Parasitology Practice Questions

Q: A 40-year-old man presents with muscle pain, fever, and difficulty breathing. He reports having eaten undercooked pork recently. Muscle biopsy shows encysted larvae. What is the most likely organism?

Trichinella spiralis. ***Trichinella spiralis*** - The patient's symptoms of **muscle pain** and **fever**, coupled with a history of consuming **undercooked pork**, are classic for **trichinellosis**. - **Muscle biopsy showing encysted larvae** definitively confirms infection by *Trichinella spiralis*, as these larvae invade muscle tissue. *Taenia solium* - This organism causes **taeniasis** (intestinal infection) and **cysticercosis** (tissue infection) from consuming undercooked pork. - While it can lead to muscle cysts in cysticercosis, the primary presentation with **muscle pain, fever, and encysted larvae strongly points to trichinellosis**, and the larvae morphology would differ. *Toxocara canis* - This nematode causes **visceral larval migrans** (VLM) when its eggs are ingested, typically from contact with dog feces, leading to larva migration through internal organs. - While it can cause some systemic symptoms and tissue invasion, it's not associated with **undercooked pork consumption** or the characteristic **encysted larvae in muscle** seen with *Trichinella*. *Ancylostoma duodenale* - This is a **hookworm** found in various regions, and infection typically occurs through skin penetration by larvae in contaminated soil, not ingestion of undercooked pork. - It primarily causes **iron deficiency anemia** and gastrointestinal symptoms, not profound muscle pain, fever, or muscle larvae as described.

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

Q: A 6-year-old child presents with severe diarrhea and dehydration. Stool examination reveals the presence of trophozoites with characteristic 'falling leaf' motility. What is the most likely causative agent?

Giardia lamblia. ***Giardia lamblia*** - The presence of **trophozoites** with characteristic **'falling leaf' motility** in stool is pathognomonic for *Giardia lamblia* infection. - *Giardia* infection typically causes **severe diarrhea** and **dehydration**, especially in children. *Entamoeba histolytica* - This parasite causes **amoebic dysentery** and forms **trophozoites** that exhibit **directional motility** with pseudopods, not a 'falling leaf' motion. - Stool examination might reveal **red blood cells** ingested by the trophozoites, indicating intestinal tissue invasion. *Cryptosporidium parvum* - This parasite is characterized by producing **oocysts** that are **acid-fast** and are typically identified via modified acid-fast staining of stool. - It does not produce trophozoites with 'falling leaf' motility; instead, it causes **self-limiting diarrhea** in immunocompetent individuals. *Cyclospora cayetanensis* - Similar to *Cryptosporidium*, **oocysts** of *Cyclospora* are **acid-fast** and larger, found in contaminated food or water. - It causes **prolonged watery diarrhea** but does not present with trophozoites exhibiting 'falling leaf' motility.

Q: Which parasite is the most common cause of chronic malabsorptive diarrhea worldwide and is characterized by pear-shaped trophozoites?

Giardia lamblia. ***Giardia lamblia*** - *Giardia lamblia* is explicitly associated with **chronic malabsorptive diarrhea** globally and is known for its distinctive **pear-shaped trophozoites** with flagella. - It causes **giardiasis**, which often leads to symptoms like **steatorrhea**, abdominal cramps, and weight loss due to malabsorption in the small intestine. *Strongyloides stercoralis* - While *Strongyloides stercoralis* can cause diarrheal illness, it is more commonly associated with **cutaneous larvae currens** and **autoinfection cycles**, leading to chronic infections that can disseminate, particularly in immunocompromised individuals. - It does not typically present with the classic pear-shaped trophozoite and is not the most common cause of malabsorptive diarrhea worldwide, unlike *Giardia*. *Ascaris lumbricoides* - *Ascaris lumbricoides* primarily causes **intestinal obstruction** or **malnutrition** in heavy infections, and its life cycle involves a lung phase, but it is not commonly linked to chronic malabsorptive diarrhea or pear-shaped trophozoites. - Adult worms are large roundworms and are identified by their macroscopic appearance rather than microscopic trophozoite morphology. *Ancylostoma duodenale* - *Ancylostoma duodenale*, a **hookworm**, is known for causing **iron deficiency anemia** due to chronic blood loss from the intestinal attachment sites. - It does not typically cause malabsorptive diarrhea as its primary clinical manifestation and is morphologically distinct from the pear-shaped trophozoites of *Giardia*.

Q: A 45-year-old female presents with abdominal pain, jaundice, and fever. An abdominal CT scan reveals multiple liver abscesses, and aspiration yields anchovy paste-like pus. What is the most likely etiological agent responsible for these findings?

Entamoeba histolytica. ***Entamoeba histolytica*** - The combination of **abdominal pain**, **jaundice**, **fever**, and **multiple liver abscesses** with **anchovy paste-like pus** is highly characteristic of an **amebic liver abscess** caused by *Entamoeba histolytica* [1]. - This parasite is acquired through the **fecal-oral route** and can invade the intestinal wall, leading to extraintestinal dissemination, most commonly to the liver [1]. *Echinococcus granulosus* - This pathogen causes **hydatid cysts**, which are typically **single**, large cysts with an **acellular fluid** rather than a purulent or "anchovy paste-like" consistency [2]. - While it can cause abdominal pain and jaundice, the pus description is not consistent with **hydatid disease** [2]. *Staphylococcus aureus* - **Pyogenic liver abscesses** caused by *Staphylococcus aureus* typically present with **fever** and **abdominal pain** but commonly involve **multiple smaller abscesses** and the pus is typically **creamy** and foul-smelling, not resembling "anchovy paste." - These are often associated with a primary source of infection, such as **bacteremia**. *Klebsiella pneumoniae* - **Klebsiella pneumoniae** is a common cause of **pyogenic liver abscesses**, particularly in patients with **diabetes mellitus**, and can present with fever, abdominal pain, and jaundice [2]. - However, the pus from a *Klebsiella* abscess is typically **thick and purulent** but lacks the distinctive **"anchovy paste" appearance** seen in amebic abscesses.

Q: An immunocompromised patient presents with diarrhea, abdominal pain, and weight loss. Stool microscopy reveals acid-fast oocysts. Evaluate the diagnostic results to identify the most likely causative agent.

Cryptosporidium. ***Cryptosporidium*** - The presence of **acid-fast oocysts** in stool microscopy from an **immunocompromised patient** with diarrhea, abdominal pain, and weight loss is highly characteristic of *Cryptosporidium* infection. - *Cryptosporidium* causes severe, prolonged, and potentially life-threatening diarrhea in immunocompromised individuals, particularly in HIV/AIDS patients. *Giardia lamblia* - *Giardia* is a cause of **diarrhea** and malabsorption, but its cysts are typically identified by microscopy as **oval and non-acid-fast**. - While it can infect immunocompromised patients, the microscopic finding of acid-fast oocysts rules out *Giardia*. *Entamoeba histolytica* - *Entamoeba histolytica* causes **amoebic dysentery** and **liver abscesses**, characterized by passage of trophozoites and cysts in stool. - Its cysts are **non-acid-fast** and have a distinct morphology with chromatoid bodies and multiple nuclei. *Cyclospora cayetanensis* - *Cyclospora* also causes prolonged diarrhea and its oocysts are **acid-fast**, similar to *Cryptosporidium*. - However, *Cyclospora* oocysts are typically **larger** (8-10 µm) than *Cryptosporidium* oocysts (4-6 µm), and while both can cause similar symptoms, *Cryptosporidium* is statistically more common in immunocompromised individuals and represents the most likely diagnosis with acid-fast oocysts in this clinical context.

Q: A 28-year-old man presents with fever and lymphadenopathy. A blood smear shows trypomastigotes with an undulating membrane. Which disease is most likely?

African sleeping sickness. ***African sleeping sickness*** - The presence of **trypomastigotes** with an **undulating membrane** on a blood smear is a characteristic finding in African sleeping sickness, caused by *Trypanosoma brucei*. - Initial symptoms like **fever** and **lymphadenopathy** ("Winterbottom's sign") are common in the hemolymphatic stage of the disease, preceding neurological involvement. *Leishmaniasis* - Leishmaniasis is caused by *Leishmania* species, which appear as **amastigotes** (intracellular, non-flagellated forms) within macrophages, not as trypomastigotes with undulating membranes in blood smears. - While it can cause fever and lymphadenopathy (visceral leishmaniasis), the morphology of the parasite on blood smear is distinctly different. *Malaria* - Malaria is caused by *Plasmodium* species, which manifest as various red blood cell stages (ring forms, trophozoites, schizonts, gametocytes) on blood smears, but not as **trypomastigotes** with undulating membranes. - Though fever is a prominent symptom, lymphadenopathy is less common, and the parasite morphology is key for differentiation. *Chagas disease* - Chagas disease, caused by *Trypanosoma cruzi*, presents as **trypomastigotes** in the blood during the acute phase. However, *T. cruzi* trypomastigotes are typically C-shaped and lack a prominent undulating membrane visible on routine blood smears, unlike *T. brucei*. - While fever and lymphadenopathy can occur, the distinct undulating membrane points away from *T. cruzi*.

Q: Which organism is most commonly involved in the pathogenesis of diarrhea in HIV-infected patients and is diagnosed by modified acid-fast staining of fecal matter?

Cryptosporidium. ***Cryptosporidium*** - **Cryptosporidium parvum** is a common cause of **chronic diarrheal illness** in **HIV-infected patients**, particularly those with a low **CD4 count**. - Its oocysts are **acid-fast**, allowing for diagnosis using **modified acid-fast staining** of fecal samples. *Giardia lamblia* - While *Giardia lamblia* can cause **diarrhea** in immunocompromised individuals, it is not typically the **most common opportunistic infection** in HIV-infected patients. - Diagnosis is primarily by identifying **cysts** or **trophozoites** in stool using **wet mounts** or **antigen tests**, not typically modified acid-fast staining. *Isospora belli* - *Isospora belli* is an **opportunistic parasite** that can cause **diarrhea** in HIV patients and its oocysts are also **acid-fast**. - However, it is **less common** than *Cryptosporidium* as a primary cause of severe, chronic diarrhea in this population. *Entamoeba histolytica* - *Entamoeba histolytica* causes **amoebic dysentery** characterized by bloody diarrhea, but it is not typically an **opportunistic infection** strongly linked to HIV-related immunodeficiency. - Diagnosis involves identifying **trophozoites** or **cysts** in stool, and they are **not acid-fast**.

Q: A 30-year-old female presents with abdominal pain, diarrhea, and weight loss. Stool microscopy reveals ova with a characteristic lateral spine. What is the most likely diagnosis?

Schistosomiasis. ***Schistosomiasis*** - The presence of **ova with a characteristic lateral spine** in stool microscopy is pathognomonic for *Schistosoma mansoni*, a common cause of **schistosomiasis**. - **Abdominal pain**, **diarrhea**, and **weight loss** are consistent symptoms of intestinal schistosomiasis. *Giardiasis* - Caused by *Giardia lamblia*, which presents as **cysts or trophozoites** in stool, not ova with spines. - Symptoms often include **malabsorption**, **bloating**, and **greasy stools**. *Amebiasis* - Caused by *Entamoeba histolytica*, identified by **trophozoites with ingested red blood cells** or **cysts** in stool. - Can cause **dysentery** (bloody diarrhea) or **amoebic liver abscess**, without characteristic spined ova. *Ascariasis* - Caused by *Ascaris lumbricoides*, which produces **mammillated or decorticated eggs** in stool, not spined ova. - Symptoms can include **abdominal discomfort** and **intestinal obstruction** with heavy worm burdens.

Question 1

A 40-year-old man presents with muscle pain, fever, and difficulty breathing. He reports having eaten undercooked pork recently. Muscle biopsy shows encysted larvae. What is the most likely organism?

Accepted Answer

Trichinella spiralis

Answer

Taenia solium

Answer

Toxocara canis

Answer

Ancylostoma duodenale

Question 2

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

Question 3

A 6-year-old child presents with severe diarrhea and dehydration. Stool examination reveals the presence of trophozoites with characteristic 'falling leaf' motility. What is the most likely causative agent?

Accepted Answer

Giardia lamblia

Answer

Entamoeba histolytica

Answer

Cryptosporidium parvum

Answer

Cyclospora cayetanensis

Question 4

Which parasite is the most common cause of chronic malabsorptive diarrhea worldwide and is characterized by pear-shaped trophozoites?

Accepted Answer

Giardia lamblia

Answer

Strongyloides stercoralis

Answer

Ascaris lumbricoides

Answer

Ancylostoma duodenale

Question 5

A 45-year-old female presents with abdominal pain, jaundice, and fever. An abdominal CT scan reveals multiple liver abscesses, and aspiration yields anchovy paste-like pus. What is the most likely etiological agent responsible for these findings?

Accepted Answer

Entamoeba histolytica

Answer

Staphylococcus aureus

Answer

Klebsiella pneumoniae

Answer

Echinococcus granulosus

Question 6

Which stool examination technique is considered the most effective for detecting Giardia lamblia based on diagnostic sensitivity and specificity?

Accepted Answer

Formalin-ether concentration method for stool examination.

Answer

ELISA method for detecting Giardia lamblia.

Answer

Microscopic examination of stool samples.

Answer

PCR-based molecular techniques for Giardia lamblia.

Question 7

An immunocompromised patient presents with diarrhea, abdominal pain, and weight loss. Stool microscopy reveals acid-fast oocysts. Evaluate the diagnostic results to identify the most likely causative agent.

Accepted Answer

Cryptosporidium

Answer

Giardia lamblia

Answer

Entamoeba histolytica

Answer

Cyclospora cayetanensis

Question 8

A 28-year-old man presents with fever and lymphadenopathy. A blood smear shows trypomastigotes with an undulating membrane. Which disease is most likely?

Accepted Answer

African sleeping sickness

Answer

Leishmaniasis

Answer

Malaria

Answer

Chagas disease

Question 9

Which organism is most commonly involved in the pathogenesis of diarrhea in HIV-infected patients and is diagnosed by modified acid-fast staining of fecal matter?

Accepted Answer

Cryptosporidium

Answer

Giardia lamblia

Answer

Isospora belli

Answer

Entamoeba histolytica

Question 10

A 30-year-old female presents with abdominal pain, diarrhea, and weight loss. Stool microscopy reveals ova with a characteristic lateral spine. What is the most likely diagnosis?

Accepted Answer

Schistosomiasis

Answer

Giardiasis

Answer

Amebiasis

Answer

Ascariasis

Parasitology — MCQs

Parasitology — MCQs

On this page

Practice by Chapter

Want unlimited practice?