Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 261: A 33-year-old woman presents with chronic diarrhea. Microscopic identification of which of the following stages of the organism in a fecal sample would provide the strongest evidence for cryptosporidiosis?

A. Egg
B. Cyst
C. Oocyst (Correct Answer)
D. Merozoite

Explanation: **Explanation:** **1. Why Oocyst is Correct:** *Cryptosporidium hominis* and *C. parvum* are intracellular protozoan parasites. The **oocyst** is the diagnostic, infectious, and environmental stage of the parasite. In cryptosporidiosis, these thick-walled oocysts are excreted in the feces of the infected host. They are highly characteristic because they are **acid-fast**, appearing as bright red/pink spheres (4–6 µm) against a blue-green background on a Modified Ziehl-Neelsen stain. Identifying these oocysts in a stool sample is the gold standard for microscopic diagnosis. **2. Why Other Options are Incorrect:** * **A. Egg:** This term refers to the life cycle stage of helminths (worms), such as *Ascaris* or *Taenia*. Protozoa like *Cryptosporidium* do not produce eggs. * **B. Cyst:** This is the resting/infective stage for other intestinal protozoa like *Entamoeba histolytica* or *Giardia lamblia*. *Cryptosporidium* specifically utilizes an oocyst, which is the product of sexual reproduction (sporogony). * **C. Merozoite:** These are intermediate stages produced during asexual reproduction (schizogony) within the host's intestinal epithelial cells. While they exist in the life cycle, they are not typically excreted in stool or used for routine diagnosis. **3. NEET-PG High-Yield Pearls:** * **Clinical Presentation:** A leading cause of self-limiting diarrhea in immunocompetent individuals but causes **severe, life-threatening, voluminous watery diarrhea** in HIV/AIDS patients (CD4 count <200 cells/mm³). * **Staining:** Use **Modified Ziehl-Neelsen (Acid-Fast) stain** or Kinyoun’s stain. * **Transmission:** Fecal-oral route; notorious for being **chlorine-resistant**, often leading to outbreaks in swimming pools. * **Treatment:** **Nitazoxanide** is the drug of choice for immunocompetent patients; HAART (to boost CD4 count) is the priority for HIV patients.

Question 262: Primary amoebic meningoencephalitis is caused by which of the following organisms?

A. Naegleria fowleri (Correct Answer)
B. Entamoeba histolytica
C. Escherichia coli
D. Babesia coli

Explanation: **Explanation:** **Primary Amoebic Meningoencephalitis (PAM)** is a rapidly fatal central nervous system infection caused by **Naegleria fowleri**, often referred to as the "brain-eating amoeba." 1. **Why Naegleria fowleri is correct:** * **Pathogenesis:** It is a free-living thermophilic amoeba found in warm freshwater. Infection occurs when contaminated water is forcefully inhaled into the nasal cavity (e.g., during swimming or diving). * **Route:** The trophozoites penetrate the **nasal mucosa**, cross the **cribriform plate**, and travel along the **olfactory nerves** to reach the brain, causing acute, fulminant hemorrhagic necrosis of the brain tissue. 2. **Why the other options are incorrect:** * **Entamoeba histolytica:** Causes intestinal amoebiasis and liver abscesses. While it can rarely cause brain abscesses, it does not cause the specific clinical entity known as PAM. * **Escherichia coli:** A common cause of neonatal meningitis, but it is a bacterium, not an amoeba. * **Babesia coli:** Likely a distractor (confusing *Babesia microti*, a blood parasite, with *Balantidium coli*, a ciliate). Neither causes PAM. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Rapid onset of headache, fever, and meningeal signs in a patient with a recent history of swimming in stagnant water. * **Diagnosis:** CSF microscopy shows **motile trophozoites** (wet mount). Note: Cysts are never seen in brain tissue, only trophozoites. * **Treatment of Choice:** **Amphotericin B** (often combined with Rifampicin or Miltefosine). * **Differential:** Do not confuse with **Granulomatous Amoebic Encephalitis (GAE)**, which is caused by *Acanthamoeba* or *Balamuthia* and typically follows a more chronic course in immunocompromised hosts.

Question 263: Malarial pigment is formed by?

A. Parasite
B. Bilirubin
C. Hemoglobin (Correct Answer)
D. Any of the above

Explanation: **Explanation:** The correct answer is **Hemoglobin**. **Why Hemoglobin is correct:** Malarial pigment, also known as **hemozoin**, is a dark brown, granular byproduct of the digestion of host erythrocytes by *Plasmodium* parasites. Within the red blood cell, the parasite consumes host **hemoglobin** to obtain essential amino acids. However, this process releases free **heme**, which is toxic to the parasite. To detoxify it, the parasite polymerizes the reactive heme into an insoluble, non-toxic crystalline form called hemozoin. This pigment is a hallmark of malarial infection and is visible under light microscopy within the parasite's food vacuole. **Why other options are incorrect:** * **Parasite:** While the parasite *creates* the pigment, the pigment itself is chemically derived from the host's hemoglobin, not the parasite's own cellular structures. * **Bilirubin:** Bilirubin is a breakdown product of heme metabolism in the human liver/spleen. While malaria causes hemolysis leading to indirect hyperbilirubinemia (jaundice), it is not the constituent of the malarial pigment found inside the RBC. **High-Yield Clinical Pearls for NEET-PG:** * **Schüffner’s dots:** These are different from malarial pigment; they are morphological changes (stippling) in the RBC membrane seen specifically in *P. vivax* and *P. ovale*. * **Detection:** Hemozoin is birefringent under polarized light. * **Drug Mechanism:** Chloroquine works by inhibiting the biocrystallization of heme into hemozoin, leading to a buildup of toxic heme that kills the parasite. * **Phagocytosis:** After the RBC ruptures, malarial pigment is taken up by monocytes and macrophages (reticuloendothelial system), often leading to a slate-grey discoloration of the spleen and liver in chronic cases.

Question 264: Which antigen is used in the rapid antigen test for Leishmania?

A. HRP-1
B. HRP-2
C. rk-39 (Correct Answer)
D. p-24

Explanation: **Explanation:** **Correct Answer: C. rk-39** The rapid diagnostic test (RDT) for Visceral Leishmaniasis (Kala-azar) detects antibodies against the **rk-39 antigen**. This is a recombinant protein consisting of 39 amino acids from a kinesin-like gene found in *Leishmania donovani*. It is highly sensitive and specific for active Visceral Leishmaniasis. In an immunochromatographic strip test (ICT), the presence of anti-rk39 antibodies in the patient's serum indicates infection. It is particularly useful in field settings due to its rapid results and lack of requirement for sophisticated laboratory equipment. **Analysis of Incorrect Options:** * **A & B (HRP-1 & HRP-2):** Histidine-Rich Proteins (specifically HRP-2) are antigens used in RDTs for **Malaria**, specifically to detect *Plasmodium falciparum*. * **D (p-24):** This is a structural protein (capsid antigen) of the **HIV** virus. The p24 antigen assay is used for early diagnosis of HIV infection during the window period before antibodies develop. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The definitive diagnosis of Kala-azar remains the demonstration of **LD bodies** (Amastigotes) in splenic or bone marrow aspirates. Splenic aspirate is more sensitive but carries a higher risk of hemorrhage. * **rk-39 Limitation:** It can remain positive for several months even after a successful cure; therefore, it cannot be used to distinguish between a relapse and a past infection. * **Culture:** *Leishmania* is grown on **NNN (Novy-MacNeal-Nicolle) medium**, where the promastigote form is seen. * **Montenegro Test:** A delayed hypersensitivity skin test that is **negative** in active Visceral Leishmaniasis (due to deficient cell-mediated immunity) but becomes positive after recovery.

Question 265: KFD (Kyasanur Forest Disease) is transmitted by which vector?

A. Fleas
B. Mites
C. Ticks (Correct Answer)
D. Mosquitoes

Explanation: **Explanation:** **Kyasanur Forest Disease (KFD)**, popularly known as "Monkey Fever," is a viral hemorrhagic fever caused by the KFD virus (Family: *Flaviviridae*). **Why Ticks are the correct answer:** The primary vector for KFD is the **Hard Tick (*Haemaphysalis spinigera*)**. The virus is maintained in a cycle involving small mammals (shrews, rats) and monkeys. Humans act as accidental, dead-end hosts when they are bitten by infected tick nymphs while visiting forest areas. **Analysis of Incorrect Options:** * **Fleas:** These are vectors for diseases like **Plague** (*Yersinia pestis*) and Endemic Typhus. * **Mites:** Specifically, the Trombiculid mite (chigger) is the vector for **Scrub Typhus** (*Orientia tsutsugamushi*). * **Mosquitoes:** These transmit a wide range of viral diseases (Dengue, Zika, Yellow Fever) and parasitic diseases (Malaria, Filariasis), but they do not transmit KFD. **High-Yield Clinical Pearls for NEET-PG:** * **Geographic Distribution:** Endemic to the Western Ghats of India (first identified in Shimoga district, Karnataka). * **Host:** Monkeys (Langurs and Bonnet macaques) are highly susceptible; their sudden death in a forest is often the first sign of a KFD outbreak. * **Clinical Presentation:** Characterized by sudden onset high fever, headache, severe myalgia, and hemorrhagic manifestations. * **Diagnosis:** PCR (early stage) or IgM ELISA (later stage). * **Prevention:** A **formalin-inactivated vaccine** is available for people in endemic areas.

Question 266: Which of the following organisms does NOT cause arthritis?

A. Wuchereria bancrofti
B. Echinococcus granulosus
C. Dracunculus medinensis
D. Trichuris trichiura (Correct Answer)

Explanation: **Explanation:** The correct answer is **Trichuris trichiura** (Whipworm). This organism primarily inhabits the large intestine (caecum) and is not associated with joint involvement. Its clinical manifestations are strictly gastrointestinal, ranging from asymptomatic infection to chronic diarrhea, iron-deficiency anemia, and, classically, **rectal prolapse** in children with heavy worm burdens. **Why the other options are incorrect:** * **Wuchereria bancrofti:** While primarily causing lymphatic filariasis (elephantiasis), it can cause **filarial arthritis**, typically involving the knee joint. This occurs due to the presence of microfilariae or adult worms triggering an immune-mediated inflammatory response in the synovium. * **Echinococcus granulosus:** This parasite causes Hydatid disease. While cysts most commonly form in the liver and lungs, they can occur in the **bones and joints** (1-2% of cases). Bone involvement often leads to pathological fractures or secondary joint destruction and arthritis. * **Dracunculus medinensis:** The Guinea worm typically emerges from the lower limbs. If the worm ruptures during extraction or if the blister becomes secondarily infected, it can lead to **septic arthritis** or "sterile" inflammatory arthritis if the worm dies near a joint. **High-Yield Clinical Pearls for NEET-PG:** * **Trichuris trichiura:** Look for "barrel-shaped eggs with bipolar plugs" in stool microscopy. * **Parasites causing Arthritis:** Apart from the options above, *Strongyloides stercoralis* and *Toxocara canis* are also known to occasionally cause reactive joint symptoms. * **Dracunculus medinensis:** The intermediate host is the **Cyclops** (water flea). It is currently targeted for global eradication.

Question 267: What is the most common cause of disseminated cutaneous leishmaniasis?

A. L. donovani
B. L. tropica
C. L. mexicana (Correct Answer)
D. L. brasilensis

Explanation: **Explanation:** **Disseminated Cutaneous Leishmaniasis (DCL)** is a rare clinical manifestation characterized by numerous non-ulcerating nodules across the body, resembling lepromatous leprosy. It occurs due to a specific **T-cell energy** (lack of cell-mediated immunity) against *Leishmania* antigens. 1. **Why L. mexicana is correct:** In the New World (Americas), the **_L. mexicana_ complex** (specifically *L. mexicana amazonensis*) is the most common cause of DCL. It is characterized by a negative Montenegro skin test, indicating a failure of the host's immune response to contain the parasite, leading to uncontrolled cutaneous spread. 2. **Analysis of Incorrect Options:** * **L. donovani:** This is the primary agent of **Visceral Leishmaniasis (Kala-azar)**. While it can cause Post-Kala-azar Dermal Leishmaniasis (PKDL), it does not typically cause DCL. * **L. tropica:** This is a major cause of **Old World Cutaneous Leishmaniasis** (oriental sore). While *L. aethiopica* is the Old World cause of DCL, *L. tropica* usually causes localized, healing ulcers. * **L. braziliensis:** This species is the classic cause of **Mucocutaneous Leishmaniasis (Espundia)**, involving the destruction of nasopharyngeal tissues, rather than disseminated nodules. **NEET-PG High-Yield Pearls:** * **Montenegro Skin Test:** Positive in localized cutaneous and mucocutaneous forms; **Negative** in Visceral and Disseminated Cutaneous Leishmaniasis. * **Chiclero’s Ulcer:** A specific clinical variant caused by *L. mexicana* affecting the ear pinna. * **DCL vs. PKDL:** DCL occurs in patients who never had visceral disease; PKDL occurs as a sequel to treated *L. donovani* infection.

Question 268: A patient with a retroviral infection presents with frequent diarrheal episodes. Stool examination reveals acid-fast oocysts of size 4-6 microns. What is the probable diagnosis?

A. Toxoplasma gondii
B. Cryptosporidium parvum (Correct Answer)
C. Coccidioides immitis
D. Sarcocystis

Explanation: ### Explanation The correct answer is **Cryptosporidium parvum**. **Why it is correct:** This case highlights a classic presentation of opportunistic infection in an immunocompromised host (retroviral infection/HIV). The diagnosis is confirmed by two key morphological features: 1. **Size:** The oocysts are small, measuring **4–6 microns**. 2. **Staining:** They are **acid-fast**, appearing red against a blue background in a Modified Ziehl-Neelsen (Kinyoun) stain. In HIV patients, *Cryptosporidium* causes chronic, profuse, watery diarrhea that can lead to severe dehydration and malabsorption. **Why the other options are incorrect:** * **Toxoplasma gondii:** While common in HIV patients, it typically presents as CNS lesions (ring-enhancing lesions on CT). Its oocysts are not usually found in human stool as humans are intermediate hosts. * **Coccidioides immitis:** This is a dimorphic fungus, not a parasite. It primarily causes pulmonary infections ("Valley Fever") and is characterized by spherules containing endospores in tissue, not acid-fast oocysts in stool. * **Sarcocystis:** While it can cause diarrhea, its oocysts (or sporocysts) are much larger (approx. 15–19 microns) and it is a less common cause of chronic diarrhea in HIV compared to *Cryptosporidium*. **NEET-PG High-Yield Pearls:** * **Differential Diagnosis of Acid-Fast Oocysts:** * *Cryptosporidium:* 4–6 µm (Smallest) * *Cyclospora:* 8–10 µm (Twice the size of Cryptosporidium) * *Cystoisospora:* 25–30 µm (Largest, oval-shaped) * **Treatment:** Nitazoxanide is the drug of choice in immunocompetent patients; however, in HIV patients, the most effective management is **HAART** to improve CD4 counts. * **Transmission:** Fecal-oral route; highly resistant to chlorination.

Question 269: What is the infective form of Entamoeba histolytica?

A. Uninucleate cyst
B. Binucleate cyst
C. Quadrinucleate cyst (Correct Answer)
D. All of the above

Explanation: ### Explanation **Correct Answer: C. Quadrinucleate cyst** **Medical Concept:** *Entamoeba histolytica* exists in two main stages: the **trophozoite** (vegetative/invasive form) and the **cyst** (survival/infective form). The process of encystation occurs in the intestinal lumen. A young cyst begins as a **uninucleate** form, matures into a **binucleate** form, and finally becomes a **mature quadrinucleate cyst**. The **mature quadrinucleate cyst** is the infective stage because it is resistant to gastric acid and environmental stressors. Upon ingestion, it undergoes **excystation** in the small intestine, where each of the four nuclei undergoes a final division to produce eight small metacystic trophozoites. **Why other options are incorrect:** * **A & B (Uninucleate and Binucleate cysts):** These are immature stages of the parasite. If ingested, they are generally not hardy enough to survive the gastric environment or lack the developmental maturity to complete the life cycle effectively in the host. * **D (All of the above):** Only the mature, four-nucleated stage is recognized as the definitive infective form in clinical parasitology. **High-Yield Clinical Pearls for NEET-PG:** * **Mode of Transmission:** Fecal-oral route (contaminated food/water). * **Diagnostic Stage:** Both trophozoites (in acute dysentery) and cysts (in chronic cases/carriers) can be found in stool. * **Morphology:** The mature cyst contains **blunt-ended chromatoid bars** (composed of ribosomes) and 1–4 nuclei with central karyosomes. * **Invasive Form:** The **trophozoite** is the form responsible for causing "flask-shaped ulcers" in the colon and extra-intestinal manifestations like Amoebic Liver Abscess (anchovy sauce pus). * **Treatment:** Metronidazole/Tinidazole (for trophozoites) followed by a luminal amoebicide like Diloxanide furoate or Paromomycin (to eradicate cysts).

Question 270: The cystic form is seen in man in all of the following except:

A. E. histolytica
B. Giardia
C. Trichomonas (Correct Answer)
D. Toxoplasma

Explanation: **Explanation:** The correct answer is **Trichomonas** because it is one of the few medically important protozoa that exists **only in the trophozoite stage**. It does not possess a cystic stage in its life cycle. **1. Why Trichomonas is the correct answer:** * *Trichomonas vaginalis* (and other species like *T. tenax* and *T. hominis*) exists solely as a pear-shaped, flagellated trophozoite. * Because it lacks a resistant cyst form, it cannot survive long outside the human host. This explains why it is primarily transmitted through direct mucosal contact (sexual intercourse) rather than contaminated food or water. **2. Why the other options are incorrect:** * **E. histolytica:** This intestinal amoeba has both a trophozoite (invasive) and a cyst (infective) stage. The quadrinucleate cyst is the diagnostic and infective form found in human feces. * **Giardia lamblia:** This flagellate exists as a teardrop-shaped trophozoite and an oval cyst. The cyst is highly resistant and is the form responsible for transmission via the fecal-oral route. * **Toxoplasma gondii:** This coccidian parasite has multiple stages in humans, including **tissue cysts** (containing bradyzoites) and oocysts (though oocysts are only shed by the definitive host, cats). Tissue cysts are a hallmark of chronic infection in humans. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *T. vaginalis* is characterized by 4 anterior flagella and an **undulating membrane** that extends half the length of the body. * **Motility:** It exhibits a characteristic **"jerky" or "twitching" motility** on wet mount microscopy. * **Clinical Sign:** In females, it causes "Strawberry Cervix" (colpitis macularis) due to punctate hemorrhages. * **Treatment:** Metronidazole is the drug of choice (treat both partners to prevent ping-pong infection).

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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