Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 591: Erythrocytic schizogony of Plasmodium liberates which stage?

A. Sporozoites
B. Trophozoites
C. Merozoites (Correct Answer)
D. Phanerozoites

Explanation: **Explanation:** The life cycle of *Plasmodium* involves two distinct phases of asexual reproduction (schizogony) in the human host: the **Exo-erythrocytic stage** (liver) and the **Erythrocytic stage** (RBCs). In the erythrocytic stage, a merozoite infects a red blood cell, matures into a ring-form **trophozoite**, and then develops into a **schizont**. When the schizont matures, the RBC ruptures, liberating a new generation of **merozoites** into the bloodstream. These released merozoites then infect fresh RBCs, repeating the cycle. The clinical paroxysm (fever and chills) in malaria coincides with this synchronous rupture of RBCs and the release of merozoites and metabolic byproducts. **Analysis of Options:** * **A. Sporozoites:** These are the infective forms of the parasite. They are inoculated into the human host by the bite of an infected female *Anopheles* mosquito. * **B. Trophozoites:** This is the active, feeding stage of the parasite *inside* the RBC. It does not represent the stage liberated upon schizogony; rather, it matures into the schizont. * **D. Phanerozoites:** This term refers to the exo-erythrocytic stages found in the tissues (liver) of birds/animals in certain species. In humans, the equivalent is the pre-erythrocytic schizont. **High-Yield Clinical Pearls for NEET-PG:** * **Hypnozoites:** These are dormant liver stages found only in *P. vivax* and *P. ovale*, responsible for **relapses**. * **Recrudescence:** Seen in *P. falciparum* and *P. malariae* due to the persistence of low-level parasitemia in the blood (not liver). * **Schüffner’s dots:** Seen in RBCs infected with *P. vivax* and *P. ovale*. * **Maurer’s clefts:** Seen in RBCs infected with *P. falciparum*.

Question 592: Which of the following parasitic eggs is typically identified in a stool microscopy examination?

A. Ascaris lumbricoides
B. Trichuris trichiura (Correct Answer)
C. Enterobius vermicularis
D. Hymenolepis nana

Explanation: ### Explanation The correct answer is **Trichuris trichiura (Option B)**. **Why it is correct:** *Trichuris trichiura* (Whipworm) resides in the large intestine. The adult females release eggs directly into the intestinal lumen, which are then passed out in the feces. These eggs are highly characteristic in stool microscopy: they are **barrel-shaped** with **bipolar mucus plugs**, making them a classic finding in a routine stool examination. **Analysis of Incorrect Options:** * **Enterobius vermicularis (Pinworm):** This is the most common "distractor" for this question. Adult females migrate out of the anus at night to deposit eggs on the **perianal skin** rather than in the stool. Therefore, stool microscopy is usually negative (sensitivity <5%). The gold standard for diagnosis is the **NIH swab** or **Scotch tape test**. * **Ascaris lumbricoides:** While *Ascaris* eggs are found in stool, the question asks which is "typically identified" or most characteristic. In many competitive exams, if *Trichuris* and *Enterobius* are compared, the focus is on the diagnostic method. However, note that *Ascaris* eggs (bile-stained, mamillated) are common, but *Trichuris* is the classic "stool-positive" textbook example often contrasted with *Enterobius*. * **Hymenolepis nana:** While eggs are found in stool, it is less frequently tested in this specific diagnostic context compared to the soil-transmitted helminths. **NEET-PG High-Yield Pearls:** * **Bile-stained eggs:** *Ascaris*, *Trichuris*, *Taenia* (Mnemonic: **ATT**). * **Non-bile stained eggs:** *Enterobius*, *Ancylostoma* (Hookworm), *H. nana*. * **Autoinfection:** Seen in *H. nana*, *Strongyloides stercoralis*, and *Enterobius*. * **Rectal Prolapse:** A classic clinical complication of heavy *Trichuris trichiura* infection in children.

Question 593: What is the second intermediate host of Diphyllobothrium latum?

A. Cylops
B. Man
C. Snail
D. Fresh water fish (Correct Answer)

Explanation: ### Explanation *Diphyllobothrium latum* (the Fish Tapeworm) is the longest tapeworm infecting humans, requiring two intermediate hosts to complete its complex life cycle. **Why the correct answer is right:** The life cycle begins when eggs are passed in feces and hatch into **coracidia** in water. These are ingested by the **1st intermediate host (Cyclops)**, where they develop into **procercoid larvae**. When a **freshwater fish** (such as pike or perch) eats the infected Cyclops, the procercoid transforms into the **plerocercoid larva** (sparganum) in the fish's muscle. This plerocercoid is the infective stage for humans; thus, the freshwater fish serves as the **2nd intermediate host**. **Analysis of Incorrect Options:** * **A. Cyclops:** This is the **1st intermediate host** where the procercoid larva develops. * **B. Man:** Humans (and other fish-eating mammals) serve as the **definitive host**, harboring the adult worm in the small intestine. * **C. Snail:** Snails are common intermediate hosts for trematodes (flukes) like *Schistosoma* or *Fasciola*, but they play no role in the life cycle of *D. latum*. **Clinical Pearls for NEET-PG:** 1. **Vitamin B12 Deficiency:** *D. latum* has a high affinity for Vitamin B12. Chronic infection leads to **Megaloblastic Anemia** and subacute combined degeneration of the spinal cord. 2. **Infective Stage:** The **Plerocercoid larva** (found in undercooked freshwater fish). 3. **Diagnostic Feature:** Operculated, non-embryonated eggs in stool (resembling fluke eggs). 4. **Treatment:** Praziquantel is the drug of choice.

Question 594: In malaria, what causes Schüffner's dots?

A. Disintegrated parts of the parasite
B. The intracellular space not occupied by the parasite
C. Pigment released from the breakdown of hemoglobin (Correct Answer)
D. Gametocytes

Explanation: **Explanation:** **Schüffner’s dots** are fine, round, reddish-pink granules seen in the cytoplasm of red blood cells (RBCs) infected with *Plasmodium vivax* and *Plasmodium ovale*. **1. Why Option C is Correct:** The correct answer is the **pigment released from the breakdown of hemoglobin**. As the malarial parasite grows within the erythrocyte, it consumes host hemoglobin. This metabolic process results in the formation of malarial pigment (hemozoin) and causes structural changes in the RBC membrane. Schüffner’s dots represent multiple **caveola-vesicle complexes** formed by the invagination of the RBC plasma membrane. These are essentially morphological manifestations of the parasite's transport system, carrying proteins and metabolic byproducts (derived from hemoglobin digestion) to the host cell surface. **2. Why Other Options are Incorrect:** * **Option A:** Schüffner’s dots are host-cell modifications, not fragments of the parasite itself. * **Option B:** The dots are specific morphological changes within the cytoplasm, not merely "empty space." * **Option D:** Gametocytes are the sexual stages of the parasite; while they may be present in a cell containing Schüffner’s dots, they do not constitute the dots themselves. **3. NEET-PG High-Yield Pearls:** * **Species Specificity:** Schüffner’s dots are characteristic of **P. vivax** and **P. ovale**. * **Other Stipplings:** * **Maurer’s Clefts:** Large, coarse dots seen in **P. falciparum**. * **Ziemann’s Stippling:** Fine dust-like dots seen in **P. malariae**. * **RBC Size:** In *P. vivax* and *P. ovale* infections, the infected RBCs are typically **enlarged/hypertrophied**, whereas in *P. falciparum* and *P. malariae*, they remain normal or smaller in size.

Question 595: In malaria, where does the sexual cycle occur?

A. Sporozoites to gametocytes
B. Gametocytes to sporozoites (Correct Answer)
C. Occurs in humans
D. Responsible for relapse

Explanation: **Explanation:** The life cycle of *Plasmodium* involves two hosts: humans (intermediate host) and the female *Anopheles* mosquito (definitive host). **1. Why Option B is Correct:** The **sexual cycle (Sporogony)** occurs entirely within the mosquito. It begins when the mosquito ingests male and female **gametocytes** from an infected human. In the mosquito's gut, these mature into gametes, undergo fertilization to form a zygote (ookinete), and eventually produce thousands of **sporozoites** (the infective stage for humans). Thus, the transition from gametocytes to sporozoites represents the completion of the sexual phase. **2. Analysis of Incorrect Options:** * **Option A (Sporozoites to gametocytes):** This represents the **asexual cycle (Schizogony)** which occurs in humans. It includes the exo-erythrocytic (liver) and erythrocytic (RBC) stages. * **Option C (Occurs in humans):** Humans host the asexual cycle only. The sexual cycle requires the cooler body temperature and specific physiological environment of the mosquito. * **Option D (Responsible for relapse):** Relapse is caused by **hypnozoites** (dormant liver stages), specifically seen in *P. vivax* and *P. ovale*. This is a feature of the asexual cycle, not the sexual cycle. **Clinical Pearls for NEET-PG:** * **Definitive Host:** Female *Anopheles* mosquito (where sexual fusion occurs). * **Intermediate Host:** Humans (where asexual multiplication occurs). * **Infective form to humans:** Sporozoites (found in mosquito salivary glands). * **Infective form to mosquito:** Gametocytes (found in human peripheral blood). * **Exflagellation:** This process refers to the formation of male microgametes in the mosquito's midgut—a high-yield term often associated with the sexual cycle.

Question 596: What is the infective form of Plasmodium falciparum for mosquitoes?

A. Merozoites
B. Sporozoites
C. Gametocytes (Correct Answer)
D. Trophozoites

Explanation: ### Explanation The life cycle of *Plasmodium falciparum* involves two hosts: the human (asexual cycle/schizogony) and the female *Anopheles* mosquito (sexual cycle/sporogony). **Why Gametocytes are correct:** After several rounds of asexual reproduction in human RBCs, some merozoites differentiate into male (**microgametocytes**) and female (**macrogametocytes**). These are the only forms capable of surviving the mosquito's digestive tract. When a mosquito bites an infected human, it ingests these gametocytes, which then undergo fertilization in the mosquito's midgut to initiate the sexual cycle. Therefore, gametocytes are the **infective stage for the vector**. **Why other options are incorrect:** * **Merozoites:** These are released from ruptured liver cells (exo-erythrocytic) or RBCs (erythrocytic). They are responsible for infecting new RBCs within the human host but are digested if ingested by a mosquito. * **Sporozoites:** This is the **infective form for humans**. They are stored in the mosquito's salivary glands and injected into the human bloodstream during a blood meal. * **Trophozoites:** This is the metabolically active feeding stage within the human RBC (e.g., the "ring form"). While present in the blood, they do not initiate infection in the mosquito. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *P. falciparum* gametocytes are characteristically **crescent or banana-shaped**, unlike the round gametocytes of other species. * **Primaquine:** This is the drug of choice for its **gametocidal** action against *P. falciparum*, used to prevent the transmission of malaria back to mosquitoes. * **Exflagellation:** This process occurs in the mosquito's midgut, where the microgametocyte rapidly divides to form eight flagellated microgametes.

Question 597: The pathogenicity of Entamoeba histolytica is indicated by which of the following?

A. Zymodeme pattern (Correct Answer)
B. Size
C. Nuclear pattern
D. ELISA test

Explanation: **Explanation:** The differentiation between pathogenic and non-pathogenic strains of *Entamoeba* is a high-yield concept in parasitology. **1. Why Zymodeme Pattern is Correct:** *Entamoeba histolytica* (pathogenic) is morphologically identical to *Entamoeba dispar* (non-pathogenic). To distinguish them, **Zymodeme analysis** is used. This involves the study of isoenzyme patterns (electrophoretic mobility of enzymes like hexokinase and glucose-phosphate isomerase). There are approximately 22 zymodemes identified; pathogenic strains typically belong to **Zymodeme II**, while non-pathogenic strains belong to Zymodeme I. **2. Why Other Options are Incorrect:** * **Size (B):** Both *E. histolytica* and *E. dispar* fall within the same size range (10–60 µm for trophozoites). While the "Small Race" (*E. hartmanni*) is distinguished by size, it does not determine the pathogenicity of *E. histolytica*. * **Nuclear Pattern (C):** Both species share the classic "cartwheel" appearance (central karyosome and fine peripheral chromatin). Nuclear morphology cannot differentiate between pathogenic and commensal species. * **ELISA test (D):** While ELISA is used for diagnosis (detecting antigens in stool or antibodies in serum), it is a diagnostic tool rather than an intrinsic biological indicator of the strain's pathogenicity. **High-Yield Clinical Pearls for NEET-PG:** * **Morphological Distinction:** The only morphological feature that indicates pathogenicity in a stool sample is the presence of **ingested RBCs (erythrophagocytosis)** within the trophozoite. * **Molecular Gold Standard:** PCR is now the preferred method to differentiate *E. histolytica* from *E. dispar*. * **Cyst Stage:** The mature cyst of *E. histolytica* is **quadrinucleate** (4 nuclei) with rounded chromatoid bars.

Question 598: Which of the following is not a cestode?

A. Diphyllobothrium latum
B. Taenia saginata
C. Schistosoma mansonia (Correct Answer)
D. Echinococcus granulosus

Explanation: **Explanation:** The correct answer is **Schistosoma mansoni** because it belongs to the class **Trematoda (Flukes)**, not Cestoda. Helminths are broadly classified into: 1. **Cestodes (Tapeworms):** Segmented, ribbon-like, hermaphroditic, and lack a digestive tract (e.g., *Taenia*, *Echinococcus*). 2. **Trematodes (Flukes):** Unsegmented, leaf-shaped, and usually hermaphroditic (except *Schistosoma*, which is dioecious/bisexual). 3. **Nematodes (Roundworms):** Cylindrical and unsegmented. **Analysis of Options:** * **Schistosoma mansoni (Correct):** It is a blood fluke. Unlike most trematodes, Schistosomes are **non-hermaphroditic** (separate sexes) and do not have an operculated egg. It primarily causes intestinal schistosomiasis and portal hypertension. * **Diphyllobothrium latum (Incorrect):** Known as the "Fish Tapeworm," it is the longest cestode infecting humans. It is unique among cestodes for having an operculated egg and causing **Vitamin B12 deficiency** (megaloblastic anemia). * **Taenia saginata (Incorrect):** Known as the "Beef Tapeworm." It is a classic cestode characterized by the absence of a rostellum and hooks (unarmed scolex). * **Echinococcus granulosus (Incorrect):** Known as the "Dog Tapeworm," it causes **Hydatid cyst** disease in humans (accidental intermediate hosts). **NEET-PG High-Yield Pearls:** * **All cestodes** require an intermediate host **except** *Hymenolepis nana* (smallest tapeworm), which can complete its life cycle in a single host. * **Schistosoma** is the only trematode that infects via **cercarial skin penetration**; others are typically ingested. * *S. haematobium* is strongly associated with **Squamous Cell Carcinoma of the bladder**.

Question 599: In which stage of filariasis are microfilaria seen in peripheral blood?

A. Tropical eosinophilia
B. Early adenolymphangitis stage (Correct Answer)
C. Late adenolymphangitis stage
D. Elephantiasis

Explanation: ### Explanation The presence of microfilariae in peripheral blood depends on the clinical stage of the infection and the host's immune response. **1. Why "Early adenolymphangitis stage" is correct:** During the **early/acute stage** of lymphatic filariasis (characterized by lymphangitis and lymphadenitis), adult worms are active in the lymphatics and are producing **microfilariae**. These microfilariae circulate in the peripheral blood (often with nocturnal periodicity), making this the primary stage for diagnosis via blood film. **2. Why the other options are incorrect:** * **Tropical Pulmonary Eosinophilia (TPE):** This is a hypersensitivity reaction to the filarial antigens. The body’s immune system rapidly clears microfilariae from the circulation, trapping them in the lungs. Therefore, **microfilariae are characteristically absent** in peripheral blood. * **Late adenolymphangitis stage:** As the disease progresses, repeated inflammatory episodes lead to the death of adult worms and the development of lymphatic obstruction. * **Elephantiasis:** This is the **chronic obstructive stage**. By this point, the lymphatic vessels are fibrosed and the adult worms are usually dead or calcified. Consequently, microfilariae disappear from the blood, making diagnosis reliant on clinical presentation or antibody/antigen detection. --- ### High-Yield NEET-PG Pearls * **Diagnostic Gold Standard:** Demonstration of microfilariae in a **peripheral blood smear** (collected between 10 PM and 2 AM for *W. bancrofti*). * **Occult Filariasis:** Refers to conditions like TPE where the disease is present but microfilariae cannot be found in the blood. * **Drug of Choice:** **Diethylcarbamazine (DEC)** is the mainstay of treatment; however, it is contraindicated in TPE if the patient has a high microfilarial load of *Loa loa* due to the risk of encephalopathy. * **Key Lab Finding in TPE:** Massively elevated serum **IgE levels** and absolute eosinophil count (>3000/µL).

Question 600: A 20-year-old male patient presents with fever, intermittent abdominal pain, and dysuria. He has a history of swimming in a freshwater lake 10 days prior. Urine microscopy findings are provided. Which of the following is the causative agent?

A. Schistosoma mansoni
B. Schistosoma hematobium (Correct Answer)
C. Schistosoma japonicum
D. Schistosoma mekongi

Explanation: **Explanation:** The clinical presentation of fever, abdominal pain, and **dysuria** following freshwater exposure (swimming) is classic for **Schistosomiasis** (Snail fever). The key diagnostic clue is the site of involvement: **urinary symptoms (dysuria)** point directly toward **Schistosoma haematobium**. **1. Why Schistosoma haematobium is correct:** * **Habitat:** Unlike other species, *S. haematobium* adult worms reside in the **vesical and pelvic venous plexuses**. * **Pathogenesis:** Eggs penetrate the bladder wall to be excreted in urine, causing inflammation, hematuria, and dysuria. * **Morphology:** It is characterized by eggs with a **terminal spine**, which would be visible on urine microscopy. **2. Why other options are incorrect:** * **S. mansoni & S. japonicum:** These species primarily inhabit the **inferior and superior mesenteric veins**, respectively. They cause intestinal and hepatic schistosomiasis (bloody diarrhea, portal hypertension) rather than urinary symptoms. *S. mansoni* eggs have a **lateral spine**, while *S. japonicum* eggs are rounded with a **rudimentary lateral knob**. * **S. mekongi:** Similar to *S. japonicum*, it causes intestinal disease and is restricted to the Mekong River basin in Southeast Asia. **Clinical Pearls for NEET-PG:** * **Intermediate Host:** Freshwater snails (*Bulinus* species for *S. haematobium*). * **Infective Stage:** **Cercaria** (enters via skin penetration). * **Diagnostic Stage:** Eggs in urine (terminal spine) or stool. * **Chronic Complication:** *S. haematobium* is a known risk factor for **Squamous Cell Carcinoma of the bladder**. * **Drug of Choice:** Praziquantel.

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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