Parasitology Practice Questions

Q: Slow growing alveolar like tumor in liver is caused by:

E. multilocularis. **Correct: E. multilocularis** - *Echinococcus multilocularis* causes **alveolar echinococcosis**, characterized by a slow-growing, infiltrative, and destructive lesion in the liver with a sponge-like, alveolar appearance - This parasite's larval stage forms multiple small cysts that are not encapsulated, leading to a tumor-like mass, often described as having an "alveolar" structure - The growth pattern mimics a malignant tumor, with irregular borders and infiltration into surrounding liver tissue *Incorrect: Amoebic liver abscess* - An amoebic liver abscess is caused by **Entamoeba histolytica** and typically presents as a single or multiple well-defined, encapsulated lesions filled with "anchovy paste-like" pus, which is distinct from an alveolar tumor - These abscesses usually develop relatively quickly (acute to subacute) and are often associated with fever and right upper quadrant pain, unlike the slow, asymptomatic growth of alveolar echinococcosis *Incorrect: E. granulosus* - *Echinococcus granulosus* causes **cystic echinococcosis** (hydatid disease), which typically presents as a single, large, unilocular cyst with a clear, laminated membrane, often surrounded by a fibrous capsule - Unlike *E. multilocularis*, *E. granulosus* cysts tend to be well-defined, spherical, and expansile rather than infiltrative and alveolar - The key distinguishing feature is the **cystic** (unilocular or with daughter cysts) versus **alveolar** (multiple small vesicles) morphology *Incorrect: Cysticercus cellulosae* - *Cysticercus cellulosae* is the larval form of **Taenia solium** and causes cysticercosis, which primarily affects muscles, subcutaneous tissue, brain, and eyes - While it forms cysts, these are typically small, solitary, and calcify in various tissues, not forming a large, infiltrative, alveolar tumor-like mass in the liver - Hepatic cysticercosis is extremely rare

Q: Based on the microscopic image showing an oval cyst containing 4 nuclei with visible fibrils under stool microscopy examination, identify the parasitic organism:

Giardia lamblia. ***Giardia lamblia*** - The presence of a cyst with **4 nuclei** and visible **fibrils** (remnants of flagella) seen on stool microscopy is pathognomonic for *Giardia lamblia*. - *Giardia* cysts are **oval-shaped** and typically contain **two to four nuclei** with characteristic curved fibrils visible within the cyst wall. - The **axostyle** (median body) is a characteristic feature of the **trophozoite** stage, not the cyst stage. *Taenia solium* - *Taenia solium* is a **tapeworm**; its eggs are typically **spherical** with a thick, radially striated embryophore and contain a six-hooked oncosphere, not a multi-nucleated cyst with fibrils. - The larval stage (**cysticercus**) can be found in human tissues, but its cysts do not appear in stool samples in this form. *Entamoeba dispar* - *Entamoeba dispar* cysts typically have **one to four nuclei** but **lack fibrils**; they are morphologically indistinguishable from *Entamoeba histolytica* cysts without molecular methods. - While it can be found in stool, its nuclear and internal structure differs significantly from the described parasitic organism. *Balantidium coli* - *Balantidium coli* cysts are much **larger** than *Giardia* cysts, typically **pear-shaped to ovoid**, and contain a single large **kidney-shaped macronucleus** and a smaller micronucleus. - This parasite is also characterized by the presence of **cilia** on its trophozoite stage.

Q: A known HIV patient on anti-retroviral therapy presented with diarrhea of six months duration. Stool microscopy showed 10-30 micrometer cysts, Kinyoun stain was positive. What is the most likely diagnosis?

Cystoisospora. ***Cystoisospora*** - **Cystoisospora belli** is a common cause of chronic diarrhea in **HIV-infected patients** and presents with oocysts typically measuring **20-30 µm** (the described 10-30 micrometer range encompasses the typical size). - The **Kinyoun acid-fast stain** is characteristically positive for *Cystoisospora* oocysts, which is the key diagnostic feature supporting this diagnosis. - This organism is particularly important in **immunocompromised patients** on ART who present with chronic diarrhea. *Balantidium coli* - **Balantidium coli** is a large ciliate, and its cysts are significantly larger (typically **40-60 µm**) than those described in the patient's stool. - While it can cause diarrhea in immunocompromised individuals, it is **not acid-fast** and therefore would not be positive with the Kinyoun stain. *Cryptosporidium* - **Cryptosporidium** is another common cause of chronic diarrhea in HIV patients, and its oocysts are **acid-fast positive** with the Kinyoun stain. - However, **Cryptosporidium** oocysts are much smaller, typically **4-6 µm**, which is inconsistent with the 10-30 micrometer cysts observed in this case. *Strongyloides* - **Strongyloides stercoralis** is a nematode that can cause chronic diarrhea and hyperinfection in immunocompromised patients, but it primarily produces **larvae** (rhabditiform and filariform) in stool, not cysts. - Its diagnostic forms are **not acid-fast**, and its morphology in stool microscopy is distinctly different from the described cysts.

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.

Question 1

Slow growing alveolar like tumor in liver is caused by:

Accepted Answer

E. multilocularis

Answer

Amoebic liver abscess

Answer

E. granulosus

Answer

Cysticercus cellulosae

Question 2

Based on the microscopic image showing an oval cyst containing 4 nuclei with visible fibrils under stool microscopy examination, identify the parasitic organism:

Accepted Answer

Giardia lamblia

Answer

Taenia solium

Answer

Balantidium coli

Answer

Entamoeba dispar

Question 3

A known HIV patient on anti-retroviral therapy presented with diarrhea of six months duration. Stool microscopy showed 10-30 micrometer cysts, Kinyoun stain was positive. What is the most likely diagnosis?

Accepted Answer

Cystoisospora

Answer

Balantidium coli

Answer

Cryptosporidium

Answer

Strongyloides

Question 4

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 5

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 6

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 7

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 8

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 9

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 10

Which of the following organisms show parthenogenesis?

Accepted Answer

Strongyloides

Answer

Ascaris

Answer

Trichuris

Answer

Ancylostoma

Parasitology — MCQs

Parasitology — MCQs

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