Parasitology Practice Questions

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.

Q: A 35-year-old heterosexual patient diagnosed with HIV had a history of chronic watery diarrhea. A colonoscopic biopsy is shown below. What is the most probable diagnosis?

Cryptosporidiosis. ***Cryptosporidiosis*** - The image likely shows **intracellular parasitic organisms** on the luminal surface of the intestinal epithelium, consistent with *Cryptosporidium parvum*. - In an HIV-positive patient with **chronic watery diarrhea**, cryptosporidiosis is a common and often severe opportunistic infection. *Giardiasis* - Caused by *Giardia lamblia*, which typically presents as **trophozoites or cysts** in the stool or duodenum, not usually as intracellular organisms. - While it can cause chronic diarrhea, the characteristic biopsy findings would differ significantly. *Cytomegalovirus (CMV) infection* - CMV colitis presents with characteristic large **intranuclear and intracytoplasmic inclusions** within infected cells, often with associated inflammation and ulceration. - While CMV is common in HIV, the described image is more suggestive of a parasitic infection. *Microsporidiosis* - Microsporidia are also **intracellular parasites** that can cause chronic diarrhea in HIV patients and are very small (1-4 µm). - However, they are typically much **smaller than Cryptosporidium** and require specialized stains (e.g., modified trichrome) to be easily visualized, and the image provided is more typical of *Cryptosporidium*.

Q: A 15-year-old boy presented with fever and chills for 3 days. On examination, he was found to have delayed skin pinch time and dry oral mucosa. Identify the pathogen involved based on the provided peripheral blood smear image.

Plasmodium vivax. ***Plasmodium vivax*** - The image shows **enlarged red blood cells** infected with various stages of *Plasmodium vivax*, including trophozoites and schizonts displaying **ameboid forms**. - The presence of **Schüffner's dots**, though not distinctly visible in this specific resolution, is characteristic of *P. vivax* infection. - *P. vivax* preferentially infects **reticulocytes** and young red blood cells, leading to the characteristic RBC enlargement. *Babesia* - *Babesia* infection typically presents with **ring forms** in red blood cells that lack pigment and often form **tetrads** (Maltese cross appearance), which are not seen here. - While it can cause fever and chills, the morphology of the parasites in the image is inconsistent with *Babesia*. *Plasmodium falciparum* - *P. falciparum* characteristically presents with **multiple small ring forms** in a single red blood cell and **crescent-shaped gametocytes**. - It infects red blood cells of all ages, does not typically enlarge the red blood cells, and early trophozoites (*ring forms*) are the most common stage seen in peripheral blood, which differs from the image. *Salmonella typhi* - *Salmonella typhi* is a bacterium that causes **typhoid fever** and is a systemic infection. - It does not infect red blood cells or present with intraerythrocytic parasites on a peripheral blood smear; diagnosis is typically made by **blood culture**.

Q: Identify the organism shown in the image.

Human whipworm (Trichuris trichiura). ***Human whipworm (Trichuris trichiura)*** - This organism is characterized by its **whip-like morphology**, with a long, thin anterior end and a thicker posterior end. - It is known to cause **trichuriasis**, primarily affecting the large intestine, leading to symptoms like **diarrhea** and **abdominal pain**. *Hookworm (Ancylostoma duodenale)* - This organism has a **hook-like mouth** structure and predominantly attaches to the **small intestine** to feed on blood. - Commonly associated with **iron deficiency anemia** and does not exhibit the distinct whip-like shape of Trichuris trichiura. *Lung fluke (Paragonimus)* - Characterized as a **trematode**, it affects the lungs and is typically transmitted through **contaminated freshwater** or undercooked crab. - Symptoms include **coughing** and **hemoptysis**, which are unrelated to the gastrointestinal symptoms of Trichuris trichiura. *Threadworm (Strongyloides)* - This organism is a **nematode** known for its **autoinfection** cycle, leading to symptoms like **persistent cough** and **pruritic rash**. - It does not resemble the whip-like structure of Trichuris trichiura and affects different body systems.

Question 1

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 2

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 3

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 4

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 5

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 6

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 7

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

Question 8

A 35-year-old heterosexual patient diagnosed with HIV had a history of chronic watery diarrhea. A colonoscopic biopsy is shown below. What is the most probable diagnosis?

Accepted Answer

Cryptosporidiosis

Answer

Giardiasis

Answer

Cytomegalovirus (CMV) infection

Answer

Microsporidiosis

Question 9

A 15-year-old boy presented with fever and chills for 3 days. On examination, he was found to have delayed skin pinch time and dry oral mucosa. Identify the pathogen involved based on the provided peripheral blood smear image.

Accepted Answer

Plasmodium vivax

Answer

Babesia

Answer

Plasmodium falciparum

Answer

Salmonella typhi

Question 10

Identify the organism shown in the image.

Accepted Answer

Human whipworm (Trichuris trichiura)

Answer

Hookworm (Ancylostoma duodenale)

Answer

Lung fluke (Paragonimus)

Answer

Threadworm (Strongyloides)

Parasitology — MCQs

Parasitology — MCQs

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