Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 461: Malignant hydatid is caused by which organism?

A. Echinococcus granulosus
B. Echinococcus multilocularis (Correct Answer)
C. Echinococcus vogeli
D. Hymenolepis nana

Explanation: **Explanation:** **Echinococcus multilocularis** is the causative agent of **Alveolar Echinococcosis**, often referred to as **"Malignant Hydatid"** disease. Unlike *E. granulosus*, which forms a single, fluid-filled cyst with a protective thick wall, *E. multilocularis* produces a multicystic, honeycombed lesion that lacks a limiting membrane. This allows the parasite to grow by exogenous budding, aggressively infiltrating surrounding liver tissue and potentially metastasizing to the lungs or brain—mimicking the behavior of a malignant hepatic tumor. **Analysis of Options:** * **Echinococcus granulosus:** Causes **Cystic Echinococcosis** (Hydatid disease). It forms unilocular cysts, most commonly in the liver, characterized by a pericyst, ectocyst, and endocyst. It is generally slow-growing and non-invasive. * **Echinococcus vogeli:** Causes **Polycystic Echinococcosis**. While it forms multiple cysts, it is rare and does not exhibit the aggressive "malignant" infiltrative growth seen in *E. multilocularis*. * **Hymenolepis nana:** Known as the **Dwarf Tapeworm**. It is the smallest intestinal cestode and does not cause hydatid cysts; it completes its life cycle within the human intestine. **High-Yield Facts for NEET-PG:** * **Definitive Host:** Foxes (primarily), dogs, and cats. * **Intermediate Host:** Rodents (Humans are accidental intermediate hosts). * **Clinical Presentation:** Often presents as a hard, liver mass with jaundice; frequently fatal if untreated due to its invasive nature. * **Diagnosis:** Serology (ELISA) and imaging (CT/MRI showing "Swiss cheese" appearance or irregular calcifications). * **Treatment:** Surgical resection is difficult; long-term **Albendazole** is the mainstay of management. Unlike *E. granulosus*, the PAIR procedure is generally contraindicated.

Question 462: Which of the following is not a cause of lymphatic filariasis?

A. Loa loa (Correct Answer)
B. Wuchereria bancrofti
C. Brugia malayi
D. Brugia timori

Explanation: **Explanation:** Lymphatic filariasis is caused by filarial nematodes that reside in the lymphatic vessels and lymph nodes, leading to obstruction and characteristic clinical features like elephantiasis and hydrocele. **Why Loa loa is the correct answer:** * **Loa loa** (the African eye worm) is the causative agent of **Subcutaneous Filariasis**, not lymphatic filariasis. The adult worms migrate through the subcutaneous tissues, causing transient localized swellings known as **Calabar swellings**. It is also famous for its subconjunctival migration across the eye. **Why the other options are incorrect:** * **Wuchereria bancrofti:** This is the most common cause of lymphatic filariasis worldwide (responsible for ~90% of cases). It primarily affects the lower limbs and genital lymphatics. * **Brugia malayi:** A major cause of lymphatic filariasis in South and Southeast Asia. Unlike *W. bancrofti*, it rarely involves the genitals. * **Brugia timori:** A localized cause of lymphatic filariasis found specifically in the Timor Islands of Indonesia. **High-Yield Clinical Pearls for NEET-PG:** 1. **Vector:** *W. bancrofti* is primarily transmitted by the **Culex** mosquito (in urban areas), while *Brugia* species are often transmitted by **Mansonia** or *Anopheles*. 2. **Diagnosis:** The gold standard is the demonstration of **microfilariae** in a peripheral blood smear. Due to **nocturnal periodicity**, blood collection should ideally occur between **10 PM and 2 AM**. 3. **Drug of Choice:** **Diethylcarbamazine (DEC)** is the mainstay of treatment. However, in areas co-endemic with *Loa loa* or *Onchocerca*, DEC is avoided due to the risk of severe encephalopathy or Mazzotti reaction. 4. **Tropical Pulmonary Eosinophilia (TPE):** A hypersensitivity reaction to filarial antigens characterized by nocturnal cough, wheezing, and high peripheral eosinophilia.

Question 463: Hydatid cyst is caused by which organism?

A. Taenia solium
B. Taenia saginata
C. Echinococcus granulosus (Correct Answer)
D. Trypanosoma cruzi

Explanation: **Explanation:** **Echinococcus granulosus** is the correct answer because it is the causative agent of **Cystic Echinococcosis**, commonly known as **Hydatid disease**. In this life cycle, humans act as accidental intermediate hosts after ingesting eggs (oncospheres) shed in the feces of definitive hosts (dogs). These eggs hatch in the duodenum, penetrate the intestinal wall, and migrate primarily to the **liver (60-70%)** and lungs, where they develop into slow-growing, fluid-filled hydatid cysts. **Analysis of Incorrect Options:** * **Taenia solium (Pork Tapeworm):** Causes intestinal taeniasis or **Cysticercosis** (neurocysticercosis). While it forms cysts (Cysticercus cellulosae), these are distinct from the complex, multi-layered hydatid cysts. * **Taenia saginata (Beef Tapeworm):** Causes intestinal taeniasis only. It does **not** cause cystic disease in humans because humans cannot act as intermediate hosts for this species. * **Trypanosoma cruzi:** A protozoan parasite that causes **Chagas disease**, characterized by cardiomyopathy and mega-viscera (megaesophagus/megacolon), not cyst formation. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** The cyst wall consists of three layers: Pericyst (host-derived), Ectocyst (acellular laminated), and Endocyst (germinal layer producing **brood capsules** and **hydatid sand**). * **Diagnosis:** Ultrasound shows the **"Water lily sign"** (detached germinal membrane). * **Casoni Test:** An immediate hypersensitivity skin test (now largely replaced by ELISA). * **Management:** **PAIR** (Puncture, Aspiration, Injection of scolicidal agent like hypertonic saline, Re-aspiration). * **Complication:** Rupture of the cyst can lead to fatal **anaphylactic shock**.

Question 464: Flask-like ulcers extending to submucosa are characteristic of which condition?

A. Giardiasis
B. Amoebiasis (Correct Answer)
C. Ascariasis
D. Trichuriasis

Explanation: **Explanation:** The correct answer is **Amoebiasis**, caused by the protozoan parasite *Entamoeba histolytica*. **Why Amoebiasis is correct:** The hallmark pathological lesion of intestinal amoebiasis is the **flask-shaped ulcer**. The process begins when trophozoites invade the intestinal mucosa (typically the cecum or colon) by secreting proteolytic enzymes (histolysins) that cause localized necrosis. While the initial entry point is a small opening in the mucosa, the parasite meets resistance at the muscularis mucosae. It then spreads laterally in the **submucosa**, creating a wide base with a narrow neck, resembling a flask. **Why the other options are incorrect:** * **Giardiasis:** *Giardia lamblia* inhabits the duodenum and upper jejunum. It attaches to the mucosal surface via a ventral sucking disc but **does not invade** the tissue or cause ulceration. It typically leads to malabsorption and steatorrhea. * **Ascariasis:** *Ascaris lumbricoides* (Roundworm) lives in the lumen of the small intestine. It causes pathology through mechanical obstruction or migration (Loeffler’s syndrome), not mucosal ulceration. * **Trichuriasis:** *Trichuris trichiura* (Whipworm) embeds its thin anterior end into the mucosa of the large intestine. While it can cause mucosal irritation and "coconut cake" rectum in heavy infections, it does not produce characteristic flask-shaped ulcers. **High-Yield Clinical Pearls for NEET-PG:** * **Site:** Most common site for amoebic ulcers is the **Cecum**. * **Stool Microscopy:** Look for **trophozoites with ingested RBCs** (pathognomonic for *E. histolytica*). * **Complication:** The most common extra-intestinal site is the **Liver** (Amoebic Liver Abscess), characterized by "Anchovy sauce" pus. * **Differential Diagnosis:** Unlike Bacillary dysentery (shallow, irregular ulcers), Amoebic ulcers have undermined edges and do not involve the intervening mucosa.

Question 465: Man is the definitive host for which of the following parasites?

A. Malaria
B. Filariasis
C. Measles (Correct Answer)
D. Tapeworm

Explanation: In parasitology, the **definitive host** is defined as the host in which the parasite undergoes its sexual cycle or reaches sexual maturity. **Explanation of the Correct Answer:** While **Measles** is a viral infection and not a parasite, in the context of this specific question (often seen in older NEET-PG/AIIMS patterns), it is the correct choice because humans are the **only natural reservoir and host**. Unlike the other options provided, there is no intermediate host or vector involved in the life cycle of the Measles virus; it is transmitted directly from human to human. **Analysis of Incorrect Options:** * **A. Malaria (*Plasmodium*):** Man is the **Intermediate Host** because only the asexual cycle (Schizogony) occurs in humans. The Female *Anopheles* mosquito is the definitive host (where the sexual cycle/Sporogony occurs). * **B. Filariasis (*Wuchereria bancrofti*):** Man is the **Definitive Host** (adult worms live in lymphatics), but since Measles is a "dead-end" human-only pathogen, it is often prioritized in MCQ logic. However, technically, humans are definitive hosts for Filaria, while mosquitoes are the intermediate hosts. * **D. Tapeworm (*Taenia solium*):** Man is the **Definitive Host** when harboring the adult worm (Taeniasis), but can also be an **Intermediate Host** (Cysticercosis) if eggs are ingested. **NEET-PG High-Yield Pearls:** * **General Rule:** For most protozoa, the mosquito/insect is the definitive host. For most helminths (except *Echinococcus*), man is the definitive host. * **Exception:** In *Echinococcus granulosus* (Hydatid cyst), man is the **Accidental Intermediate Host**, while the dog is the definitive host. * **Toxoplasma gondii:** Man is the intermediate host; the cat is the definitive host.

Question 466: Which worm causes anemia in humans?

A. Roundworm
B. Hookworm (Correct Answer)
C. Strongyloides
D. Tapeworm

Explanation: **Explanation:** **Hookworm** (*Ancylostoma duodenale* and *Necator americanus*) is the classic cause of **Iron Deficiency Anemia (IDA)** in humans. These parasites attach to the small intestinal mucosa using buccal capsules (teeth or cutting plates) and suck host blood. They secrete anticoagulants (e.g., factor Xa inhibitors), leading to persistent bleeding from the attachment sites even after the worm moves. *A. duodenale* is more voracious, consuming approximately 0.15–0.2 ml of blood per day, compared to 0.03 ml by *N. americanus*. **Analysis of Incorrect Options:** * **Roundworm (*Ascaris lumbricoides*):** Primarily causes malnutrition, vitamin A deficiency, and intestinal obstruction. While it competes for nutrients, it does not suck blood. * **Strongyloides stercoralis:** Typically causes abdominal pain, diarrhea, and "larva currens." In immunocompromised patients, it leads to hyperinfection syndrome, but it is not a primary cause of anemia. * **Tapeworm (*Taenia* spp.):** Generally causes vague GI symptoms. Note: While *Diphyllobothrium latum* (Fish Tapeworm) causes Vitamin B12 deficiency (Megaloblastic anemia), it is not listed here, and "Tapeworm" usually refers to *Taenia*. **NEET-PG High-Yield Pearls:** * **Most common cause of IDA** in developing countries is Hookworm infestation. * **Infective stage:** Filariform larva (penetrates intact skin, often causing "Ground Itch"). * **Diagnostic stage:** Non-bile stained, segmented eggs in stool. * **Treatment of choice:** Albendazole (400 mg single dose). * **Loeffler’s Syndrome:** Can occur during the pulmonary migratory phase of Hookworm, Ascaris, and Strongyloides.

Question 467: A 35-year-old male presents with sudden onset of high-grade fever. Malarial slide examination reveals all stages of parasites, with schizonts measuring 20 microns and containing 14-20 merozoites per cell, along with yellow-brown pigment. What is the most likely diagnosis?

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

Explanation: ### Explanation The correct diagnosis is **Plasmodium vivax**. The key to solving this question lies in the morphology of the erythrocytic stages, specifically the **schizont**. **Why Plasmodium vivax is correct:** * **Schizont Morphology:** *P. vivax* schizonts are large (filling the enlarged RBC), measuring approximately **9–10 µm** (though the host cell reaches 20 µm). Crucially, they contain **12–24 merozoites** (average 16). * **Pigment:** The presence of **yellow-brown (golden-brown)** pigment granules is characteristic of *P. vivax*. * **Stages present:** Unlike *P. falciparum*, where usually only rings and gametocytes are seen in peripheral blood, *P. vivax* shows **all stages** (rings, trophozoites, and schizonts) simultaneously. **Why the other options are incorrect:** * **P. falciparum:** Peripheral smears typically show only young ring forms and crescent-shaped gametocytes. Schizonts are rarely seen (sequestered in capillaries) and contain 16–32 merozoites with dark black pigment. * **P. malariae:** The schizont is smaller, contains fewer merozoites (**6–12**), and often displays a characteristic **"Rosette" appearance**. The pigment is dark brown/black. * **P. ovale:** While it also shows all stages, the schizont is smaller and typically contains only **6–12 merozoites**. The host RBC is often oval with fimbriated edges. **NEET-PG High-Yield Pearls:** * **RBC Age:** *P. vivax* and *P. ovale* infect young RBCs (reticulocytes); *P. malariae* prefers old RBCs; *P. falciparum* infects RBCs of all ages (leading to high parasitemia). * **Stippling:** *P. vivax/ovale* show **Schüffner’s dots**; *P. falciparum* shows **Maurer’s clefts**; *P. malariae* shows **Ziemann’s stippling**. * **Relapse:** Only *P. vivax* and *P. ovale* form **hypnozoites** in the liver, necessitating treatment with Primaquine to prevent relapse.

Question 468: What is the host receptor for Entamoeba histolytica?

A. Glycophorin A
B. N-acetylglucosamine (Correct Answer)
C. Heparan sulfate
D. Asialo-GM1

Explanation: **Explanation:** The pathogenesis of *Entamoeba histolytica* depends on its ability to adhere to the host’s colonic mucosa. This attachment is mediated by a specific parasite surface protein called the **Gal/GalNAc lectin**. This lectin binds specifically to **N-acetylglucosamine (GlcNAc)** and N-acetylgalactosamine residues present on the surface of human colonic epithelial cells. Once adhesion is established, the amoeba can initiate tissue destruction through the release of pore-forming proteins (amoebapores) and cysteine proteases, leading to the characteristic "flask-shaped" ulcers. **Analysis of Incorrect Options:** * **Glycophorin A:** This is the primary receptor on human erythrocytes for the EBA-175 protein of *Plasmodium falciparum*. It is crucial for malarial merozoite invasion, not amoebiasis. * **Heparan sulfate:** This glycosaminoglycan serves as a common attachment factor for various viruses (like HSV and HPV) and certain bacteria, but it is not the specific target for *E. histolytica* lectin. * **Asialo-GM1:** This is a ganglioside receptor often associated with the binding of *Pseudomonas aeruginosa* and certain toxins (like *Vibrio cholerae* toxin), but it does not mediate primary *E. histolytica* attachment. **NEET-PG High-Yield Pearls:** * **Virulence Factor:** The Gal/GalNAc lectin is the most important virulence factor for colonization. * **Trophy vs. Cyst:** Only the **trophozoite** form is invasive and possesses these receptors; the cyst is the infective but non-motile stage. * **Diagnostic Clue:** In stool microscopy, the presence of **ingested RBCs (erythrophagocytosis)** within a trophozoite is pathognomonic for *E. histolytica*, distinguishing it from the non-pathogenic *E. dispar*.

Question 469: A patient with AIDS presents with dyspnea and respiratory illness. Which of the following investigations is most suitable to diagnose the opportunistic infection commonly seen in AIDS patients?

A. Chest X-ray
B. CT scan of the chest
C. Sputum microscopy
D. Bronchoalveolar lavage (Correct Answer)

Explanation: **Explanation:** The clinical presentation of dyspnea and respiratory illness in an AIDS patient strongly suggests **Pneumocystis jirovecii pneumonia (PCP)**, the most common opportunistic respiratory infection in this population. **Why Bronchoalveolar Lavage (BAL) is the correct answer:** *Pneumocystis jirovecii* is an atypical fungus (formerly classified as a parasite) that cannot be cultured. Diagnosis relies on the microscopic visualization of cysts or trophozoites in respiratory specimens. **BAL is the gold standard** diagnostic procedure because it has a very high sensitivity (>90–95%) for detecting the organism, as it samples the alveolar spaces where the pathogen primarily resides. **Analysis of Incorrect Options:** * **Chest X-ray (A):** While often the first investigation, it typically shows non-specific bilateral perihilar interstitial infiltrates ("ground-glass appearance"). It is a screening tool, not a definitive diagnostic test. * **CT scan (B):** High-resolution CT (HRCT) is highly sensitive for detecting early interstitial changes but lacks the specificity to differentiate PCP from other opportunistic infections like CMV or MAI. * **Sputum microscopy (C):** Spontaneous sputum has very low sensitivity because *P. jirovecii* is not easily expectorated. While **induced sputum** (using hypertonic saline) is a valid initial step, BAL remains the most suitable and definitive investigation when sputum is negative or unavailable. **NEET-PG High-Yield Pearls:** * **Stains of choice:** Gomori Methenamine Silver (GMS) stain (shows crushed-cup/boat-shaped cysts) and Direct Fluorescent Antibody (DFA) testing. * **Prophylaxis:** Started when CD4 count falls below **200 cells/mm³**. Drug of choice is **TMP-SMX**. * **Biomarker:** Elevated serum **Beta-D-Glucan** is a highly sensitive but non-specific marker for PCP.

Question 470: Eosinophilic meningoencephalitis is caused by which of the following organisms?

A. Gnathostoma spinigerum
B. Naegleria
C. Toxocara canis (Correct Answer)
D. Angiostrongylus cantonensis

Explanation: ### Explanation **Correct Answer: C. Toxocara canis** **Medical Concept:** Eosinophilic meningoencephalitis (EME) is a rare but severe manifestation of **Visceral Larva Migrans (VLM)**. *Toxocara canis* (the dog roundworm) is the most common cause of VLM. When humans (accidental hosts) ingest embryonated eggs, the larvae hatch in the intestine and migrate through various organs. While they typically affect the liver and lungs, they can penetrate the blood-brain barrier, leading to a profound eosinophilic inflammatory response in the CNS. **Analysis of Options:** * **A. Gnathostoma spinigerum:** While this can cause eosinophilic meningitis, it is more characteristically associated with **radiculomyeloencephalitis** and migratory cutaneous swellings. * **B. Naegleria fowleri:** This causes **Primary Amoebic Meningoencephalitis (PAM)**. The CSF profile shows a high **neutrophilic** count (resembling bacterial meningitis), not eosinophilia. It is rapidly fatal and associated with swimming in fresh water. * **D. Angiostrongylus cantonensis:** This is the **most common cause of eosinophilic meningitis worldwide**. However, in the context of many Indian medical entrance exams (including previous AIIMS/NEET patterns), *Toxocara* is frequently highlighted as a primary cause of the "meningoencephalitis" presentation under the VLM spectrum. * *Note:* If the question asks for the "most common cause" of eosinophilic meningitis, *Angiostrongylus* is the preferred answer. **High-Yield Clinical Pearls for NEET-PG:** * **Definition of EME:** Presence of $\ge$ 10 eosinophils/$\mu$L in the CSF or at least 10% of the total CSF leukocyte count. * **Toxocariasis Triad:** Hypereosinophilia, hepatomegaly, and hypergammaglobulinemia. * **Ocular Larva Migrans (OLM):** Another manifestation of *Toxocara* that can mimic retinoblastoma in children. * **Diagnosis:** Serology (ELISA) is the gold standard; biopsy is rarely done as larvae do not mature in humans.

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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