Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 841: Flask shaped ulcers in colon are caused by:-

A. Entamoeba histolytica (Correct Answer)
B. Giardia lamblia
C. Enterobius vermicularis
D. H. Pylori

Explanation: ***Entamoeba histolytica*** - *Entamoeba histolytica* is a protozoan parasite that invades the **colonic mucosa**, leading to characteristic **flask-shaped ulcers**. - These ulcers are formed as the trophozoites penetrate the epithelium and spread laterally through the submucosa, creating a narrow neck at the mucosal surface and a wider base below. *Giardia lamblia* - *Giardia lamblia* causes **giardiasis**, primarily affecting the **small intestine** and leading to malabsorption and diarrhea. - It typically does not cause invasive disease or ulcer formation in the colon, instead, it attaches to the intestinal villi. *Enterobius vermicularis* - *Enterobius vermicularis* is an intestinal nematode (pinworm) that causes **enterobiasis**, commonly manifesting as **perianal itching**, especially at night. - It is a non-invasive parasite and does not cause ulcers in the colon. *H. Pylori* - *H. pylori* is a bacterium primarily associated with infections of the **stomach** and **duodenum**, causing gastritis, peptic ulcers, and increasing the risk of gastric cancer. - It does not colonize the colon or cause flask-shaped ulcers.

Question 842: Slow growing alveolar like tumor in liver is caused by:

A. Amoebic liver abscess
B. E. granulosus
C. Cysticercus cellulosae
D. E. multilocularis (Correct Answer)

Explanation: **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

Question 843: 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?

A. Balantidium coli
B. Cryptosporidium
C. Cystoisospora (Correct Answer)
D. Strongyloides

Explanation: ***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.

Question 844: A wet mount of vaginal discharge shows motile, flagellated protozoa. Which organism is most likely causing this infection?

A. Gardnerella vaginalis
B. Candida albicans
C. Lactobacillus
D. Trichomonas vaginalis (Correct Answer)

Explanation: ***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.

Question 845: A blood smear from a patient with cyclic fevers shows Maltese cross formations within red blood cells. Which organism is the likely cause?

A. Leishmania donovani
B. Plasmodium vivax
C. Plasmodium falciparum
D. Babesia microti (Correct Answer)

Explanation: ***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.

Question 846: A 30-year-old man with HIV presents with chronic diarrhea. Stool examination reveals small, spherical, acid-fast oocysts. What is the causative agent?

A. Entamoeba histolytica
B. Cryptosporidium parvum (Correct Answer)
C. Isospora belli
D. Giardia lamblia

Explanation: ***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.

Question 847: Which is the primary host cell for Plasmodium vivax during its human life cycle?

A. Red blood cells
B. Macrophages
C. Endothelial cells
D. Liver cells (Correct Answer)

Explanation: ***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*.

Question 848: 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.

A. Assertion is true, reason is true but reason is not the correct explanation of the assertion.
B. Assertion is true, reason is false.
C. Assertion is false, reason is true.
D. Assertion is true, reason is true and reason is the correct explanation of the assertion. (Correct Answer)

Explanation: ***Assertion is true, reason is true and reason is the correct explanation of the assertion.*** - **Plasmodium falciparum** exhibits **cytoadherence**, where infected RBCs (containing mature trophozoites and schizonts) bind to endothelial cells of capillaries and venules in various organs (brain, heart, lungs, kidneys). - This sequestration in deep vascular beds prevents these parasite stages from circulating in the **peripheral blood**, which is why schizonts are rarely seen in routine peripheral blood smears. - The reason directly explains the assertion - cytoadherence is the mechanism causing absence of schizonts from peripheral circulation. *Assertion is true, reason is true but reason is not the correct explanation of the assertion.* - This is **incorrect** because cytoadherence is indeed the direct cause and correct explanation for why schizonts are not seen in peripheral blood. - The phenomenon of sequestration through cytoadherence is the established pathophysiological mechanism. *Assertion is true, reason is false.* - This is **incorrect** because both the assertion and reason are true. - **Cytoadherence** to vascular endothelium is a well-established mechanism of *P. falciparum* pathogenesis. *Assertion is false, reason is true.* - This is **incorrect** because the assertion is **true** - *P. falciparum* schizonts are indeed absent from peripheral blood smears. - Only ring forms and occasionally gametocytes are seen in peripheral blood.

Question 849: Which of the following organisms show parthenogenesis?

A. Ascaris
B. Trichuris
C. Strongyloides (Correct Answer)
D. Ancylostoma

Explanation: ***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 850: A patient develops watery diarrhea after attending a wedding. Stool examination reveals acid-fast oocysts. Which organism is most likely responsible?

A. Giardia lamblia
B. Cryptosporidium hominis (Correct Answer)
C. Entamoeba histolytica
D. Cyclospora cayetanensis

Explanation: ***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.

Practice by Chapter

Classification of Parasites

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Intestinal Protozoa

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Blood and Tissue Protozoa

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Malaria Parasites

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Leishmaniasis

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Intestinal Helminths: Nematodes

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Tissue Nematodes

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Trematodes

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Cestodes

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Ectoparasites

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Antiparasitic Drugs

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Laboratory Diagnosis of Parasitic Infections

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