Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 631: In Toxoplasmosis, what role does a human typically play in the parasite's life cycle?

A. Paratenic host
B. Intermediate host (Correct Answer)
C. Definitive host
D. Temporary host

Explanation: ### Explanation **1. Why "Intermediate Host" is Correct:** In parasitology, the **Definitive Host** is where the parasite undergoes its sexual cycle, while the **Intermediate Host** is where the asexual cycle occurs. * **Definitive Host:** Members of the **Felidae family (cats)**. Sexual reproduction (gametogony and oocyst formation) occurs in the intestinal epithelium of cats. * **Intermediate Host:** Humans and other mammals/birds. In humans, the parasite undergoes asexual multiplication, existing as **tachyzoites** (active infection) and **bradyzoites** (latent infection in tissue cysts). Since humans do not support the sexual stage, they are strictly intermediate hosts. **2. Why Other Options are Incorrect:** * **A. Paratenic host:** A "transport" host where the parasite survives but does not undergo any development. In Toxoplasmosis, the parasite actively multiplies and changes forms (tachyzoites to bradyzoites), so humans are not paratenic hosts. * **C. Definitive host:** As mentioned, this is reserved for cats. Humans never harbor the sexual stage of *Toxoplasma gondii*. * **D. Temporary host:** This term usually refers to hosts visited by parasites for a short duration (like leeches or mosquitoes). Toxoplasma remains in the human body for life (encysted), making this term inapplicable. **3. NEET-PG High-Yield Pearls:** * **Infective Stages:** Humans are infected via **Oocysts** (from cat feces), **Tissue cysts** (undercooked meat), or **Tachyzoites** (transplacental transmission). * **Dead-end Host:** Humans are also considered "accidental" or "dead-end" hosts because the cycle usually stops with us (unless human flesh is consumed by a carnivore). * **Diagnosis:** Sabin-Feldman Dye Test (Gold Standard), though rarely used now. IgM/IgG ELISA is the routine choice. * **Congenital Toxoplasmosis Triad:** Chorioretinitis, Hydrocephalus, and Intracranial calcifications.

Question 632: Which of the following is NOT a water-borne parasite?

A. Ascariasis
B. Enterobius
C. Giardiasis
D. Strongyloides (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Strongyloides**. The primary distinction here lies in the **mode of transmission**. Water-borne parasites are typically transmitted via the **fecal-oral route** through the ingestion of contaminated water containing infective cysts or eggs. 1. **Why Strongyloides is the correct answer:** *Strongyloides stercoralis* is primarily a **soil-transmitted helminth**. Infection occurs via **penetration of intact skin** (usually the feet) by filariform larvae present in contaminated soil. It is not considered a classic water-borne pathogen because ingestion is not its primary route of entry. 2. **Analysis of Incorrect Options:** * **Giardiasis (C):** *Giardia lamblia* is the most common protozoan cause of water-borne diarrheal outbreaks. Its cysts are highly resistant to chlorine and are frequently transmitted through contaminated drinking or recreational water. * **Ascariasis (A) & Enterobius (B):** Both are transmitted via the fecal-oral route. While often food-borne or transmitted via contaminated hands (especially *Enterobius* via autoinoculation), their eggs can contaminate water sources, leading to water-borne transmission in areas with poor sanitation. ### NEET-PG Clinical Pearls: * **Strongyloides Unique Feature:** It is the only helminth that can cause **autoinfection**, leading to "Hyperinfection Syndrome" in immunocompromised patients (especially those on steroids). * **Diagnostic Clue:** For *Strongyloides*, we look for **larvae in stool**, not eggs (unlike *Ascaris*). * **Ground Itch:** This refers to the pruritic dermatitis at the site of larval penetration (seen in *Strongyloides* and Hookworms). * **Chlorine Resistance:** *Giardia* and *Cryptosporidium* are high-yield examples of water-borne parasites resistant to standard chlorination.

Question 633: Which of the following are characteristic features of Giardiasis?

A. Malabsorption (Correct Answer)
B. Cyst with 4 nuclei
C. Trophozoite with four nuclei
D. Common in hypogammaglobulinemia

Explanation: **Explanation:** **Giardia lamblia** is a flagellated protozoan that primarily inhabits the duodenum and upper jejunum. It is a leading cause of diarrheal disease worldwide. **1. Why Malabsorption is correct:** The hallmark of chronic Giardiasis is **malabsorption**, particularly of fats (steatorrhea) and fat-soluble vitamins. The trophozoites use a ventral sucking disc to attach to the intestinal villi. This mechanical barrier, combined with the induction of apoptosis in enterocytes and the blunting of villi, leads to a reduction in the absorptive surface area. This results in foul-smelling, greasy stools that float. **2. Analysis of Incorrect Options:** * **Cyst with 4 nuclei:** While a mature cyst of *Giardia* does contain 4 nuclei, this is also a characteristic of *Entamoeba histolytica*. In the context of NEET-PG questions, "Malabsorption" is the more specific clinical "characteristic feature" associated with the pathology of Giardiasis. * **Trophozoite with four nuclei:** This is morphologically incorrect. The *Giardia* trophozoite is pear-shaped and characteristically **binucleated** (two nuclei), giving it a "monkey-face" or "owl-eye" appearance. * **Common in hypogammaglobulinemia:** While patients with IgA deficiency are indeed more susceptible to chronic Giardiasis, it is a predisposing factor/association rather than a "characteristic feature" of the disease itself. **High-Yield Clinical Pearls for NEET-PG:** * **Habitat:** Duodenum (Acidic environment). * **Morphology:** Trophozoite is heart-shaped with 4 pairs of flagella and 2 nuclei. * **Diagnosis:** Stool microscopy (cysts/trophozoites), **Entero-test** (String test), or ELISA for fecal antigens. * **Drug of Choice:** Tinidazole or Metronidazole. * **Key Association:** Common in campers/hikers drinking unfiltered stream water ("Beaver fever").

Question 634: A patient complains of diarrhea. Stool examination shows ova. Which of the following cannot be the cause?

A. Cryptosporidium
B. Opisthorchis viverni (Correct Answer)
C. Isospora
D. E. histolytica

Explanation: ### Explanation The core concept tested here is the distinction between **parasitic life stages** (Ova vs. Cysts/Oocysts) found in stool samples. **Why Opisthorchis viverrini is the correct answer:** *Opisthorchis viverrini* (Southeast Asian liver fluke) is a trematode. Trematodes typically produce **eggs (ova)** that are shed in the feces. Therefore, finding "ova" in a stool examination is a characteristic finding for this parasite. **Note on the Question Construction:** There appears to be a technical discrepancy in the provided key. In standard parasitology: * **A, C, and D** are Protozoa. Protozoa do **not** produce "ova" (eggs); they produce **Cysts** (*E. histolytica*) or **Oocysts** (*Cryptosporidium, Isospora*). * **B** is a Helminth (Trematode), which **does** produce **Ova**. If the question asks which **cannot** be the cause of "ova" in stool, the answer should technically be the protozoans (A, C, or D). However, if the question implies which organism is *not* typically associated with simple watery diarrhea but rather biliary pathology, or if there is a typo in the options provided, *Opisthorchis* stands out as the only helminth. **Analysis of Options:** * **Cryptosporidium & Isospora (Cystoisospora):** These are Coccidian protozoa. They cause diarrhea and are identified by **Oocysts** (acid-fast positive), not ova. * **E. histolytica:** An amoeba that causes dysentery. It sheds **Cysts** and **Trophozoites** in stool, never ova. * **Opisthorchis viverrini:** A fluke that resides in the bile ducts. It sheds **embryonated eggs (ova)** in stool. **NEET-PG High-Yield Pearls:** 1. **Ova vs. Cysts:** Only Helminths (Platyhelminthes and Nematodes) produce **Ova**. Protozoa produce **Cysts/Oocysts**. 2. **Acid-Fast Staining:** *Cryptosporidium, Isospora,* and *Cyclospora* are **modified acid-fast positive**. 3. **Opisthorchis/Clonorchis:** Strongly associated with **Cholangiocarcinoma** (bile duct cancer). 4. **E. histolytica:** Look for "Quadrinucleated cysts" or "Trophozoites with ingested RBCs" (erythrophagocytosis).

Question 635: Which organism causes biliary duct obstruction?

A. Clonorchis sinensis (Correct Answer)
B. Ankylostoma duodenale
C. Strongyloides stercales
D. Enterobius vermicularis

Explanation: **Explanation:** **Clonorchis sinensis** (the Chinese Liver Fluke) is the correct answer because its primary habitat in the human body is the **biliary passages**. After ingestion of undercooked freshwater fish containing metacercariae, the larvae migrate to the distal bile ducts. Chronic infection leads to mechanical obstruction, inflammatory hyperplasia of the biliary epithelium, and periductal fibrosis. This can manifest clinically as biliary colic, jaundice, and cholangitis. **Analysis of Incorrect Options:** * **Ankylostoma duodenale (Hookworm):** Resides in the small intestine (jejunum) and attaches to the mucosa to suck blood, primarily causing iron-deficiency anemia, not biliary obstruction. * **Strongyloides stercoralis:** Primarily inhabits the mucosal glands of the duodenum and upper jejunum. It is known for its autoinfection cycle and hyperinfection syndrome in immunocompromised patients. * **Enterobius vermicularis (Pinworm):** Lives in the cecum and appendix. Its most common clinical presentation is perianal pruritus (itching) due to nocturnal egg deposition. **High-Yield Clinical Pearls for NEET-PG:** * **Carcinogenesis:** Chronic *Clonorchis sinensis* infection is a major risk factor for **Cholangiocarcinoma** (bile duct cancer). It is classified as a Group 1 carcinogen by the IARC. * **Intermediate Hosts:** 1st host: Snail (Parafossarulus); 2nd host: Cyprinidae family fish. * **Diagnosis:** Identification of characteristic "operculated eggs with a small knob" in stool or biliary aspirate. * **Treatment:** Praziquantel is the drug of choice. * **Other Biliary Flukes:** *Opisthorchis viverrini* and *Fasciola hepatica* can also cause similar biliary pathology.

Question 636: Nocturnal pruritus ani is characteristic of which of the following?

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

Explanation: **Explanation:** **Enterobius vermicularis** (Pinworm or Seatworm) is the correct answer because of its unique life cycle. Unlike most intestinal nematodes, the gravid female Enterobius migrates out of the anus, typically at night (**nocturnal migration**), to deposit eggs on the perianal and perineal skin. The movement of the worm and the sticky substance used to adhere the eggs cause intense local irritation, leading to **nocturnal pruritus ani** (perianal itching). This often results in sleep disturbance and irritability in children. **Analysis of Incorrect Options:** * **Ascaris lumbricoides:** Primarily causes Loeffler’s syndrome (pulmonary phase) and intestinal obstruction. It does not migrate to the perianal region. * **Trichuris trichiura (Whipworm):** Associated with mucosal damage in the large intestine. Heavy infections typically lead to chronic diarrhea and **rectal prolapse**, but not perianal itching. * **Ancylostoma duodenale (Hookworm):** Known for causing iron deficiency anemia due to blood-sucking and "ground itch" at the site of larval skin penetration (usually the feet). **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The investigation of choice is the **NIH Swab** or **Scotch Tape Test** (Cellophane tape test) performed early in the morning before bathing. Eggs are rarely found in routine stool samples. * **Morphology:** Eggs are characteristic **Planoconvex** (D-shaped). * **Transmission:** Autoinfection is common due to the finger-to-mouth transfer of eggs following scratching (fecal-oral). * **Treatment:** **Albendazole** or Mebendazole is used. It is crucial to treat the **entire family** simultaneously to prevent reinfection.

Question 637: Cysticercosis is caused by:

A. Taenia solium (Correct Answer)
B. Taenia saginata
C. Echinococcus
D. None of the above

Explanation: **Explanation:** The correct answer is **Taenia solium**. **1. Why Taenia solium is correct:** Cysticercosis occurs when a human becomes the **accidental intermediate host** by ingesting the **eggs** of *Taenia solium* (pork tapeworm) via contaminated food or water (fecal-oral route) or through autoinfection. Once ingested, the eggs hatch into oncospheres, penetrate the intestinal wall, and disseminate to tissues (brain, muscles, eyes) where they develop into larvae called **Cysticercus cellulosae**. *Note:* Ingesting the *larvae* in undercooked pork causes intestinal Taeniasis, but ingesting the *eggs* causes Cysticercosis. **2. Why other options are incorrect:** * **Taenia saginata (Beef tapeworm):** Humans are the definitive hosts and only develop intestinal infections. *T. saginata* does **not** cause cysticercosis in humans because the eggs are not infectious to us; the intermediate stage occurs only in cattle. * **Echinococcus:** This genus (e.g., *E. granulosus*) causes **Hydatid disease** (Cystic Echinococcosis), characterized by slow-growing unilocular cysts, typically in the liver or lungs. **NEET-PG High-Yield Pearls:** * **Neurocysticercosis (NCC):** The most common cause of adult-onset seizures in developing countries. * **Diagnosis:** MRI/CT shows "starry sky" appearance (multiple calcified cysts) or a "scolex within a cyst" (pathognomonic). * **Drug of Choice:** Albendazole is preferred over Praziquantel for NCC as it has better CNS penetration. * **Morphology:** *T. solium* has a rostellum with hooks (armed), whereas *T. saginata* does not (unarmed).

Question 638: All of the following are true about Toxoplasma infection EXCEPT:

A. May occur due to ingestion of oocyst from cat's faeces
B. May spread by organ transplantation
C. Toxoplasmosis is usually symptomatic in immunocompetent persons (Correct Answer)
D. Infection is severe and progressive in immunocompromised hosts

Explanation: **Explanation:** *Toxoplasma gondii* is an obligate intracellular protozoan with a complex life cycle involving felids (cats) as definitive hosts and mammals/birds as intermediate hosts. **Why Option C is the correct answer (The Exception):** In immunocompetent individuals, primary infection with *Toxoplasma* is **asymptomatic in approximately 80-90% of cases**. When symptoms do occur, they are typically mild and self-limiting, resembling a mononucleosis-like syndrome (fever, malaise, and painless cervical lymphadenopathy). The parasite then remains dormant as tissue cysts (bradyzoites) for the life of the host. **Analysis of other options:** * **Option A:** Ingestion of **sporulated oocysts** from soil or water contaminated with cat feces is a primary mode of transmission. * **Option B:** Transmission can occur via **organ transplantation** (especially heart or kidney) if the donor has latent tissue cysts, or via blood transfusion. * **Option D:** In immunocompromised hosts (e.g., HIV/AIDS with CD4 <100 cells/µL), the infection is severe. It usually results from the **reactivation** of latent cysts, leading to life-threatening Toxoplasmic Encephalitis (the most common presentation) or disseminated disease. **High-Yield Clinical Pearls for NEET-PG:** * **Definitive Host:** Cat; **Intermediate Host:** Humans/Man. * **Infective Stages:** Oocysts (from cats), Tissue cysts (in undercooked meat), and Tachyzoites (transplacental). * **Congenital Toxoplasmosis Triad:** Chorioretinitis, Hydrocephalus, and Intracranial calcifications. * **Diagnosis:** Sabin-Feldman Dye Test (Gold Standard); Ring-enhancing lesions on CT/MRI in AIDS patients. * **Drug of Choice:** Pyrimethamine + Sulfadiazine.

Question 639: All of the following organisms cause malabsorption, EXCEPT?

A. Giardiasis
B. Ancylostoma duodenale (Correct Answer)
C. Strongyloides stercoralis
D. Capillaria philippinensis

Explanation: **Explanation:** The core concept tested here is the distinction between parasites that cause **malabsorption** (by damaging the intestinal mucosa or creating a physical barrier) and those that cause **iron-deficiency anemia** (by blood loss). **Why Ancylostoma duodenale is the correct answer:** *Ancylostoma duodenale* (Hookworm) primarily resides in the small intestine, where it attaches to the mucosa using its teeth/cutting plates to suck blood. Its primary clinical manifestation is **Iron Deficiency Anemia (IDA)** and protein loss (hypoalbuminemia) due to chronic hemorrhage. It does not typically cause generalized malabsorption of fats, carbohydrates, or vitamins. **Why the other options are incorrect:** * **Giardia lamblia:** The classic cause of malabsorption. It coats the duodenal mucosa (blunting villi), leading to "steatorrhea" (foul-smelling, fatty stools) and deficiency of fat-soluble vitamins (A, D, E, K). * **Strongyloides stercoralis:** In heavy infections, these larvae penetrate the intestinal wall, causing significant mucosal inflammation, villous atrophy, and a malabsorption syndrome similar to Celiac disease. * **Capillaria philippinensis:** This parasite causes "intestinal capillariasis," characterized by severe mucosal damage leading to massive protein-losing enteropathy and electrolyte imbalance (malabsorption). **NEET-PG High-Yield Pearls:** 1. **Hookworm Fact:** *A. duodenale* sucks more blood (0.15–0.2 ml/day) than *Necator americanus* (0.03 ml/day). 2. **Malabsorption List:** Other parasites causing malabsorption include *Cryptosporidium parvum*, *Isospora belli*, and *Microsporidia* (especially in HIV patients). 3. **D-Xylose Test:** Often used to differentiate mucosal malabsorption (like Giardiasis) from pancreatic insufficiency.

Question 640: Which of the following is NOT true about Cryptosporidium?

A. Spores are chlorine resistant
B. Spores are acid-fast
C. Spores are greater than 100 micrometers in size (Correct Answer)
D. Enzyme immunoassay is a diagnostic method

Explanation: **Explanation:** *Cryptosporidium parvum* is a significant protozoan parasite causing self-limiting diarrhea in immunocompetent individuals and life-threatening, chronic watery diarrhea in immunocompromised patients (especially those with HIV/AIDS). **1. Why Option C is the correct answer (The False Statement):** The infective stage of *Cryptosporidium* is the **oocyst** (not technically a "spore," though the terms are sometimes used interchangeably in casual contexts). These oocysts are remarkably small, measuring only **4–6 micrometers (µm)** in diameter. Stating they are greater than 100 µm is biologically incorrect; for comparison, a human red blood cell is about 7 µm. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** Oocysts are highly **chlorine-resistant** due to their thick, multi-layered wall. This makes them a common cause of waterborne outbreaks in public swimming pools and municipal water supplies. * **Option B:** They are **acid-fast**. In the laboratory, they are typically identified using the **Modified Kinyoun’s Acid-Fast stain**, where they appear as bright red/pink spherical bodies against a blue background. * **Option D:** **Enzyme Immunoassay (EIA)** or ELISA for detecting copro-antigens (antigens in stool) is a standard, highly sensitive diagnostic method used in clinical laboratories. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Fecal-oral route (contaminated water is the most common vehicle). * **Diagnosis:** Modified Ziehl-Neelsen stain (Acid-fast), Immunofluorescence (Gold Standard), and PCR. * **Treatment:** **Nitazoxanide** is the drug of choice for immunocompetent patients; in HIV patients, the priority is **HAART** to restore CD4 counts. * **Morphology:** Look for the "4-4-4" rule: 4 µm size, 4 sporozoites inside, and Acid-fast.

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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