film shows enlarged red cells with Schüffner's dots, amoeboid trophozoites and schizonts with about 16 merozoites. The species is most likely:
1. APlasmodium ovale
2. BPlasmodium malariae
3. CPlasmodium knowlesi
4. DPlasmodium vivax
Answer and explanation in LearnFree account
Question 16 · Medium
A sheathed microfilaria with nuclei running in a continuous row to the tip of the tail is found in blood drawn at midday. The most likely species is:
1. AMansonella perstans
2. BBrugia malayi
3. CWuchereria bancrofti
4. DLoa loa
Answer and explanation in LearnFree account
Question 17 · Hard
A stool for ova and parasites arrives 2 days after the patient had a barium enema. What is the best action?
1. AExamine a permanent stained smear only
2. BRecollect after the barium has cleared
3. CReport it as no parasites seen
4. DConcentrate it before making slides
Answer and explanation in LearnFree account
Question 18 · Hard
A traveller back from Malaysian Borneo has daily fevers. The thin film shows 3% parasitemia in normal-sized red cells without stippling, with rings, band forms and schizonts of up to 16 merozoites. Which species is most likely?
1. APlasmodium falciparum
2. BPlasmodium knowlesi
3. CPlasmodium malariae
4. DPlasmodium vivax
Answer and explanation in LearnFree account
Question 19 · Easy
Enterobius vermicularis eggs are best found in a sample taken:
1. AFrom a midday urine sediment
2. BFrom stool preserved in formalin
3. CFrom an evening blood film
4. DFrom perianal skin with tape on waking
Answer and explanation in LearnFree account
Question 20 · Medium
A permanent stain shows trophozoites 5-15 µm with one or two nuclei, each with its chromatin split into 4-8 granules and no peripheral chromatin. No cysts are seen. Which organism is this?
1. ADientamoeba fragilis
2. BEndolimax nana
3. CIodamoeba bütschlii
4. DEntamoeba hartmanni
Answer and explanation in LearnFree account
Question 21 · Medium
Round oocysts 8-10 µm across stain from pale to deep red with modified acid-fast stain and glow blue under UV light. Which organism is this?
1. AEnterocytozoon bieneusi
2. BCystoisospora belli
3. CCyclospora cayetanensis
4. DCryptosporidium species
Answer and explanation in LearnFree account
Question 22 · Medium
A stool concentrate shows round eggs 30-35 µm with a thick, radially striated shell and a six-hooked embryo. How should they be reported?
1. ATaenia species eggs
2. BTaenia solium eggs
3. CTaenia saginata eggs
4. DHymenolepis nana eggs
Answer and explanation in LearnFree account
Question 23 · Medium
Compared with formalin-ethyl acetate sedimentation, zinc sulfate flotation is more likely to miss:
1. AHymenolepis nana eggs
2. BThin-shelled hookworm eggs
3. CGiardia duodenalis cysts
4. DOperculated trematode eggs
Answer and explanation in LearnFree account
Question 24 · Hard
A 60-year-old man who grew up in rural Jamaica takes high-dose prednisone. He develops E. coli bacteremia, wheezing and lung infiltrates. A sputum wet mount shows motile larvae about 500 µm long with a notched tail. The most likely parasite is:
1. ANecator americanus
2. BAscaris lumbricoides
3. CStrongyloides stercoralis
4. DParagonimus westermani
Answer and explanation in LearnFree account
Question 25 · Medium
A bone marrow aspirate shows macrophages packed with oval bodies 2-4 µm long, each with a nucleus and a small rod-shaped kinetoplast. These are:
1. AHistoplasma yeast cells
2. BTalaromyces yeast cells
3. CLeishmania amastigotes
4. DToxoplasma tachyzoites
Answer and explanation in LearnFree account

11 more Parasitology questions

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Study Microbiology

Parasitology

On a thin film, 46 of 2,000 red cells counted contain P. falciparum rings, and 4 of these cells hold two rings each. What is the percent parasitemia?

A 70-year-old man from coastal Massachusetts, who had a splenectomy, has fever and hemolysis in July. He has not travelled. His thin film shows small rings, some outside red cells, and tetrads, with no pigment or gametocytes. The most likely cause is:

Two weeks after successful treatment of P. falciparum malaria, a patient's HRP2 rapid antigen test is still positive, but repeated thick and thin films show no parasites. The best explanation is:

A man from the Philippines who eats raw freshwater crabs has chronic cough and hemoptysis. Three AFB smears are negative. Sputum shows golden-brown operculated eggs about 90 µm long, thickened at the end opposite the operculum. The most likely parasite is:

Rhabditiform larvae in fresh stool have a short buccal canal and a large genital primordium. Which parasite is this?

A Giemsa-stained blood film shows C-shaped trypomastigotes with a large kinetoplast at the posterior end. Which organism is most likely?

Which Plasmodium stage does an infected female Anopheles mosquito inject into a person?

A 34-year-old man with advanced HIV has chronic watery diarrhea and blood eosinophilia. A modified acid-fast stain of stool shows red, ellipsoidal oocysts about 25 × 12 µm, each holding a round sporoblast. The most likely organism is:

A cyst 18 µm across has eight nuclei with eccentric karyosomes and uneven peripheral chromatin, plus splinter-shaped chromatoidal bodies. It is:

A man who returned from Ghana 10 days ago has fever, but his first thick and thin films are negative. What should the laboratory advise?

A thin film shows enlarged red cells with Schüffner's dots, amoeboid trophozoites and schizonts with about 16 merozoites. The species is most likely:

A sheathed microfilaria with nuclei running in a continuous row to the tip of the tail is found in blood drawn at midday. The most likely species is:

A stool for ova and parasites arrives 2 days after the patient had a barium enema. What is the best action?

A traveller back from Malaysian Borneo has daily fevers. The thin film shows 3% parasitemia in normal-sized red cells without stippling, with rings, band forms and schizonts of up to 16 merozoites. Which species is most likely?

Enterobius vermicularis eggs are best found in a sample taken:

A permanent stain shows trophozoites 5-15 µm with one or two nuclei, each with its chromatin split into 4-8 granules and no peripheral chromatin. No cysts are seen. Which organism is this?

Round oocysts 8-10 µm across stain from pale to deep red with modified acid-fast stain and glow blue under UV light. Which organism is this?

A stool concentrate shows round eggs 30-35 µm with a thick, radially striated shell and a six-hooked embryo. How should they be reported?

Compared with formalin-ethyl acetate sedimentation, zinc sulfate flotation is more likely to miss:

A 60-year-old man who grew up in rural Jamaica takes high-dose prednisone. He develops E. coli bacteremia, wheezing and lung infiltrates. A sputum wet mount shows motile larvae about 500 µm long with a notched tail. The most likely parasite is:

A bone marrow aspirate shows macrophages packed with oval bodies 2-4 µm long, each with a nucleus and a small rod-shaped kinetoplast. These are: