Parasitology Practice Questions

Q: A 7-year-old child presented with intermittent abdominal cramps and loose stools. Stool examination revealed eggs measuring 100µm. Which of the following is NOT a cause of these symptoms?

Opisthorchis viverni. ### Explanation The core of this question lies in the **morphometry of trematode eggs**. The stool examination reveals large eggs measuring **100µm**. **1. Why Opisthorchis viverrini is the correct answer:** *Opisthorchis viverrini* (and *Clonorchis sinensis*) are known as small liver flukes. Their eggs are among the smallest of the helminths, typically measuring **25–30 µm** in length. They are operculated with a characteristic "knob" at the posterior pole. Since the question specifies an egg size of 100µm, *Opisthorchis* is excluded as a possible cause. **2. Analysis of Incorrect Options (Large Egg Producers):** * **Fasciola gigantica:** This is a giant liver fluke. Its eggs are large, ovoid, and operculated, typically measuring **160–190 µm** (even larger than *F. hepatica*). * **Echinostoma iliocanum:** This intestinal fluke produces large, operculated, yellowish-brown eggs measuring approximately **80–120 µm**. * **Gastrodiscoides hominis:** A common intestinal fluke in parts of India (Assam). It produces large, rhomboidal/ovoid operculated eggs measuring **150 × 60–70 µm**. **Clinical Pearls for NEET-PG:** * **Large Eggs (>100 µm):** Think *Fasciola* spp., *Fasciolopsis buski*, *Gastrodiscoides hominis*, and *Schistosoma* spp. * **Small Eggs (<35 µm):** Think *Opisthorchis*, *Clonorchis*, and *Heterophyes*. * **Gastrodiscoides hominis:** High-yield for its association with **pig reservoirs** and its prevalence in the Brahmaputra valley. * **Opisthorchis/Clonorchis:** Strongly associated with **Cholangiocarcinoma** (bile duct cancer) due to chronic biliary inflammation.

Q: Which of the following statements about Entamoeba histolytica is true?

It is antigenically different from Entamoeba dispar.. **Explanation:** *Entamoeba histolytica* and *Entamoeba dispar* are part of a species complex that are morphologically identical but genetically and pathologically distinct. **1. Why Option C is Correct:** While they look the same under a microscope, *E. histolytica* (pathogenic) and *E. dispar* (non-pathogenic) are **antigenically and genetically distinct**. They can be differentiated using biochemical methods (isoenzyme analysis/zymodemes), molecular methods (PCR), or by detecting specific fecal antigens (EIA) targeting the Gal/GalNAc lectin unique to *E. histolytica*. **2. Why Other Options are Incorrect:** * **Option A:** In reality, approximately **90% of *E. histolytica* infections are asymptomatic**. Only about 10% progress to clinical disease (amoebic dysentery or extraintestinal abscesses). * **Option B:** They are **morphologically indistinguishable** in both cyst and trophozoite stages. The only morphological clue for *E. histolytica* is the presence of **ingested RBCs** (erythrophagocytosis) in the trophozoite, which indicates invasive disease. Without RBCs, they cannot be told apart by light microscopy. **High-Yield Clinical Pearls for NEET-PG:** * **Infective Stage:** Quadrinucleate cyst. * **Pathognomonic Feature:** Trophozoites with ingested RBCs (Erythrophagocytosis). * **Flask-shaped ulcers:** Typical lesion seen in the colon. * **Anchovy sauce pus:** Characteristic appearance of aspirated material from an Amoebic Liver Abscess. * **Drug of Choice:** Metronidazole/Tinidazole (for invasive disease) followed by a luminal amoebicide like Diloxanide furoate or Paromomycin (to eradicate cysts).

Q: Which of the following is NOT true about Cryptosporidium?

Spores are larger than those of other protozoa.. **Explanation:** **1. Why Option D is the correct answer:** *Cryptosporidium parvum* is characterized by its **small size**. The oocysts (spores) measure approximately **4–6 µm** in diameter. In contrast, other intestinal coccidia are significantly larger: *Cyclospora cayetanensis* measures 8–10 µm, and *Cystoisospora belli* is the largest, measuring roughly 25–30 µm. Therefore, the statement that they are larger than other protozoa is morphologically incorrect. **2. Why the other options are incorrect (True statements about Cryptosporidium):** * **Option A:** Oocysts are **Acid-fast**. They stain bright red against a blue background in a Modified Ziehl-Neelsen (Kinyoun) stain, which is a key diagnostic feature. * **Option B:** Oocysts are highly **chlorine-resistant**. Standard water chlorination levels do not kill them; they require filtration (less than 1 micron) or boiling to be eliminated from water supplies. * **Option C:** The **sporulated oocyst** is both the diagnostic form (found in stool) and the infective form. Unlike *Cystoisospora*, which requires time outside the host to mature, *Cryptosporidium* oocysts are "ready-to-infect" immediately upon excretion, leading to autoinfection. **Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Causes self-limiting watery diarrhea in immunocompetent hosts but **chronic, life-threatening "cholera-like" diarrhea** in HIV/AIDS patients (CD4 count <200 cells/mm³). * **Stain of Choice:** Modified Acid-Fast stain. * **Treatment:** **Nitazoxanide** is the drug of choice for immunocompetent patients; for AIDS patients, the primary management is HAART to restore CD4 counts. * **Transmission:** Fecal-oral route; common cause of waterborne outbreaks in swimming pools and daycare centers.

Q: What is the most common cause of cutaneous larva migrans?

Ankylostoma caninum. **Explanation:** **Cutaneous Larva Migrans (CLM)**, also known as "creeping eruption," is a zoonotic infestation caused by the larvae of animal hookworms. 1. **Why Option A is Correct:** **Ankylostoma caninum** (the dog hookworm) is the **most common** cause of CLM worldwide. **Ankylostoma braziliense** (found in both dogs and cats) is another frequent cause. Humans are accidental "dead-end" hosts; the larvae penetrate the skin (usually through contact with contaminated soil or sand) but lack the enzymes necessary to penetrate the basement membrane and reach the circulation. Consequently, they remain confined to the epidermis, migrating aimlessly and creating the characteristic serpiginous, erythematous, pruritic tracks. 2. **Why Other Options are Incorrect:** * **Options B & C (Toxocara canis/cati):** These are the causative agents of **Visceral Larva Migrans (VLM)** and Ocular Larva Migrans. Unlike CLM, these larvae migrate through internal organs rather than the skin. * **Option D (Gnathostoma):** This causes **Larva Migrans Profunda**. Unlike the superficial tracks of CLM, Gnathostomiasis typically presents as deep, migratory subcutaneous swellings (panniculitis). **High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** Intensely pruritic, linear or serpiginous tracks, most commonly on the feet (ground itch). * **Rate of Migration:** The larvae move at a rate of roughly **1–2 cm per day**. * **Löffler’s Syndrome:** Rare systemic involvement where larvae reach the lungs, causing transient pulmonary infiltrates and peripheral eosinophilia. * **Treatment of Choice:** **Oral Albendazole** (400 mg for 3–7 days) or **Ivermectin** (200 µg/kg single dose). Topical Thiabendazole is also an option.

Q: Chromidial bars are absent in which stage of cysts in Entamoeba?

Quadrinucleate. ### Explanation The presence and morphology of **chromidial bars** (aggregates of ribosomes) are crucial diagnostic features in the life cycle of *Entamoeba histolytica*. **Why Quadrinucleate is the correct answer:** As the cyst of *Entamoeba histolytica* matures, it undergoes nuclear division while simultaneously consuming its stored food reserves. Chromidial bars and the glycogen mass are prominent in the early stages (uninucleate and binucleate) to provide energy and protein synthesis machinery for maturation. By the time the cyst reaches the **mature quadrinucleate stage** (the infective stage), these stored reserves—including the chromidial bars—are typically **completely absorbed and disappear**. **Analysis of Incorrect Options:** * **A & B (Uninucleate & Binucleate):** These are immature stages. They characteristically contain a large glycogen mass and prominent, cigar-shaped chromidial bars with rounded ends. * **D (Octanucleate):** This stage is characteristic of *Entamoeba coli*, not *E. histolytica*. In *E. coli*, chromidial bars are often present even in the mature (octanucleate) stage, appearing splinter-like or filamentous with jagged ends. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *E. histolytica* chromidial bars are **cigar-shaped** (rounded ends), whereas *E. coli* bars are **splinter-like** (pointed ends). * **Infective Stage:** The mature **quadrinucleate cyst** is the infective form for humans. * **Diagnostic Point:** The presence of ingested RBCs (erythrophagocytosis) in a trophozoite is pathognomonic for *E. histolytica*, distinguishing it from the commensal *E. dispar*. * **Nuclear Feature:** Look for "cartwheel appearance" (central karyosome and peripheral chromatin) in *E. histolytica*.

Q: Which of the following statements is NOT true about Kala-Azar?

Man has the flagellar stage of the organism. ### Explanation **Correct Answer: D. Man has the flagellar stage of the organism** **Why Option D is NOT true:** The causative agent of Kala-azar, *Leishmania donovani*, exists in two distinct morphological forms. The **flagellar stage (Promastigote)** is found only in the vector (Sandfly) and in artificial cultures. In the human host (Man), the parasite exists exclusively in the **aflagellar stage (Amastigote)**, also known as Leishman-Donovan (LD) bodies, residing within the reticuloendothelial system (macrophages). **Analysis of Other Options:** * **A. Sandfly is the vector:** This is true. The disease is transmitted by the bite of the female *Phlebotomus argentipes*. * **B. Man is the only reservoir in India:** This is true. Unlike in other parts of the world (where dogs or rodents may be reservoirs), Indian Kala-azar is **anthroponotic**, meaning humans are the sole reservoir of infection. * **C. Aldehyde test is diagnostic:** While technically a non-specific screening tool, in the context of classic medical exams, the Napier’s Aldehyde test is associated with Kala-azar. It depends on the increase in serum gamma globulins. Note: It only becomes positive after 3 months of illness. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Demonstration of LD bodies in **splenic aspirate** (highest sensitivity) or bone marrow aspirate. * **Drug of Choice:** **Liposomal Amphotericin B** (single dose of 10mg/kg is the current WHO recommendation for India). * **PKDL (Post Kala-azar Dermal Leishmaniasis):** A non-ulcerative condition appearing years after "cure"; it serves as a major parasite reservoir in the community. * **Elimination Goal:** To reduce the annual incidence to less than **1 case per 10,000 population** at the block level.

Q: Stool examination in a patient reveals a specific finding. What is the likely route of infection of this parasite?

Ingestion of food contaminated with the egg or larva. ***Ingestion of food contaminated with the egg or larva*** - **Ascaris lumbricoides** transmission occurs through the **fecal-oral route** when contaminated food or water containing **embryonated eggs** is ingested. - The eggs survive in soil for months and become **infective after 2-4 weeks** of maturation in the environment. *Insect bite* - Vector-borne transmission is characteristic of parasites like **Plasmodium** (malaria) or **Leishmania**. - **Ascaris** does not require an **arthropod vector** for transmission and spreads directly through contaminated environments. *Ingestion of improperly cooked beef* - This route is specific to **Taenia saginata** (beef tapeworm) which forms **cysticerci** in cattle muscle. - **Ascaris** does not have an **intermediate host** and does not require meat consumption for transmission. *Contact with contaminated water during swimming* - This describes **percutaneous transmission** seen in **Schistosoma** species where **cercariae** penetrate intact skin. - **Ascaris eggs** cannot penetrate skin and must be **ingested orally** to cause infection.

Q: A pregnant lady with AIDS presents with diarrhea. Stool examination reveals acid-fast positive cysts. Which organism is likely responsible for this infection?

Isospora belli. ### Explanation The clinical presentation of chronic diarrhea in an immunocompromised patient (AIDS) combined with the finding of **acid-fast positive cysts** is characteristic of the Coccidian group of parasites. **Why Isospora belli is correct:** * **Acid-fast staining:** *Isospora belli* (now *Cystoisospora belli*) produces large, oval oocysts (approx. 25–30 μm) that are **acid-fast positive** (modified Ziehl-Neelsen stain). * **Clinical Context:** It is a common cause of severe, watery diarrhea in HIV/AIDS patients. While *Cryptosporidium parvum* and *Cyclospora* are also acid-fast, *Isospora* is a classic high-yield answer for this description in competitive exams. **Why other options are incorrect:** * **Microsporidia:** These are tiny intracellular fungi (formerly classified as parasites). While they cause diarrhea in AIDS, they are **not** acid-fast; they require special stains like Modified Trichrome or Calcofluor White. * **Giardia lamblia:** This is a flagellate that causes malabsorptive "steatorrhea." It is identified by pear-shaped trophozoites or oval cysts on iodine mount, but it is **not acid-fast**. * **Entamoeba histolytica:** This causes amoebic dysentery (bloody diarrhea). Cysts are characterized by having 1–4 nuclei and chromatoid bodies, but they do **not** retain acid-fast stains. **High-Yield Clinical Pearls for NEET-PG:** 1. **Acid-fast Parasites (The "Big Three"):** *Cryptosporidium* (Small, spherical), *Cyclospora* (Mid-sized, spherical), and *Isospora* (Large, oval). 2. **Treatment of Choice:** Unlike most protozoa (treated with Metronidazole), *Isospora belli* is treated with **Trimethoprim-Sulfamethoxazole (TMP-SMX)**. 3. **Autofluorescence:** *Isospora* and *Cyclospora* oocysts show blue-green autofluorescence under UV microscopy.

Q: The Montenegro test is performed to diagnose which of the following conditions?

Kala-azar. **Explanation:** The **Montenegro test** (also known as the Leishmanin skin test) is a delayed-type hypersensitivity (Type IV) reaction used to detect prior exposure to *Leishmania* parasites, the causative agents of **Kala-azar** (Visceral Leishmaniasis). **Why Kala-azar is correct:** The test involves the intradermal injection of a suspension of killed *Leishmania* promastigotes. A positive result (induration ≥5 mm after 48–72 hours) indicates a cell-mediated immune response. It is important to note that the test is typically **negative** during active visceral leishmaniasis (due to suppressed cell-mediated immunity) but becomes **positive** after successful treatment or in cases of Cutaneous Leishmaniasis and Post-Kala-azar Dermal Leishmaniasis (PKDL). **Why other options are incorrect:** * **Hydatid disease:** Diagnosed primarily via imaging (USG/CT) and the **Casoni’s test** (though now largely replaced by serology like ELISA). * **Taeniasis:** Diagnosed by stool microscopy for eggs or proglottids; no specific skin test is used for routine diagnosis. * **Filariasis:** Diagnosed by demonstrating microfilariae in peripheral blood (night samples) or using the ICT (Immunochromatographic test) for circulating filarial antigen. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Sandfly (*Phlebotomus argentipes*). * **Drug of Choice:** Liposomal Amphotericin B (current standard). * **RK-39 Immunochromatographic test:** The rapid diagnostic test of choice for active Kala-azar in field settings. * **Aldehyde Test (Napier’s test):** Based on hypergammaglobulinemia; it is non-specific but used for screening.

Question 1

A 7-year-old child presented with intermittent abdominal cramps and loose stools. Stool examination revealed eggs measuring 100µm. Which of the following is NOT a cause of these symptoms?

Accepted Answer

Opisthorchis viverni

Answer

Fasciola gigantica

Answer

Echinostoma iliocanum

Answer

Gastrodiscoides hominis

Question 2

Which of the following statements about Entamoeba histolytica is true?

Accepted Answer

It is antigenically different from Entamoeba dispar.

Answer

It is mostly symptomatic.

Answer

It can be differentiated morphologically from Entamoeba dispar.

Answer

All of the above.

Question 3

Which of the following is NOT true about Cryptosporidium?

Accepted Answer

Spores are larger than those of other protozoa.

Answer

Spores are acid fast.

Answer

Spores are chlorine resistant.

Answer

The diagnostic form is the sporulated oocyst.

Question 4

What is the most common cause of cutaneous larva migrans?

Accepted Answer

Ankylostoma caninum

Answer

Toxocara canis

Answer

Toxocara cati

Answer

Gnathostoma

Question 5

Chromidial bars are absent in which stage of cysts in Entamoeba?

Accepted Answer

Quadrinucleate

Answer

Uninucleate

Answer

Binucleate

Answer

Octanucleate

Question 6

Which of the following statements is NOT true about Kala-Azar?

Accepted Answer

Man has the flagellar stage of the organism

Answer

Sandfly is the vector

Answer

Man is the only reservoir in India

Answer

Aldehyde test is diagnostic

Question 7

Stool examination in a patient reveals a specific finding. What is the likely route of infection of this parasite?

Accepted Answer

Ingestion of food contaminated with the egg or larva

Answer

Insect bite

Answer

Ingestion of improperly cooked beef

Answer

Contact with contaminated water during swimming

Question 8

A pregnant lady with AIDS presents with diarrhea. Stool examination reveals acid-fast positive cysts. Which organism is likely responsible for this infection?

Accepted Answer

Isospora belli

Answer

Microsporidia

Answer

Giardia lamblia

Answer

Entamoeba histolytica

Question 9

The Montenegro test is performed to diagnose which of the following conditions?

Accepted Answer

Kala-azar

Answer

Hydatid disease

Answer

Taeniasis

Answer

Filariasis

Question 10

Which of the following organisms presents with acid-fast oocysts?

Accepted Answer

All of the above

Answer

Cryptosporidium

Answer

Isospora

Answer

Cyclospora

Parasitology — MCQs

Parasitology — MCQs

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