Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 931: A 30-year-old patient presented with features of acute meningoencephalitis in the casualty. His CSF on wet mount microscopy revealed motile unicellular microorganisms. The most likely organism is:

A. Acanthamoeba
B. Trypanosoma cruzi
C. Naegleria fowleri (Correct Answer)
D. Entamoeba histolytica

Explanation: ***Naegleria fowleri*** - This organism causes **primary amoebic meningoencephalitis (PAM)**, which is an acute, fulminant infection primarily in healthy individuals exposed to contaminated fresh water. - Its presence in **CSF wet mount microscopy** as **motile unicellular microorganisms** is a classic diagnostic feature. *Acanthamoeba* - This amoeba typically causes **granulomatous amoebic encephalitis (GAE)**, which is a subacute to chronic infection, often in immunocompromised individuals. - While it can be found in CSF, its presentation is less acute, and it's more commonly associated with **keratitis** in contact lens wearers. *Trypanosoma cruzi* - This protozoan causes **Chagas disease**, which can involve the central nervous system, particularly in the chronic phase. - However, it is typically diagnosed by identifying **trypomastigotes in blood smears** or tissue biopsies, not as motile unicellular organisms in CSF wet mount in acute meningoencephalitis. *Entamoeba histolytica* - This parasite causes **amoebic dysentery** and **liver abscesses**. - Central nervous system involvement is rare and usually occurs as part of disseminated disease, and the organism is typically identified via serology, stool examination, or aspiration of abscesses, not as motile forms in CSF in acute meningoencephalitis.

Question 932: What is the primary reservoir of protozoan infection caused by Toxoplasma gondii in relation to human infection?

A. Cats (Correct Answer)
B. Dogs
C. Rats
D. Sheep

Explanation: ***Cats*** - **Cats** are the definitive host for *Toxoplasma gondii*, meaning the parasite can complete its sexual life cycle only within felines. - They shed **oocysts** in their feces, which are highly infectious and a primary source of human infection. *Dogs* - Dogs can become infected with *T. gondii* by eating infected tissues, but they do not excrete infectious **oocysts** and are therefore not significant in human transmission. - They are considered an intermediate host, similar to humans, not a primary reservoir. *Rats* - **Rats** (and other rodents) are intermediate hosts for *T. gondii*, typically getting infected by ingesting oocysts. - They can transmit the infection to cats when cats prey on them, but they do not directly transmit the parasite to humans as a primary reservoir. *Sheep* - **Sheep** are also intermediate hosts for *T. gondii*, often becoming infected by grazing on contaminated pastures. - Humans can acquire the infection by consuming undercooked meat from infected sheep, but sheep themselves do not shed oocysts and are not the primary reservoir.

Question 933: Filariasis is caused due to lymphatic obstruction by which of the following?

A. V. bancrofti (Correct Answer)
B. C. sinensis
C. S. haematobium
D. P. vivax

Explanation: ***V. bancrofti*** - **Wuchereria bancrofti** is a parasitic nematode that causes **lymphatic filariasis**, leading to lymphatic obstruction. - The adult worms reside in the lymphatic vessels and disrupt normal lymphatic drainage, resulting in chronic **lymphedema** and **elephantiasis**. *P. vivax* - **Plasmodium vivax** is a species of parasite that causes **malaria**, a disease characterized by fever, chills, and anemia. - It primarily infects **red blood cells** and liver cells, and does not cause lymphatic obstruction. *C. sinensis* - **Clonorchis sinensis** is a **liver fluke** that causes **clonorchiasis**, an infection of the biliary ducts. - It leads to inflammation and fibrosis of the bile ducts and is not associated with lymphatic obstruction. *S. haematobium* - **Schistosoma haematobium** is a **blood fluke** that causes **urinary schistosomiasis**. - It infects the **bladder** and urinary tract, leading to hematuria and fibrosis, but does not cause lymphatic obstruction.

Question 934: The culture medium used for Entamoeba histolytica is

A. Robinson's medium
B. CLED medium
C. Blood agar
D. TYI-S-33 medium (Correct Answer)

Explanation: ***TYI-S-33 medium*** - **TYI-S-33 (Trypticase-Yeast extract-Iron-Serum) medium** is the most commonly used axenic culture medium for *Entamoeba histolytica*, meaning it supports the growth of the amoeba in the absence of other microorganisms. - This medium provides the necessary nutrients, including **trypticase**, **yeast extract**, **iron**, and **serum**, for the proliferation and maintenance of *E. histolytica* cultures in vitro. - It is the gold standard for axenic cultivation in research and diagnostic laboratories. *Robinson's medium* - **Robinson's medium** is a diphasic medium, meaning it has both solid and liquid phases, and is used for culturing *Entamoeba histolytica* but it is a **xenic** culture medium, meaning it requires the presence of other microorganisms (bacterial flora). - This medium was historically important but is less commonly used for routine culture and has been largely replaced by axenic media like TYI-S-33 for research and diagnostic purposes. *CLED medium* - The **Cystine-Lactose-Electrolyte-Deficient (CLED) medium** is a differential culture medium primarily used for the isolation and enumeration of urinary pathogens. - It is designed to prevent **swarming of Proteus species** and to differentiate lactose fermenters from non-fermenters, but it is not suitable for growing *Entamoeba histolytica*. *Blood agar* - **Blood agar** is a general-purpose enriched agar medium used for the isolation and cultivation of a wide variety of fastidious and non-fastidious bacteria. - It is particularly useful for demonstrating **hemolytic reactions** of bacteria, which are important for bacterial identification, but it does not support the growth of parasitic protozoa like *Entamoeba histolytica*.

Question 935: Which one of the following statements is false?

A. Young adult males of low socioeconomic status are most commonly affected by invasive amoebiasis
B. A low iron content in the diet predisposes to invasive amoebiasis (Correct Answer)
C. The presence of ingested RBC is seen only in E. histolytica
D. The pathogenic and non-pathogenic strains of E. histolytica can be differentiated by the electrophoretic study of zymodemes

Explanation: ***A low iron content in the diet predisposes to invasive amoebiasis*** - This statement is false because **high iron levels**, not low, are associated with increased susceptibility to invasive amoebiasis and more severe disease. - **Iron serves as a growth factor** for *E. histolytica*, enhancing its virulence and ability to cause infection. *The presence of ingested RBC is seen only in E. histolytica* - The presence of **ingested red blood cells (RBCs)** within the trophozoites is a characteristic and important diagnostic feature of **invasive *E. histolytica***. - This feature helps distinguish it from non-pathogenic amoebae like *E. dispar*, which do not ingest RBCs. *Young adult males of low socioeconomic status are most commonly affected by invasive amoebiasis* - **Young adult males** are often at higher risk due to factors such as occupational exposure and certain social behaviors. - **Low socioeconomic status** is a major determinant due to poor sanitation, contaminated water sources, and inadequate hygiene, facilitating transmission. *The pathogenic and non-pathogenic strains of E. histolytica can be differentiated by the electrophoretic study of zymodemes* - **Zymodeme analysis**, which involves studying isoenzyme patterns, was historically used to differentiate between pathogenic *E. histolytica* and non-pathogenic *E. dispar*. - While molecular methods are now more common, zymodeme studies provided a biochemical basis for distinguishing these strains before advanced genetic techniques were widely available.

Question 936: Which Plasmodium species is known for causing severe malaria with rapid multiplication of infected red blood cells?

A. Plasmodium ovale
B. Plasmodium vivax
C. Plasmodium falciparum (Correct Answer)
D. Plasmodium malariae

Explanation: ***Plasmodium falciparum*** - This species is responsible for the most severe forms of malaria, including **cerebral malaria**, due to its ability to infect red blood cells of all ages. - It exhibits **rapid asexual multiplication**, leading to a high parasitic load and significant erythrocyte destruction. *Plasmodium ovale* - This species typically causes a relatively **mild form of malaria** and can remain dormant in the liver as **hypnozoites**. - It primarily infects **young, enlarged red blood cells** and has a lower parasitic burden compared to *P. falciparum*. *Plasmodium vivax* - While it can also cause **recurrent infections** (due to hypnozoites), *P. vivax* usually leads to less severe disease than *P. falciparum*. - It predominantly infects **reticulocytes (young red blood cells)**, which limits the overall parasitic load. *Plasmodium malariae* - Known for causing **quartan malaria**, characterized by a 72-hour fever cycle, and can lead to **nephrotic syndrome**. - It primarily infects **older, senescent red blood cells** and has the lowest parasitic multiplication rate among the human *Plasmodium* species.

Question 937: What is the investigation of choice for diagnosing amoebiasis?

A. Serological tests
B. Colonoscopy
C. Stool examination for cysts and trophozoites (Correct Answer)
D. Microscopy combined with serological tests

Explanation: **Stool examination for cysts and trophozoites** - **Direct microscopic examination** of fresh stool samples allows for the identification of *Entamoeba histolytica* **trophozoites** (in diarrheic stool) or **cysts** (in formed stool), which is crucial for diagnosis. - This method is **cost-effective** and readily available, making it the primary diagnostic approach for confirming the presence of the parasite. *Serological tests* - Serological tests like **ELISA** detect antibodies against *E. histolytica*, which can be helpful in diagnosing **extraintestinal amoebiasis** (e.g., amoebic liver abscess). - They are often not definitive for acute intestinal amoebiasis as antibodies may persist for a long time after infection or be absent in early stages. *Colonoscopy* - **Colonoscopy** with biopsy can visualize and sample **amoebic ulcers** in the colon, which can be useful in cases of severe dysentery or suspected colitis. - It is an **invasive procedure** and not the first-line investigation for routine diagnosis of amoebiasis, especially when stool examination is available. *Microscopy combined with serological tests* - While both can be used, **stool microscopy remains the gold standard** for diagnosing active intestinal infection. - The combination of **microscopy and serology** is generally employed in situations where intestinal amoebiasis is suspected but microscopy is repeatedly negative or when **extraintestinal amoebiasis** is suspected.

Question 938: What is the most common trophozoite infection transmitted by sexual intercourse?

A. Entamoeba histolytica
B. Trichomonas vaginalis (Correct Answer)
C. Treponema pallidum
D. Giardia intestinalis

Explanation: ***Trichomonas vaginalis*** (Correct Answer) - *T. vaginalis* is a **flagellated protozoan** which exists only in the **trophozoite stage** and is the most common non-viral sexually transmitted infection globally. - It causes **trichomoniasis**, an infection of the urogenital tract, characterized by vaginitis in women and often asymptomatic urethritis in men. - As a trophozoite infection transmitted sexually, it perfectly fits the question criteria. *Entamoeba histolytica* (Incorrect) - This protozoan causes **amoebiasis**, an intestinal infection typically transmitted via the **fecal-oral route** through contaminated food or water, not sexual intercourse. - While it does exist as a **trophozoite**, it is not sexually transmitted. - It exists in both trophozoite and **cyst** forms, with cysts being the infectious stage. *Treponema pallidum* (Incorrect) - This is a **bacterium** (spirochete), not a trophozoite or protozoan, and it is the causative agent of **syphilis**. - While it is sexually transmitted, it does not fit the description of a trophozoite infection. *Giardia intestinalis* (Incorrect) - *Giardia intestinalis* (also known as **Giardia lamblia**) causes **giardiasis**, an intestinal infection typically transmitted via the **fecal-oral route** through contaminated water or food. - While it exists as **trophozoites** and cysts, it is not primarily a sexually transmitted infection. - Sexual transmission can rarely occur through oral-anal contact, but this is not the primary route.

Question 939: Which of the following is not typically seen in the peripheral smear of Plasmodium falciparum infection?

A. Maurer's dot
B. Accole
C. Schizonts (Correct Answer)
D. Banana-shaped gametocytes

Explanation: ***Schizonts*** - While present in the life cycle, **mature schizonts** (containing merozoites) of *P. falciparum* are usually sequestered in **deep tissues** and are not commonly seen in large numbers in peripheral blood smears. - Their absence in a typical peripheral smear is a key diagnostic feature differentiating *P. falciparum* from other *Plasmodium* species. *Maurer's dots* - **Maurer's dots** (large, irregular clefts and stippling) are prominent diagnostic features observed in the red blood cells infected with **late trophozoites** and young schizonts of *Plasmodium falciparum*. - They represent rough endoplasmic reticulum and ribosomes modified by the parasite within the host red blood cell. *Accolé* - The **accolé** or **appliqué form** refers to rings of *P. falciparum* that appear flattened and closely applied to the periphery of the red blood cell. - This characteristic morphology is frequently observed in **young ring forms** of *P. falciparum* and is a useful diagnostic feature on peripheral smears. *Banana-shaped gametocytes* - **Banana-shaped** or **crescent-shaped gametocytes** are the hallmark diagnostic feature of *Plasmodium falciparum* on a peripheral blood smear. - These are the sexual stages of the parasite, and their unique morphology makes them easily identifiable.

Question 940: What is the primary cause of malignant hydatid disease?

A. Malignant transformation of hydatid cysts
B. Infection with Echinococcus multilocularis (Correct Answer)
C. Hydatid disease in immunocompromised patients
D. Caused by Echinococcus granulosus

Explanation: ***Infection with Echinococcus multilocularis*** - **Malignant hydatid disease** is specifically associated with *Echinococcus multilocularis*, which causes alveolar echinococcosis. - This parasite's larval stage grows in a highly **infiltrative** and **destructive** manner, resembling a malignant tumor, hence the term "malignant hydatid disease." *Malignant transformation of hydatid cysts* - Hydatid cysts themselves are parasitic structures and do not undergo **malignant transformation** in the conventional sense of human tissue developing cancer. - The term "malignant" in this context refers to the **aggressive growth pattern** of *E. multilocularis* rather than cellular anaplasia. *Hydatid disease in immunocompromised patients* - While immunocompromised individuals may experience more severe or disseminated hydatid disease, this does not directly cause the **"malignant" characteristic** of alveolar echinococcosis. - The inherent **aggressive growth** of *Echinococcus multilocularis* is the primary factor, regardless of host immune status. *Caused by Echinococcus granulosus* - *Echinococcus granulosus* causes **cystic echinococcosis** (CE), which typically forms unilocular cysts, often considered benign. - CE is generally less aggressive and does not exhibit the **infiltrative growth pattern** characteristic of "malignant hydatid disease" caused by *E. multilocularis*.

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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