Parasitology Practice Questions

Q: In malaria, which is the definitive host?

Female Anopheles mosquito. ### Explanation In parasitology, the **definitive host** is defined as the host in which the parasite undergoes its **sexual cycle** (syngamy and sporogony). Conversely, the intermediate host is where the asexual cycle (schizogony) occurs. **1. Why Option D is Correct:** In the life cycle of *Plasmodium*, the sexual phase occurs within the **Female Anopheles mosquito**. When the mosquito ingests gametocytes from an infected human, fertilization occurs in the mosquito's midgut, leading to the formation of a zygote, ookinete, oocyst, and eventually sporozoites. Because sexual reproduction happens here, the mosquito is the definitive host. Humans serve as the **intermediate host**. **2. Why Other Options are Incorrect:** * **Option A (Male Anopheles):** Male mosquitoes do not take blood meals; they feed on plant juices and nectar. Therefore, they cannot transmit malaria or host the parasite. * **Option B (Sand fly):** This is the vector for **Leishmaniasis** (Kala-azar) and Sandfly fever. * **Option C (Tsetse fly):** This is the vector for **African Trypanosomiasis** (Sleeping Sickness). **3. High-Yield Clinical Pearls for NEET-PG:** * **Exception Rule:** In most parasitic infections, humans are the definitive host. Malaria and *Echinococcus granulosus* (Hydatid cyst) are notable exceptions where humans are the intermediate host. * **Infective Form:** The **Sporozoite** is the infective form for humans (inoculated by the mosquito). * **Diagnostic Form:** The **Gametocyte** is the infective form for the mosquito (picked up from humans). * **Relapse:** Caused by **hypnozoites** (dormant stages in the liver) seen in *P. vivax* and *P. ovale*. * **Recrudescence:** Seen in *P. falciparum* due to incomplete clearance of parasites from the blood.

Q: Which amoeba commonly causes meningoencephalitis?

Naegleria. **Explanation:** **Naegleria fowleri**, often referred to as the "brain-eating amoeba," is the correct answer. It is a free-living amoeba found in warm freshwater bodies. It enters the human body through the nasal mucosa, typically during swimming or diving, and migrates along the olfactory nerves through the cribriform plate to reach the brain. This results in **Primary Amoebic Meningoencephalitis (PAM)**, a rapidly progressive and usually fatal condition characterized by sudden onset headache, fever, and meningeal signs. **Why other options are incorrect:** * **Entamoeba histolytica:** While it is a major human pathogen, it primarily causes intestinal amoebiasis (dysentery) and amoebic liver abscesses. Brain involvement (cerebral amoebiasis) is an extremely rare, secondary complication. * **Giardia:** This is a flagellated protozoan, not an amoeba. It causes giardiasis, a malabsorptive diarrheal disease localized to the small intestine; it does not cause CNS infections. * **Escherichia coli:** This is a gram-negative bacterium, not an amoeba. While it is a common cause of neonatal meningitis, it does not fit the classification of "amoebic meningoencephalitis." **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Look for motile trophozoites in a fresh **wet mount of CSF**. * **Drug of Choice:** Amphotericin B (often used in combination with Rifampicin or Miltefosine). * **Differential:** *Acanthamoeba* and *Balamuthia* cause **Granulomatous Amoebic Encephalitis (GAE)**, which is more chronic and typically occurs in immunocompromised hosts, unlike the acute PAM caused by *Naegleria* in healthy individuals.

Q: Which tissue is most commonly affected in cysticercosis?

Muscle. **Explanation:** Cysticercosis is caused by the larval stage (*Cysticercus cellulosae*) of the pork tapeworm, **Taenia solium**. Humans become accidental intermediate hosts by ingesting eggs through contaminated food or water. **1. Why Muscle is Correct:** While Neurocysticercosis (NCC) is the most clinically significant and symptomatic presentation, **subcutaneous tissue and skeletal muscles** are statistically the **most common** sites of larval lodgment. In many cases, muscular involvement remains asymptomatic or presents as small, palpable, painless nodules, which is why it is often underdiagnosed compared to neurological involvement. **2. Analysis of Incorrect Options:** * **Brain (Option A):** This is the most common site for **symptomatic** cysticercosis and the leading cause of adult-onset seizures worldwide. However, in terms of absolute frequency of larval distribution in the body, it ranks second to muscle/subcutaneous tissue. * **Eye (Option B):** Ocular cysticercosis occurs in about 1–3% of cases. It can involve the subretinal space or vitreous, potentially causing blindness, but it is far less common than muscular or neural involvement. * **Liver (Option C):** The liver is the primary site for **Hydatid disease** (*Echinococcus granulosus*), not cysticercosis. *T. solium* larvae rarely involve the hepatic parenchyma. **High-Yield NEET-PG Pearls:** * **Definitive Host:** Human (harbors adult worm). * **Intermediate Host:** Pig (normal); Human (accidental). * **Mode of Infection for Cysticercosis:** Ingestion of eggs (fecal-oral) or autoinfection. (Note: Ingesting undercooked pork leads to intestinal Taeniasis, not cysticercosis). * **Diagnosis:** MRI is the gold standard for NCC (shows "scolex within a cyst"). * **Treatment:** Albendazole is the drug of choice (combined with steroids to prevent inflammatory response to dying larvae).

Q: What is the definitive host in Toxoplasmosis?

Cat. **Explanation:** In parasitology, the **definitive host** is defined as the host in which the parasite undergoes its **sexual reproductive cycle**. For *Toxoplasma gondii*, members of the family Felidae (domestic **cats** and their relatives) are the only definitive hosts. Within the intestinal epithelium of the cat, the parasite undergoes gametogeny and schizogony, resulting in the excretion of unsporulated **oocysts** in the feces. **Analysis of Options:** * **A. Cat (Correct):** As the definitive host, cats harbor the sexual stage. Humans and other animals become infected by ingesting sporulated oocysts from contaminated soil/water or bradyzoites in undercooked meat. * **B. Man (Incorrect):** Humans serve as **intermediate hosts**. In humans, the parasite only undergoes asexual reproduction (tachyzoites and bradyzoites) and exists in tissue cysts. Humans are usually a "dead-end" host. * **C. Pig (Incorrect):** Pigs, like sheep and rodents, are intermediate hosts. Ingesting undercooked pork containing tissue cysts is a common route of transmission to humans. * **D. Dog (Incorrect):** Dogs are accidental intermediate hosts. While they can carry the parasite in their tissues, they do not produce oocysts. **High-Yield NEET-PG Pearls:** * **Infective stage for humans:** Sporulated oocysts (from cat feces) or Bradyzoites (in tissue cysts). * **Diagnosis:** Sabin-Feldman Dye Test (Gold Standard), though rarely used now; IgM/IgG ELISA is standard. * **Congenital Toxoplasmosis Triad:** Chorioretinitis, Hydrocephalus, and Intracranial calcifications. * **HIV/Immunocompromised:** Most common cause of ring-enhancing lesions in the brain (CNS Toxoplasmosis). * **Treatment:** Pyrimethamine + Sulfadiazine (Drug of choice). Spiramycin is used in pregnancy to prevent vertical transmission.

Q: What is the band form of P. malariae?

Trophozoite stage. The correct answer is **B. Trophozoite stage**. ### **Explanation** *Plasmodium malariae* is unique among the malaria species for its specific morphological features in the peripheral blood smear. The **"Band form"** occurs during the **growing trophozoite stage**. As the parasite matures, it stretches across the diameter of the red blood cell (RBC), forming a characteristic rectangular or ribbon-like band. This happens because *P. malariae* prefers older RBCs, which are less flexible, constraining the parasite's growth into this distinct shape. ### **Why other options are incorrect:** * **Schizont stage:** In *P. malariae*, the schizont is characterized by a **"Daisy-head" or "Rosette" appearance**, typically containing 6–12 merozoites clustered around a central mass of brown pigment (hemozoin). * **Merozoite stage:** These are the small, individual daughter cells released upon the rupture of a schizont. They do not form bands; they are the invasive stage that targets new RBCs. * **Gametocyte stage:** The gametocytes of *P. malariae* are typically small, round, or oval and fill the RBC. They do not exhibit the elongated band morphology. ### **NEET-PG High-Yield Pearls for *P. malariae*:** 1. **Quartan Malaria:** It has a 72-hour erythrocytic cycle, leading to fever every fourth day. 2. **RBC Preference:** It infects only **senescent (old) RBCs**, leading to lower parasitemia levels compared to *P. falciparum*. 3. **Ziemann’s Stippling:** Fine dust-like pinkish dots may be seen in the cytoplasm of infected RBCs (unlike Schüffner’s dots in *P. vivax*). 4. **Clinical Complication:** It is classically associated with **Nephrotic Syndrome** (specifically Quartan Malarial Nephropathy) due to immune complex deposition.

Q: Black water fever is caused by?

P. falciparum. **Explanation:** **Blackwater Fever** is a severe and life-threatening complication of malaria specifically caused by **Plasmodium falciparum**. **Why P. falciparum is correct:** The condition is characterized by massive **intravascular hemolysis** (destruction of red blood cells within the blood vessels). This leads to severe hemoglobinemia and subsequent **hemoglobinuria**, which turns the urine a dark red, brown, or black color (hence the name "Blackwater"). The underlying mechanism involves a Type II hypersensitivity-like reaction, often triggered in patients with repeated infections or those treated irregularly with Quinine. The massive release of hemoglobin can lead to acute tubular necrosis and **acute renal failure**, which is the primary cause of mortality. **Why other options are incorrect:** * **P. vivax:** While it causes significant morbidity and "Benign Tertian Malaria," it rarely causes the massive intravascular hemolysis required to produce blackwater fever. * **Kala-azar / Leishmaniasis:** These are caused by *Leishmania donovani*. While they cause pancytopenia and massive splenomegaly, they do not cause acute intravascular hemolysis or hemoglobinuria. **NEET-PG High-Yield Pearls:** * **Key Triad:** Hemolysis, hemoglobinuria, and acute renal failure. * **Parasite Load:** *P. falciparum* is the most severe because it infects RBCs of all ages (unlike *P. vivax* which prefers reticulocytes). * **Sequestration:** *P. falciparum* causes RBCs to develop "knobs" (PfEMP-1), leading to cytoadherence and microvascular occlusion (Cerebral Malaria). * **Drug Association:** Historically associated with **Quinine** therapy.

Q: Chagas disease is caused by which protozoan parasite?

Trypanosoma cruzi. **Explanation:** **1. Why Trypanosoma cruzi is Correct:** Chagas disease, also known as **American Trypanosomiasis**, is caused by the flagellate protozoan *Trypanosoma cruzi*. It is primarily transmitted to humans through the feces of the **Triatomine bug** (also known as the "kissing bug" or "assassin bug"). The parasite enters the bloodstream through the bite wound or mucous membranes. It is endemic to Central and South America. **2. Why Other Options are Incorrect:** * **Trypanosoma gambiense & Trypanosoma brucei:** These species cause **African Trypanosomiasis** (Sleeping Sickness). They are transmitted by the **Tsetse fly**. *T. gambiense* causes the chronic West African form, while *T. rhodesiense* causes the acute East African form. * **Leishmania donovani:** This parasite causes **Visceral Leishmaniasis** (Kala-azar), characterized by the pentad of fever, hepatosplenomegaly, pancytopenia, lymphadenopathy, and hypergammaglobulinemia. It is transmitted by the **Sandfly** (*Phlebotomus*). **3. High-Yield Clinical Pearls for NEET-PG:** * **Acute Phase Sign:** **Romaña’s sign** (painless unilateral periorbital edema). * **Chronic Phase Complications:** Dilated cardiomyopathy, **Megaesophagus**, and **Megacolon** (due to destruction of the myenteric/Auerbach’s plexus). * **Diagnostic Morphologies:** *T. cruzi* is found as **C-shaped trypomastigotes** in peripheral blood and **amastigotes** in cardiac/smooth muscle tissues. * **Drug of Choice:** Benznidazole or Nifurtimox.

Question 1

In malaria, which is the definitive host?

Accepted Answer

Female Anopheles mosquito

Answer

Male Anopheles mosquito

Answer

Sand fly

Answer

Tsetse fly

Question 2

Which amoeba commonly causes meningoencephalitis?

Accepted Answer

Naegleria

Answer

Entamoeba histolytica

Answer

Giardia

Answer

Escherichia coli

Question 3

Which tissue is most commonly affected in cysticercosis?

Accepted Answer

Muscle

Answer

Brain

Answer

Eye

Answer

Liver

Question 4

What is the definitive host in Toxoplasmosis?

Accepted Answer

Cat

Answer

Man

Answer

Pig

Answer

Dog

Question 5

A 28-year-old man presents with vague muscle pain involving his right arm that developed several weeks after eating undercooked meat. A biopsy from the affected muscle reveals encysted forms of Trichinella spiralis. What type of white blood cell is most likely to be increased in numbers in the peripheral blood of this individual?

Accepted Answer

Eosinophil

Answer

Basophil

Answer

Macrophage

Answer

Neutrophil

Question 6

What is the band form of P. malariae?

Accepted Answer

Trophozoite stage

Answer

Schizont stage

Answer

Merozoite stage

Answer

Gametocyte stage

Question 7

Dracunculosis infection occurs through which mode of transmission?

Accepted Answer

Ingestion of water containing cyclops infected with the parasite

Answer

Ingestion of water containing the parasite directly

Answer

Ingestion of contaminated fish

Answer

Penetration of skin by the parasite

Question 8

Sputum examination is not useful in the diagnosis of which of the following parasitic infections?

Accepted Answer

Trichuris trichura

Answer

Ancylostoma duodenale

Answer

Paragonimiasis

Answer

Strongyloides stercoralis

Question 9

Black water fever is caused by?

Accepted Answer

P. falciparum

Answer

Kala azar

Answer

P. vivax

Answer

Leishmaniasis

Question 10

Chagas disease is caused by which protozoan parasite?

Accepted Answer

Trypanosoma cruzi

Answer

Trypanosoma gambiense

Answer

Trypanosoma brucei

Answer

Leishmania donovani

Parasitology — MCQs

Parasitology — MCQs

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