Parasitology Practice Questions

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

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: A patient with AIDS presents with fever and weight loss. A biopsy of a lymph node shows intracellular amastigotes. Which organism is likely responsible?

Leishmania donovani (causes visceral leishmaniasis). ***Leishmania donovani (causes visceral leishmaniasis)*** - The presence of **intracellular amastigotes** in a lymph node biopsy, combined with fever and weight loss in an **AIDS patient**, is highly suggestive of **visceral leishmaniasis** caused by *Leishmania donovani* [1]. - **Immunocompromized individuals**, particularly those with AIDS, are highly susceptible to severe and disseminated forms of leishmaniasis, leading to **visceral involvement** [1]. *Trypanosoma cruzi (causes Chagas disease)* - While *Trypanosoma cruzi* is an intracellular parasite and can be found in tissue, its amastigotes are typically found within **cardiac and smooth muscle cells**, not primarily in lymph nodes. - **Chagas disease** primarily affects the heart and gastrointestinal tract, and while amastigotes are seen, the overall clinical context and preferred tissue for biopsy differ. *Plasmodium falciparum (causes malaria)* - *Plasmodium falciparum* are **intracellular parasites** that infect **red blood cells**, not lymph nodes. - The classic presentation of malaria involves **cyclic fevers**, chills, and anemia, and the diagnosis is made by identifying parasites in **blood smears**. *Toxoplasma gondii (opportunistic infection)* - *Toxoplasma gondii* can cause disseminated disease in **AIDS patients**, particularly **encephalitis**, and forms **cysts** (bradyzoites) in various tissues including the brain, muscle, and lymph nodes. - However, the characteristic finding in *Toxoplasma* infection is usually **cysts** or **tachyzoites** in tissue, not the distinct amastigote form seen in Leishmania, and lymph node involvement is often as a toxoplasmic lymphadenitis rather than disseminated amastigotes.

Q: A patient develops watery diarrhea after attending a wedding. Stool examination reveals acid-fast oocysts. Which organism is most likely responsible?

Cryptosporidium hominis. ***Cryptosporidium hominis*** - The presence of **acid-fast oocysts** in stool is a characteristic diagnostic finding for *Cryptosporidium* infection. - The onset of **watery diarrhea** after a wedding suggests a common-source outbreak, which is **most commonly** caused by *Cryptosporidium* in waterborne or foodborne transmission. - *Cryptosporidium* is the **most frequent cause** of acid-fast oocyst-associated diarrhea in both immunocompetent and immunocompromised patients. *Giardia lamblia* - This organism causes **giardiasis**, which typically presents with **greasy, foul-smelling diarrhea** and malabsorption, rather than purely watery diarrhea. - Diagnosis involves identifying **trophozoites or cysts** in stool using microscopy, but these are **not acid-fast**. *Entamoeba histolytica* - Causes **amoebic dysentery**, characterized by **bloody diarrhea** (colitis) due to tissue invasion, and can also form liver abscesses. - Stool examination would show **trophozoites with ingested red blood cells** or cysts, which are **not acid-fast**. *Cyclospora cayetanensis* - While *Cyclospora* also causes **watery diarrhea** and produces **acid-fast oocysts**, it is **significantly less common** than *Cryptosporidium* in outbreak settings. - *Cyclospora* oocysts are **larger** (8-10 µm vs. 4-6 µm) and often **autofluorescent** under UV light, but these distinguishing features require specific mention in the clinical scenario. - In the absence of specific oocyst size or other distinguishing features, **Cryptosporidium is the most likely diagnosis** given its higher prevalence in waterborne/foodborne outbreaks.

Q: A neonate develops severe jaundice and hepatosplenomegaly shortly after birth. Laboratory tests reveal elevated IgM antibodies against Toxoplasma gondii. What is the most likely route of transmission?

Vertical transmission from mother. ***Vertical transmission from mother*** - The presence of **IgM antibodies** in a neonate indicates a **recent or active infection**, which is characteristic of congenital infections. - **Toxoplasma gondii** can be transmitted **transplacentally** from an infected mother to her fetus, leading to symptoms like jaundice and hepatosplenomegaly at birth. *Ingestion of contaminated water* - This route of transmission is typical for acquiring Toxoplasma gondii in older children or adults, usually through oocysts in contaminated water or food. - It would not explain the congenital presentation with immediate symptoms and specific IgM antibodies in a neonate shortly after birth. *Inhalation of oocysts* - While Toxoplasma gondii oocysts can be airborne, inhalation is a rare route of infection compared to ingestion, and certainly not the primary mode of congenital transmission. - This route would be more likely to cause symptoms in an individual with a compromised immune system in an environment with high oocyst load, not a neonate. *Vector-borne transmission* - Toxoplasma gondii is not transmitted by vectors (such as mosquitoes or ticks), but rather through ingestion of contaminated food/water or vertical transmission. - This route is incorrect for Toxoplasma and would not explain the congenital symptoms observed.

Q: In an HIV-positive patient presenting with headache, confusion, and fever, which of the following is the most likely causative organism for the multiple ring-enhancing lesions observed on MRI?

Toxoplasma gondii. ***Toxoplasma gondii*** - **Toxoplasmosis** is the most common cause of **multiple ring-enhancing lesions** on MRI in HIV-positive patients, especially with headache, confusion, and fever. - The lesions are typically seen in the **basal ganglia** and **corticomedullary junction**. *Cryptococcus neoformans* - Primarily causes **meningoencephalitis** in HIV-positive patients, characterized by diffuse leptomeningeal enhancement rather than ring-enhancing lesions on MRI. - While it can cause cryptococcomas (mass lesions), these are less common and typically do not present as multiple ring-enhancing lesions. *JC Virus* - Causes **progressive multifocal leukoencephalopathy (PML)**, which presents with non-enhancing white matter lesions on MRI, often without mass effect. - PML lesions are characterized by demyelination and usually do not show ring enhancement. *Cytomegalovirus* - Can cause CMV encephalitis or ventriculitis in HIV-positive patients, which often presents with periventricular enhancement and microabscesses. - While multifocal lesions can occur, classic multiple ring-enhancing lesions are not the typical presentation for CMV encephalitis.

Q: A 40-year-old man who recently traveled to South America presents with a chronic, ulcerative lesion on his arm. A biopsy reveals amastigotes within macrophages. What is the most likely diagnosis?

Cutaneous leishmaniasis. ### Cutaneous leishmaniasis - The patient's recent travel to **South America**, a region endemic for Leishmaniasis, combined with a chronic, **ulcerative lesion** and the presence of **amastigotes within macrophages** on biopsy, are classic diagnostic features [1]. - Amastigotes are the non-motile, intracellular form of Leishmania parasites found in human tissues [1]. *Cutaneous tuberculosis* - While it can manifest as chronic ulcerative lesions, the biopsy would show **granulomas with caseation necrosis** containing acid-fast bacilli, not amastigotes within macrophages. - Diagnosis typically involves isolation of *Mycobacterium tuberculosis* or molecular tests, which is not described. *Sporotrichosis* - Characterized by **nodular lesions** that can ulcerate and spread along lymphatic channels, often associated with a history of **thorn prick** or contact with decaying vegetation. - Biopsy would reveal **cigar-shaped budding yeasts** (Sporothrix schenckii), not amastigotes within macrophages. *Mycetoma* - Presents as a chronic, destructive infection primarily affecting the **feet**, characterized by **swelling, sinuses, and grains** (macroscopic aggregates of microorganisms) being discharged. - Histology would show **granulomas with characteristic grains** (fungal hyphae or bacterial filaments), significantly different from amastigotes.

Q: A patient presents with diarrhea, bloating, and weight loss. Stool analysis shows pear-shaped, flagellated protozoa. What is the most likely diagnosis?

Giardiasis. Giardiasis - The classic presentation of diarrhea, bloating, and weight loss combined with the microscopic finding of pear-shaped, flagellated protozoa [1] in stool is pathognomonic for Giardia lamblia infection [1]. - Giardia trophozoites are easily identifiable by their distinctive morphology [1] and motility in fresh stool preparations. Amebiasis - Caused by Entamoeba histolytica, which typically presents with bloody diarrhea (dysentery) [2] and abdominal pain. - Trophozoites are larger, amoeboid, and lack flagella; they may contain ingested red blood cells. Ascariasis - This is an infection by a roundworm (Ascaris lumbricoides), a multicellular parasite, not a single-celled protozoan. - Symptoms can include abdominal pain, malnutrition, or pulmonary involvement during larval migration, but pear-shaped flagellated protozoa would not be seen. Schistosomiasis - Caused by a fluke (Schistosoma species), a type of flatworm, which is also a multicellular parasite, not a protozoan. - Symptoms vary depending on the species and stage of infection but often involve urinary or intestinal inflammation and are diagnosed by finding eggs (not flagellated protozoa) in stool or urine.

Q: A 45-year-old male presents with diarrhea and abdominal pain after consuming undercooked beef. Stool examination reveals eggs. What is the most likely cause?

Taenia saginata. ***Taenia saginata*** - The consumption of **undercooked beef** (from which *Taenia saginata* is acquired) followed by **diarrhea, abdominal pain**, and the presence of **eggs in stool** is highly indicative of taeniasis caused by *Taenia saginata* (beef tapeworm). - *Taenia saginata* infections are characterized by **proglottid (tapeworm segment) passage** in stool, which can also be a key diagnostic feature, though only eggs are mentioned here. *Giardia lamblia* - This parasite causes **giardiasis**, typically spread through contaminated water or food, not specifically undercooked meat. - Symptoms include **cramps, bloating, and foul-smelling, fatty diarrhea**, but stool examination reveals **cysts or trophozoites**, not helminth eggs. *Ascaris lumbricoides* - This is a **roundworm** infection, typically acquired through contaminated soil or food, and is not specifically linked to undercooked meat. - While it can cause **abdominal pain and malnutrition**, its eggs are distinctive **oval, mammillated eggs** and are not usually associated with meat consumption in this context. *Entamoeba histolytica* - This protozoan causes **amebiasis**, usually acquired through contaminated food or water, often in areas with poor sanitation. - While it causes **diarrhea (often bloody)** and **abdominal pain**, stool examination would reveal **cysts or trophozoites**, not helminth eggs.

Q: Which pathogen is commonly responsible for causing diarrhea in patients with HIV/AIDS?

Cryptosporidium. ***Cryptosporidium*** - **Cryptosporidium** is the most common opportunistic pathogen causing diarrhea in HIV/AIDS patients, especially those with CD4 counts <200 cells/μL - Causes **chronic, severe, watery diarrhea** that can be life-threatening in immunocompromised individuals - The parasite's ability to cause disease is amplified due to impaired cell-mediated immunity, resulting in **prolonged infection** and malabsorption - Cryptosporidiosis is an AIDS-defining opportunistic infection *Giardia lamblia* - While **Giardia lamblia** causes diarrhea (giardiasis), it is not typically an opportunistic infection specific to HIV/AIDS - Affects both immunocompetent and immunocompromised individuals equally - Symptoms include **greasy, foul-smelling stools** and **abdominal cramps**, but rarely causes the severe, chronic wasting diarrhea seen with cryptosporidiosis in AIDS *Escherichia coli* - Various strains of **E. coli** can cause diarrhea in both immunocompetent and immunocompromised individuals - Not uniquely associated with severe, chronic diarrhea in HIV/AIDS as an opportunistic pathogen - Can cause traveler's diarrhea or food poisoning, but typically doesn't lead to the severe wasting syndrome characteristic of AIDS-related opportunistic infections *Clostridium difficile* - **C. difficile** infection is primarily associated with **antibiotic use** disrupting the gut microbiota, leading to pseudomembranous colitis - While HIV-positive individuals can develop C. difficile infection, it's not an AIDS-defining opportunistic infection - Related to antibiotic exposure rather than HIV-induced immunosuppression directly

Question 1

What is the primary causative agent of scabies?

Accepted Answer

Sarcoptes scabiei

Answer

Pediculus humanus

Answer

Pthirus pubis

Answer

Demodex folliculorum

Question 2

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 3

A patient with AIDS presents with fever and weight loss. A biopsy of a lymph node shows intracellular amastigotes. Which organism is likely responsible?

Accepted Answer

Leishmania donovani (causes visceral leishmaniasis)

Answer

Trypanosoma cruzi (causes Chagas disease)

Answer

Plasmodium falciparum (causes malaria)

Answer

Toxoplasma gondii (opportunistic infection)

Question 4

A patient develops watery diarrhea after attending a wedding. Stool examination reveals acid-fast oocysts. Which organism is most likely responsible?

Accepted Answer

Cryptosporidium hominis

Answer

Giardia lamblia

Answer

Entamoeba histolytica

Answer

Cyclospora cayetanensis

Question 5

A neonate develops severe jaundice and hepatosplenomegaly shortly after birth. Laboratory tests reveal elevated IgM antibodies against Toxoplasma gondii. What is the most likely route of transmission?

Accepted Answer

Vertical transmission from mother

Answer

Ingestion of contaminated water

Answer

Inhalation of oocysts

Answer

Vector-borne transmission

Question 6

In an HIV-positive patient presenting with headache, confusion, and fever, which of the following is the most likely causative organism for the multiple ring-enhancing lesions observed on MRI?

Accepted Answer

Toxoplasma gondii

Answer

Cryptococcus neoformans

Answer

JC Virus

Answer

Cytomegalovirus

Question 7

A 40-year-old man who recently traveled to South America presents with a chronic, ulcerative lesion on his arm. A biopsy reveals amastigotes within macrophages. What is the most likely diagnosis?

Accepted Answer

Cutaneous leishmaniasis

Answer

Cutaneous tuberculosis

Answer

Sporotrichosis

Answer

Mycetoma

Question 8

A patient presents with diarrhea, bloating, and weight loss. Stool analysis shows pear-shaped, flagellated protozoa. What is the most likely diagnosis?

Accepted Answer

Giardiasis

Answer

Amebiasis

Answer

Ascariasis

Answer

Schistosomiasis

Question 9

A 45-year-old male presents with diarrhea and abdominal pain after consuming undercooked beef. Stool examination reveals eggs. What is the most likely cause?

Accepted Answer

Taenia saginata

Answer

Giardia lamblia

Answer

Ascaris lumbricoides

Answer

Entamoeba histolytica

Question 10

Which pathogen is commonly responsible for causing diarrhea in patients with HIV/AIDS?

Accepted Answer

Cryptosporidium

Answer

Giardia lamblia

Answer

Escherichia coli

Answer

Clostridium difficile

Parasitology — MCQs

Parasitology — MCQs

On this page

Practice by Chapter

Want unlimited practice?