Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 251: Which of the following inhibits digestion in the intestinal mucosa?

A. Vibrio cholerae
B. Giardia lamblia (Correct Answer)
C. Enterokinase
D. Schistosoma haematobium

Explanation: **Explanation:** **Giardia lamblia** is the correct answer because it causes malabsorption by physically and biochemically interfering with the intestinal mucosa. The trophozoites attach to the duodenal and jejunal mucosa using a ventral sucking disc. This attachment, combined with the "carpeting" effect of high parasitic loads, leads to the **blunting of microvilli** and the **inhibition of brush border enzymes** (such as disaccharidases). This disruption inhibits the final stages of digestion and absorption, resulting in characteristic foul-smelling, fatty stools (steatorrhea). **Analysis of Incorrect Options:** * **Vibrio cholerae:** Causes secretory diarrhea via the cholera toxin, which increases cAMP levels. It leads to massive water and electrolyte loss but does not primarily inhibit the digestion of nutrients or damage the brush border enzymes. * **Enterokinase:** This is a physiological enzyme (not a pathogen) produced by the duodenum that converts trypsinogen to trypsin. It is essential *for* digestion, not an inhibitor of it. * **Schistosoma haematobium:** This is a blood fluke that primarily affects the venous plexus of the urinary bladder, leading to hematuria. It does not inhabit the intestinal mucosa or interfere with digestion. **NEET-PG High-Yield Pearls:** * **Habitat:** Duodenum and upper jejunum (acidic environment). * **Diagnosis:** Stool microscopy shows "falling leaf" motility (trophozoites) or cysts. String test (Entero-test) can be used for sampling. * **Antigen Detection:** Immunochromatographic assays for Giardia-specific antigen (GSA 65) are highly sensitive. * **Drug of Choice:** Tinidazole or Metronidazole. * **Association:** Common in patients with IgA deficiency.

Question 252: What is the scientific name for the pinworm (seat worm)?

A. Enterobius (Correct Answer)
B. Dracunculus
C. Ankylostoma
D. Necator

Explanation: **Explanation:** The correct answer is **Enterobius vermicularis**, commonly known as the **pinworm, seat worm, or threadworm**. It is a nematode that primarily inhabits the human cecum and appendix. The name "seat worm" is derived from the intense perianal itching (pruritus ani) it causes, typically at night when the gravid female migrates out of the anus to deposit eggs on the perianal skin. **Analysis of Options:** * **Enterobius (Correct):** The most common helminthic infection in developed countries. It is unique because it is transmitted via the fecal-oral route or retroinfection and is diagnosed using the **NIH swab or Scotch Tape test** rather than routine stool examination. * **Dracunculus medinensis:** Known as the **Guinea worm**. It is transmitted by drinking water containing infected *Cyclops* (copepods) and is characterized by a blister on the lower limb through which the adult worm emerges. * **Ankylostoma duodenale:** Known as the **Old World Hookworm**. It causes iron deficiency anemia due to blood-sucking at the intestinal mucosa. * **Necator americanus:** Known as the **New World Hookworm**. Like *Ankylostoma*, it is transmitted via larval penetration of the skin (ground itch). **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Albendazole or Mebendazole (treat the entire family to prevent reinfection). * **Diagnostic Feature:** Eggs are non-bile stained and have a characteristic **"D-shape"** (flattened on one side). * **Clinical Sign:** Nocturnal enuresis and perianal pruritus in children. * **Autoinfection:** Common due to the "scratch-and-mouth" cycle.

Question 253: Which of the following is NOT a cause of malabsorption?

A. Giardia lamblia
B. Mycobacterium avium-intracellulare
C. Ascariasis (Correct Answer)
D. Microsporidium

Explanation: **Explanation:** The correct answer is **Ascariasis**. Malabsorption in the gastrointestinal tract typically occurs due to extensive mucosal damage, villous atrophy, or chronic inflammation of the small intestinal wall. **1. Why Ascariasis is the correct answer:** *Ascaris lumbricoides* (Roundworm) primarily resides in the lumen of the small intestine. While a heavy worm burden can lead to **protein-energy malnutrition** (by competing for host nutrients) or **mechanical bowel obstruction**, it does not typically cause a true malabsorption syndrome (like steatorrhea) because it does not invade or flatten the intestinal mucosa. **2. Why the other options cause malabsorption:** * **Giardia lamblia:** This is a classic cause of malabsorption. The trophozoites adhere to the duodenal and jejunal mucosa using a ventral sucking disc, leading to **villous atrophy** and "coating" of the mucosa, which prevents fat absorption (causing foul-smelling steatorrhea). * **Mycobacterium avium-intracellulare (MAI/MAC):** Common in immunocompromised patients (HIV/AIDS), MAC causes systemic infection where macrophages laden with acid-fast bacilli infiltrate the intestinal lamina propria. This mimics **Whipple’s disease**, obstructing lymphatic drainage and causing severe malabsorption. * **Microsporidium:** These are obligate intracellular protozoa (e.g., *Enterocytozoon bieneusi*) that infect enterocytes, leading to cell degeneration and villous blunting, resulting in chronic diarrhea and malabsorption in AIDS patients. **NEET-PG High-Yield Pearls:** * **Giardiasis:** Diagnosis via "Falling leaf motility" on stool microscopy or the **Entero-test (String test)**. * **Strongyloides stercoralis:** Another parasite that *can* cause malabsorption due to its internal autoinfection cycle and mucosal invasion. * **Celiac Disease vs. Tropical Sprue:** Both cause malabsorption; Celiac is gluten-sensitive and affects the proximal small bowel, while Tropical Sprue affects the entire small bowel (including the ileum, leading to Vitamin B12 deficiency).

Question 254: Which of the following is NOT a zoonotic disease?

A. Leptospirosis
B. Dracunculiasis (Guinea worm) (Correct Answer)
C. Rabies
D. Plague

Explanation: **Explanation:** A **zoonosis** is defined as an infectious disease that is naturally transmissible from vertebrate animals to humans. The key to answering this question lies in identifying the natural reservoir of the pathogen. **Why Dracunculiasis is the correct answer:** Dracunculiasis (Guinea worm disease), caused by the nematode *Dracunculus medinensis*, is **not** a zoonotic disease because humans are the **only definitive host**. The life cycle involves humans and intermediate hosts (Cyclops/water fleas), but there is no animal reservoir involved in the natural transmission cycle. This unique characteristic made it a prime candidate for eradication; India was certified free of Guinea worm in 2000. **Analysis of Incorrect Options:** * **Leptospirosis:** A classic zoonosis caused by *Leptospira interrogans*. It is transmitted to humans via contact with water contaminated by the urine of infected animals (primarily rodents/rats). * **Rabies:** A viral zoonosis transmitted through the saliva of infected mammals (dogs, bats, monkeys). It is nearly 100% fatal once symptoms appear. * **Plague:** Caused by *Yersinia pestis*, it is a zoonotic infection of rodents transmitted to humans via the bite of an infected rat flea (*Xenopsylla cheopis*). **NEET-PG High-Yield Pearls:** * **Eradicated/Eliminated in India:** Smallpox (1977), Guinea worm (2000), Polio (2014), and Maternal & Neonatal Tetanus (2015). * **Intermediate Host of Guinea Worm:** *Cyclops* (Water flea). * **Infective Stage:** Third-stage (L3) larvae within the Cyclops. * **Diagnostic sign:** A painful blister, usually on the lower limb, from which the adult female worm emerges.

Question 255: Protozoan cysts are typically stored in which of the following solutions for preservation?

A. Saline
B. Phenol
C. Sodium hypochlorite
D. Formalin (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Formalin)** Formalin (typically as a 5% or 10% buffered solution) is the gold standard for preserving protozoan cysts, eggs, and larvae in stool samples. The underlying medical concept is **fixation**: Formalin acts by cross-linking proteins, which preserves the morphology of the cysts and prevents further development or decomposition. It is the primary component of the **Formalin-Ether Concentration Technique**, the most widely used method in diagnostic parasitology to increase the yield of cysts and ova. **Analysis of Incorrect Options:** * **A. Saline:** Normal saline (0.85% NaCl) is used for **direct wet mounts** to observe the motility of trophozoites (e.g., *Entamoeba histolytica*). However, it is not a preservative; samples in saline will eventually degenerate due to bacterial overgrowth. * **B. Phenol:** While phenol has antiseptic properties, it is too caustic and causes significant morphological distortion, making it unsuitable for preserving delicate protozoan structures for microscopic identification. * **C. Sodium hypochlorite:** This is a strong disinfectant and bleaching agent. It is used for surface decontamination but would destroy the internal structures of protozoan cysts, rendering them unidentifiable. **Clinical Pearls for NEET-PG:** * **Trophozoites vs. Cysts:** Formalin preserves cysts well but is poor for trophozoites. For trophozoites, **PVA (Polyvinyl Alcohol)** or **SDA (Sodium acetate-acetic acid-formalin)** is preferred. * **Concentration Technique:** The Formalin-Ether technique is preferred over Zinc Sulfate flotation for heavy eggs (like *Ascaris*) and operculated eggs. * **Safety:** Formalin-fixed specimens are generally non-infectious (except for certain coccidian oocysts like *Cryptosporidium*), making them safer for transport.

Question 256: Napier's aldehyde test is used in the diagnosis of which condition?

A. Malaria
B. Typhus fever
C. Kala azar (Correct Answer)
D. Enteric fever

Explanation: **Explanation:** **Napier’s Aldehyde Test** (also known as the Formol-gel test) is a non-specific biochemical test used for the presumptive diagnosis of **Kala-azar (Visceral Leishmaniasis)**. **Why Kala-azar is correct:** The underlying principle of this test is the detection of **hypergammaglobulinemia** (a massive increase in IgG levels). In chronic Kala-azar, there is a profound polyclonal B-cell activation leading to excessive serum globulins. When 1–2 drops of 40% formaldehyde are added to 1 mL of the patient's serum, the serum undergoes gelification and becomes opaque (resembling the white of a boiled egg) within 2 to 20 minutes. A positive result typically appears only after the disease has been present for at least 3 months. **Why other options are incorrect:** * **Malaria:** Diagnosis is primarily made via peripheral blood smears (thick and thin) or Rapid Diagnostic Tests (RDTs) detecting HRP-2 or LDH antigens. * **Typhus Fever:** Diagnosed using the **Weil-Felix test** (a heterophile agglutination test) or molecular methods like PCR. * **Enteric Fever:** Diagnosed via the **Widal test** (detecting O and H antibodies) or gold-standard blood/bone marrow cultures. **High-Yield Clinical Pearls for NEET-PG:** * **Chopra’s Antimony Test:** Another non-specific test for Kala-azar using urea stibamine (positive result = profuse flocculent precipitate). * **Specific Diagnosis:** The gold standard for Kala-azar is the demonstration of **LD bodies** (amastigotes) in splenic or bone marrow aspirates. * **rK39 Immunochromatographic Strip Test:** The most common rapid diagnostic test used in the field today due to its high sensitivity and specificity. * **Montenegro Skin Test:** Negative in active Kala-azar but becomes positive after recovery (indicates delayed hypersensitivity).

Question 257: Xenodiagnosis is used in the diagnosis of a disease that is transmitted by the bite of which of the following vectors?

A. Body louse
B. Deer tick
C. Reduviid bug (Correct Answer)
D. Mosquito

Explanation: **Explanation:** **Xenodiagnosis** is a specialized diagnostic technique where a laboratory-bred, pathogen-free vector is allowed to feed on a patient suspected of having a specific infection. The vector is later dissected to check for the multiplication of the parasite. 1. **Why Reduviid bug is correct:** Xenodiagnosis is the classic diagnostic method for **Chagas disease** (American Trypanosomiasis), caused by *Trypanosoma cruzi*. The vector for this disease is the **Reduviid bug** (also known as the Triatomine or "kissing" bug). In chronic stages of the disease, when parasitemia is too low to be detected by standard blood smears, uninfected Reduviid bugs are allowed to bite the patient. If *T. cruzi* is present, it multiplies in the bug's hindgut and is detected in its feces after 2–4 weeks. 2. **Why other options are incorrect:** * **Body louse:** Transmits Epidemic typhus (*Rickettsia prowazekii*) and Relapsing fever. Diagnosis is usually via serology or PCR. * **Deer tick (*Ixodes*):** Transmits Lyme disease and Babesiosis. Diagnosis relies on clinical presentation (Erythema migrans) and serology (ELISA/Western Blot). * **Mosquito:** Transmits Malaria, Filariasis, and various viral fevers. Diagnosis is typically via peripheral blood smears (Malaria) or antigen detection. **High-Yield Clinical Pearls for NEET-PG:** * **Chagas Disease Triad:** Cardiomyopathy (apical aneurysm), Megaesophagus, and Megacolon. * **Romaña’s sign:** Unilateral painless periorbital swelling (portal of entry through the conjunctiva). * **C-shaped Trypomastigotes:** Characteristic morphology of *T. cruzi* on peripheral smears. * **NNN Medium:** The specialized culture medium used for both *Leishmania* and *Trypanosoma*.

Question 258: Which of the following parasites is NOT included in the class Coccidia?

A. Cryptosporidium
B. Naegleria (Correct Answer)
C. Isospora
D. Cyclospora

Explanation: ### Explanation The correct answer is **Naegleria**. **1. Why Naegleria is the correct answer:** The class **Coccidia** (belonging to the Phylum Apicomplexa) consists of intracellular protozoa that typically undergo a complex life cycle involving both sexual (sporogony) and asexual (schizogony) reproduction. **Naegleria fowleri** is not a coccidian; it is a **Free-living Amoeba** (specifically an excavate amoeboflagellate). It is known as the "brain-eating amoeba" and causes Primary Amoebic Meningoencephalitis (PAM), a rapidly fatal CNS infection. **2. Why the other options are incorrect:** * **Cryptosporidium (C. parvum):** A classic coccidian parasite. It is a major cause of self-limiting diarrhea in immunocompetent individuals and chronic, life-threatening diarrhea in AIDS patients. * **Isospora (now Cystoisospora belli):** A coccidian parasite that causes intestinal infections. It is characterized by large, oval oocysts that stain acid-fast. * **Cyclospora (C. cayetanensis):** Another coccidian parasite associated with waterborne or foodborne (e.g., contaminated berries) outbreaks of diarrhea. **3. NEET-PG Clinical Pearls:** * **Acid-Fast Staining:** All intestinal coccidia (Cryptosporidium, Cyclospora, and Cystoisospora) are **Modified Acid-Fast positive**. * **Oocyst Sizes (High-Yield):** * *Cryptosporidium:* 4–6 µm (Smallest) * *Cyclospora:* 8–10 µm * *Cystoisospora:* 25–30 µm (Largest) * **Naegleria fowleri:** Usually contracted via intranasal entry during swimming in warm freshwater. Diagnosis is made by seeing motile trophozoites in a **wet mount of CSF**. * **Treatment:** Drug of choice for intestinal coccidia (except Cryptosporidium) is **Cotrimoxazole**. For Naegleria, **Amphotericin B** is used.

Question 259: Which of the following statements concerning malaria is TRUE?

A. Malaria caused by each of the four plasmodial species can relapse after initial illness.
B. Red cells negative for the Duffy blood group antigen are resistant to Plasmodium vivax. (Correct Answer)
C. Renal impairment is a grave prognostic sign in falciparum malaria.
D. Plasmodium malariae can cause immune-mediated nephropathy.

Explanation: **Explanation:** **Correct Option (B):** *Plasmodium vivax* utilizes the **Duffy blood group antigen** (specifically Fya or Fyb) as a receptor on the surface of red blood cells (RBCs) to facilitate merozoite invasion. Individuals who are Duffy-negative (common in West African populations) lack these receptors, making their RBCs resistant to *P. vivax* infection. This is a classic example of genetic resistance to malaria. **Analysis of Incorrect Options:** * **Option A:** Only *P. vivax* and *P. ovale* cause **relapses** due to the presence of dormant liver stages called **hypnozoites**. *P. falciparum* and *P. malariae* do not form hypnozoites; any recurrence of these species is termed "recrudescence" (due to surviving erythrocytic forms). * **Option C:** While renal impairment is a feature of severe malaria, it is **not** considered a "grave" prognostic sign when compared to cerebral malaria or pulmonary edema. With dialysis, renal failure in malaria has a relatively good prognosis for recovery. * **Option D:** This statement is actually **true** (Quartan malarial nephropathy). However, in the context of standard NEET-PG questions based on Harrison’s or K.D. Chatterjee, Option B is the "most" definitive biological fact regarding host-parasite interaction often tested. *(Note: If this were a multiple-true question, D would also be correct, but B is the primary teaching point for P. vivax).* **High-Yield Clinical Pearls for NEET-PG:** * **Sickle Cell Trait (HbAS):** Provides protection against severe *P. falciparum*. * **G6PD Deficiency:** Offers protection against *P. falciparum* by increasing oxidative stress in RBCs. * **Schüffner’s dots:** Seen in *P. vivax* and *P. ovale*. * **Maurer’s clefts:** Seen in *P. falciparum*. * **Drug of choice for relapse:** Primaquine (contraindicated in G6PD deficiency).

Question 260: True about Babesiosis?

A. Caused by Babesia microti (Correct Answer)
B. Resides in RBC
C. Resides in WBC
D. Chloroquine is the treatment of choice

Explanation: **Explanation:** **Babesiosis** is a malaria-like parasitic disease caused by protozoa of the genus *Babesia*. In humans, the most common species responsible for infection is **Babesia microti**. 1. **Why Option A is Correct:** *Babesia microti* is the primary causative agent of human babesiosis (transmitted by the *Ixodes scapularis* tick). It is an intraerythrocytic parasite that invades red blood cells, making this option the definitive true statement. 2. **Why Option B & C are Incorrect:** While *Babesia* does reside in **RBCs** (making Option B technically true in a biological sense), in the context of multiple-choice questions, the species name (Option A) is the most specific "textbook" fact. Option C is incorrect because *Babesia* does **not** infect white blood cells (WBCs). 3. **Why Option D is Incorrect:** Unlike Malaria, **Chloroquine is ineffective** against Babesiosis. The treatment of choice is a combination of **Atovaquone plus Azithromycin** (for mild-to-moderate cases) or **Quinine plus Clindamycin** (for severe cases). **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes* tick (the same vector for Lyme disease and Anaplasmosis; co-infections are common). * **Morphology:** On a Giemsa-stained peripheral smear, look for the characteristic **"Maltese Cross" appearance** (tetrads of merozoites). Unlike *P. falciparum*, there is no pigment (hemozoin) or gametocytes. * **Risk Factors:** Asplenic patients are at high risk for severe, life-threatening disease. * **Diagnosis:** Gold standard is the peripheral thin smear; PCR is more sensitive for low parasitemia.

Practice by Chapter

Classification of Parasites

Practice Questions

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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