Parasitology Practice Questions

Q: Episodes of repeated thin stools with mucus, subjective feeling of fever and lower abdominal pain, with leukocytes in stool. Which of the following is likely?

Entamoeba. ***Entamoeba*** - The presence of **leukocytes in stool** along with symptoms like repeated thin stools with mucus, subjective fever, and lower abdominal pain are characteristic of **invasive amoebiasis** caused by *Entamoeba histolytica* [1]. - *Entamoeba histolytica* is known to cause **amoebic dysentery**, which involves inflammation and ulceration of the colon, leading to the presence of inflammatory cells in the stool [1]. *Clostridium perfringens* - This bacterium typically causes **food poisoning** with symptoms of abdominal cramps and watery diarrhea, which is usually **self-limiting** and does not typically involve significant inflammation or leukocytes in stool. - While it can cause necrotizing enteritis in severe cases, the more common presentation does not align with the description of mucosal stools and leukocytes. *Staphylococcus aureus* - *S. aureus* causes **food poisoning** through the ingestion of preformed toxins, leading to rapid onset of nausea, vomiting, and non-bloody diarrhea. - This condition is typically **short-lived** and does not usually involve leukocytes in the stool or significant inflammation, primarily affecting the upper gastrointestinal tract. *Giardia* - *Giardia lamblia* causes **giardiasis**, characterized by chronic diarrhea, malabsorption, bloating, and foul-smelling stools, but generally **does not cause invasive disease** or significant inflammation in the intestinal lining [2]. - Stool analysis in giardiasis typically shows **no leukocytes** or red blood cells, as the parasite does not invade the intestinal wall [2].

Q: A wet mount of vaginal discharge shows motile, flagellated protozoa. Which organism is most likely causing this infection?

Trichomonas vaginalis. ***Trichomonas vaginalis*** - This **motile, flagellated protozoan** is directly visualized on a wet mount from vaginal discharge and is the causative agent of **trichomoniasis**. - Infection often presents with a **foamy, yellow-green discharge**, pruritus, and dysuria. *Gardnerella vaginalis* - This bacterium is associated with **bacterial vaginosis**, characterized by **"clue cells"** (epithelial cells covered in bacteria) on wet mount, and a **fishy odor**, not flagellated protozoa. - While *Gardnerella* is a common resident, its overgrowth, along with other anaerobes, leads to clinical bacterial vaginosis. *Candida albicans* - This is a **yeast** that causes **vulvovaginal candidiasis**, characterized by a **cottage cheese-like discharge** and intense itching. - On wet mount, **yeast buds and pseudohyphae** would be observed, not flagellated protozoa. *Lactobacillus* - These are **normal, beneficial bacteria** of the vaginal flora, maintaining an acidic pH. - Their presence indicates a **healthy vaginal environment**, and they do not appear as motile, flagellated protozoa.

Q: A blood smear from a patient with cyclic fevers shows Maltese cross formations within red blood cells. Which organism is the likely cause?

Babesia microti. ***Babesia microti*** - The presence of **Maltese cross formations** (tetrads of merozoites) within red blood cells on a blood smear is a **pathognomonic sign** of *Babesia microti* infection. - *Babesia microti* causes **babesiosis**, a tick-borne illness known to cause **cyclic fevers**, hemolytic anemia, and can be particularly severe in asplenic individuals. *Leishmania donovani* - *Leishmania donovani* is the causative agent of **visceral leishmaniasis** (kala-azar), which is characterized by irregular fever, hepatosplenomegaly, and pancytopenia. - Diagnosis involves finding **amastigotes** within macrophages in bone marrow, spleen, or lymph nodes, not Maltese cross formations in red blood cells. *Plasmodium vivax* - *Plasmodium vivax* causes **tertian malaria**, characterized by fevers recurring every 48 hours, and infects **young red blood cells**, leading to their enlargement with **Schüffner's dots**. - While it infects red blood cells and causes fever, it does not form Maltese cross structures; instead, ring forms, trophozoites, schizonts, and gametocytes are observed. *Plasmodium falciparum* - *Plasmodium falciparum* causes the **most severe form of malaria**, characterized by irregular fevers and the presence of **crescent-shaped gametocytes** and small, delicate ring forms in red blood cells. - It does not produce Maltese cross formations; its characteristic morphology and severe clinical presentation differentiate it from babesiosis.

Q: A 30-year-old man with HIV presents with chronic diarrhea. Stool examination reveals small, spherical, acid-fast oocysts. What is the causative agent?

Cryptosporidium parvum. ***Cryptosporidium parvum*** - This is the **most common cause** of chronic, watery diarrhea in immunocompromised individuals, particularly those with **HIV/AIDS**. - Its oocysts are **acid-fast**, small (4-6 μm), and typically **spherical**, readily identified in stool samples using modified acid-fast staining methods. - Causes self-limiting diarrhea in immunocompetent hosts but severe, persistent diarrhea in AIDS patients with CD4 count <200. *Entamoeba histolytica* - Causes **amoebic dysentery** characterized by bloody diarrhea, abdominal pain, and liver abscesses, but its cysts are **not acid-fast**. - Identification involves finding **trophozoites with ingested red blood cells** or cysts in stool, not acid-fast oocysts. *Isospora belli (Cystoisospora belli)* - Also causes chronic diarrhea in HIV patients and produces acid-fast oocysts, but its oocysts are **ellipsoidal, much larger (20-30 μm)**, and contain two sporoblasts. - While also acid-fast, its morphology in stool microscopy differs significantly from the smaller, round oocysts characteristic of *Cryptosporidium*. *Giardia lamblia* - Causes **giardiasis**, characterized by non-bloody, foul-smelling diarrhea, steatorrhea, and malabsorption. - Its cysts and trophozoites are identified in stool, but they are **not acid-fast** and have distinct pear-shaped trophozoites and oval cysts with nuclei.

Q: Which is the primary host cell for Plasmodium vivax during its human life cycle?

Liver cells. ***Liver cells*** - Following infection by an **Anopheles mosquito**, **Plasmodium sporozoites** first travel to the liver and infect hepatocytes. - In the liver cells, they undergo **asexual reproduction** (schizogony) to form merozoites, which are then released into the bloodstream. *Red blood cells* - While **merozoites** released from the liver eventually infect red blood cells, **erythrocytic stages** are not the *primary* host cell where the initial infection and multiplication take place. - The parasite causes clinical symptoms (fever, chills) during this stage, but the liver is the first human cell type infected. *Macrophages* - **Macrophages** are part of the immune system and play a role in clearing infected cells or parasites but are not the primary host cells for *Plasmodium vivax* replication. - They may phagocytose parasites, but they do not serve as a site for significant parasitic proliferation. *Endothelial cells* - **Endothelial cells** line blood vessels and are generally associated with severe complications of *Plasmodium falciparum* due to cytoadherence, but they are not primary host cells for *Plasmodium vivax* replication. - *P. vivax* does not typically cause the sequestration in endothelial cells seen with *P. falciparum*.

Q: Which of the following organisms show parthenogenesis?

Strongyloides. ***Correct: Strongyloides*** - *Strongyloides stercoralis* is known for its complex life cycle, which includes **parthenogenetic reproduction** in the free-living female generation. - The parasitic females can produce larvae directly through **parthenogenesis (reproduction without fertilization)**, enabling autoinfection. - This unique ability allows the parasite to **reproduce without a male** within and outside the human host, leading to persistent infections and hyperinfection syndrome. *Incorrect: Ascaris* - *Ascaris lumbricoides* reproduces sexually, requiring **both male and female worms** for fertilization and egg production. - There is no evidence of parthenogenetic reproduction in *Ascaris*. *Incorrect: Trichuris* - *Trichuris trichiura* (whipworm) is a **dioecious** (sexually reproducing) nematode where **separate male and female worms** are required for reproduction. - Parthenogenesis is not observed in the life cycle of *Trichuris*. *Incorrect: Ancylostoma* - *Ancylostoma duodenale* (hookworm) reproduces sexually in the human intestine. - Requires **male and female worms** to produce fertilized eggs; parthenogenesis does not occur.

Q: What is the best investigation for identifying malaria species?

Thin smear with Giemsa. ***Thin smear with Giemsa*** - A **thin smear** allows for the visualization of **parasite morphology** within red blood cells, which is crucial for distinguishing between species of *Plasmodium*. - **Giemsa stain** provides optimal contrast for identifying characteristic features such as **merozoites**, **trophozoites**, **schizonts**, and **gametocytes** of different malaria species. *Thick smear* - A **thick smear** is primarily used for **detecting the presence of malaria parasites** and for quantifying parasite density due to its higher sensitivity. - However, because red blood cells are lysed, it **does not preserve parasite morphology** well, making species identification difficult. *QBC* - **Quantitative Buffy Coat (QBC) analysis** is a rapid method for detecting malaria parasites based on their fluorescence under UV light. - While sensitive for detection, it generally **does not allow for precise species identification** due to the lack of clear morphological detail. *Thin smear with acridine orange* - A **thin smear stained with acridine orange** is used for rapid detection of parasites by fluorescence microscopy. - Similar to QBC, it is **less effective for detailed morphological examination** and specific species identification compared to Giemsa-stained thin smears.

Q: A 23-year-old male returns from a hiking trip with complaints of diarrhea, abdominal cramps, and fatigue. Stool microscopy reveals flagellated trophozoites. What is the most likely diagnosis?

Giardia lamblia. Giardia lamblia - The presence of flagellated trophozoites in a stool sample from a patient with a history of a hiking trip strongly points to Giardia lamblia infection, often acquired from contaminated water [1]. - Symptoms like diarrhea, abdominal cramps, and fatigue are characteristic of giardiasis, which can lead to malabsorption [1]. Entamoeba histolytica - While it causes diarrhea and abdominal pain, this parasite is identified by amoeboid trophozoites with pseudopods and often presents with bloody stools, not flagella [2]. - It can also cause liver abscesses, which is not indicated here [2]. Cryptosporidium hominis - This parasite is identified by its oocysts in stool samples, not flagellated trophozoites, and can cause severe, protracted watery diarrhea, particularly in immunocompromised individuals. - It does not present as a flagellated form in diagnostic samples. Trichomonas vaginalis - This is a flagellated parasite, but it primarily causes genitourinary infections and is found in the reproductive tract, not typically in stool samples causing gastrointestinal symptoms. - It is transmitted sexually and does not cause traveler's diarrhea.

Q: What is the primary causative agent of scabies?

Sarcoptes scabiei. ***Sarcoptes scabiei*** - _Sarcoptes scabiei_ var. _hominis_ is the **mite** responsible for causing scabies in humans - These mites burrow into the superficial layer of the **epidermis**, leading to intense itching and characteristic skin lesions - Scabies is transmitted through **prolonged skin-to-skin contact** and causes **nocturnal pruritus** *Pediculus humanus* - _Pediculus humanus_ refers to human lice, specifically head lice (_P. humanus capitis_) and body lice (_P. humanus humanus_) - While they cause pruritus and skin irritation (pediculosis), they are ectoparasites that do not burrow into the skin like scabies mites *Pthirus pubis* - _Pthirus pubis_, commonly known as the **pubic louse** or "crab louse," infests the coarse hair of the human body, primarily the pubic region - It causes intense itching in affected areas but does not burrow into the skin like the scabies mite *Demodex folliculorum* - _Demodex folliculorum_ are mites that commonly reside in or near **hair follicles** of mammals, including humans - While often asymptomatic, they can contribute to skin conditions such as **rosacea** and **demodicosis**, but they are not the cause of scabies

Question 1

Episodes of repeated thin stools with mucus, subjective feeling of fever and lower abdominal pain, with leukocytes in stool. Which of the following is likely?

Accepted Answer

Entamoeba

Answer

Clostridium perfringens

Answer

Staphylococcus aureus

Answer

Giardia

Question 2

A wet mount of vaginal discharge shows motile, flagellated protozoa. Which organism is most likely causing this infection?

Accepted Answer

Trichomonas vaginalis

Answer

Gardnerella vaginalis

Answer

Candida albicans

Answer

Lactobacillus

Question 3

A blood smear from a patient with cyclic fevers shows Maltese cross formations within red blood cells. Which organism is the likely cause?

Accepted Answer

Babesia microti

Answer

Leishmania donovani

Answer

Plasmodium vivax

Answer

Plasmodium falciparum

Question 4

A 30-year-old man with HIV presents with chronic diarrhea. Stool examination reveals small, spherical, acid-fast oocysts. What is the causative agent?

Accepted Answer

Cryptosporidium parvum

Answer

Entamoeba histolytica

Answer

Isospora belli

Answer

Giardia lamblia

Question 5

Which is the primary host cell for Plasmodium vivax during its human life cycle?

Accepted Answer

Liver cells

Answer

Red blood cells

Answer

Macrophages

Answer

Endothelial cells

Question 6

Assertion: Plasmodium falciparum does not show the schizont stage in the peripheral blood. Reason: This is due to cytoadherence of infected RBCs to the vascular endothelium.

Accepted Answer

Assertion is true, reason is true and reason is the correct explanation of the assertion.

Answer

Assertion is true, reason is true but reason is not the correct explanation of the assertion.

Answer

Assertion is true, reason is false.

Answer

Assertion is false, reason is true.

Question 7

Which of the following organisms show parthenogenesis?

Accepted Answer

Strongyloides

Answer

Ascaris

Answer

Trichuris

Answer

Ancylostoma

Question 8

What is the best investigation for identifying malaria species?

Accepted Answer

Thin smear with Giemsa

Answer

Thick smear

Answer

QBC

Answer

Thin smear with acridine orange

Question 9

A 23-year-old male returns from a hiking trip with complaints of diarrhea, abdominal cramps, and fatigue. Stool microscopy reveals flagellated trophozoites. What is the most likely diagnosis?

Accepted Answer

Giardia lamblia

Answer

Entamoeba histolytica

Answer

Cryptosporidium hominis

Answer

Trichomonas vaginalis

Question 10

What is the primary causative agent of scabies?

Accepted Answer

Sarcoptes scabiei

Answer

Pediculus humanus

Answer

Pthirus pubis

Answer

Demodex folliculorum

Parasitology — MCQs

Parasitology — MCQs

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