Parasitology — MCQs

Parasitology Indian Medical PG Practice Questions and MCQs

Question 551: Chronic dysentery, abdominal pain, and rectal prolapse in children is caused by which of the following parasites?

A. Enterobius vermicularis
B. Ascariasis
C. Trichuris trichura (Correct Answer)
D. Trichinella spiralis

Explanation: **Explanation:** The correct answer is **Trichuris trichiura** (Whipworm). This parasite primarily inhabits the caecum and large intestine. In heavy infections, particularly in children, the worms can be found throughout the colon and rectum. **Why Trichuris trichiura is correct:** The hallmark clinical presentation of heavy Trichuriasis is **Trichuris Dysentery Syndrome**. The worms embed their slender anterior ends into the intestinal mucosa, causing chronic inflammation, mucosal friability, and blood loss. The constant irritation and straining (tenesmus) during defecation, combined with the weakening of the pelvic floor muscles and local inflammation, lead to the classic complication of **rectal prolapse**. **Analysis of Incorrect Options:** * **Enterobius vermicularis (Pinworm):** Primarily causes perianal pruritus (itching) at night. It does not cause dysentery or rectal prolapse. * **Ascaris lumbricoides (Roundworm):** Typically causes intestinal obstruction (bolus formation) or Loeffler’s syndrome (pulmonary phase). It does not typically cause chronic dysentery. * **Trichinella spiralis:** Known for causing muscle pain (myositis), periorbital edema, and eosinophilia after ingestion of undercooked pork. It does not inhabit the lower bowel to cause prolapse. **NEET-PG High-Yield Pearls:** * **Morphology:** Adult worm is whip-like (thick posterior, thin anterior). * **Egg:** Characteristic **barrel-shaped** (lemon-shaped) with **bipolar mucus plugs**. * **Treatment:** Mebendazole or Albendazole. * **Key Association:** Always link "Rectal Prolapse + Child + Dysentery" to *Trichuris trichiura*.

Question 552: Recurrent giardiasis is associated with which of the following conditions?

A. Severe combined immunodeficiency
B. Common variable immunodeficiency (Correct Answer)
C. DiGeorge syndrome
D. C8 deficiency

Explanation: **Explanation:** The correct answer is **Common Variable Immunodeficiency (CVID)**. **1. Why CVID is the correct answer:** *Giardia lamblia* is an intestinal protozoan that colonizes the duodenum and jejunum. The body’s primary defense against *Giardia* is **Secretory IgA (sIgA)**, which prevents the attachment of trophozoites to the intestinal epithelium. CVID is characterized by hypogammaglobulinemia (specifically low IgG, IgA, and often IgM). The profound **deficiency of IgA** in these patients leads to a failure in neutralizing the parasite, resulting in chronic, recurrent, or treatment-resistant giardiasis. This is a classic association frequently tested in medical exams. **2. Analysis of Incorrect Options:** * **Severe Combined Immunodeficiency (SCID):** This involves a total lack of both T-cell and B-cell function. While these patients are susceptible to all infections, they usually present in early infancy with opportunistic infections like *Pneumocystis jirovecii* or Candidiasis, rather than isolated recurrent giardiasis. * **DiGeorge Syndrome:** This is primarily a T-cell deficiency due to thymic hypoplasia. While T-cells help B-cells produce antibodies, the specific clinical hallmark of DiGeorge is susceptibility to viral, fungal, and mycobacterial infections, not specifically *Giardia*. * **C8 Deficiency:** Deficiencies in late complement components (C5-C9) specifically predispose individuals to recurrent infections by **Neisseria** species (meningitis and gonorrhea) due to the inability to form the Membrane Attack Complex (MAC). **3. High-Yield Clinical Pearls for NEET-PG:** * **Stool Examination:** Look for "falling leaf motility" (trophozoites) or oval cysts. * **Antigen Detection:** ELISA for Giardia antigen in stool is more sensitive than microscopy. * **Drug of Choice:** Metronidazole (Tinidazole is also highly effective). * **Association:** Patients with **X-linked Agammaglobulinemia (Bruton’s)** also have a high risk of giardiasis due to lack of B-cells and IgA. * **Biopsy:** In CVID patients, intestinal biopsy may show "nodular lymphoid hyperplasia."

Question 553: What is the most virulent Plasmodium species causing malaria?

A. Vivax
B. Ovale
C. Falciparum (Correct Answer)
D. Malariae

Explanation: **Explanation:** **Plasmodium falciparum** is the most virulent species because of its unique ability to cause **sequestration** and **cytoadherence**. Unlike other species, *P. falciparum* expresses **PfEMP-1** (Plasmodium falciparum erythrocyte membrane protein 1) on the surface of infected RBCs. This protein binds to endothelial receptors (like ICAM-1 and CD36), causing infected cells to stick to capillary walls. This leads to microvascular obstruction, tissue hypoxia, and multi-organ failure, manifesting as **Cerebral Malaria**, Blackwater fever, or Acute Respiratory Distress Syndrome (ARDS). Furthermore, it can infect RBCs of all ages, leading to high levels of parasitemia (>5%). **Why other options are incorrect:** * **P. vivax & P. ovale:** These species preferentially infect **young RBCs (reticulocytes)**, limiting the total parasite load. While they cause significant morbidity and can relapse due to **hypnozoites** in the liver, they rarely cause the fatal microvascular complications seen in falciparum. * **P. malariae:** This species infects only **old RBCs**, resulting in the lowest parasitemia levels. It follows a 72-hour (quartan) cycle and is generally the most benign form, though it is associated with nephrotic syndrome in children. **High-Yield NEET-PG Pearls:** * **Maurer’s dots:** Seen in *P. falciparum* (Schüffner’s dots are seen in *P. vivax/ovale*). * **Multiple rings per RBC** and **banana-shaped gametocytes** are diagnostic hallmarks of *P. falciparum*. * **Recrudescence** is seen in *P. falciparum* (due to incomplete treatment), whereas **Relapse** is seen in *P. vivax/ovale* (due to hypnozoites).

Question 554: Peripheral smear of malaria shows which of the following findings, EXCEPT?

A. Single ring form - Vivax
B. Multiple ring form - Falciparum
C. Bar and band forms - Malariae
D. Banana shaped gametocyte - Ovale (Correct Answer)

Explanation: The correct answer is **D. Banana shaped gametocyte - Ovale**. ### **Explanation** The diagnosis of malaria via peripheral smear relies on identifying specific morphological features of the *Plasmodium* species within red blood cells (RBCs). 1. **Why Option D is correct:** Banana-shaped (crescentic) gametocytes are the pathognomonic hallmark of ***Plasmodium falciparum***, not *P. ovale*. In *P. ovale* infections, the gametocytes are typically round or oval, and the infected RBCs often show fimbriated (ragged) edges and an oval shape. 2. **Why Option A is incorrect:** *P. vivax* typically presents with a **single ring form** (trophozoite) per RBC. The infected RBC is usually enlarged and contains Schüffner’s dots. 3. **Why Option B is incorrect:** **Multiple ring forms** (multiple parasites infecting a single RBC) and the "Accole/Applique" position (rings at the periphery of the RBC) are characteristic features of ***P. falciparum*** due to its high parasitemia levels. 4. **Why Option C is incorrect:** ***P. malariae*** is known for its unique **"Band and Bar" forms**, where the growing trophozoite stretches across the diameter of the non-enlarged RBC. ### **High-Yield Clinical Pearls for NEET-PG** * **Maurer’s Dots:** Seen in *P. falciparum*. * **Schüffner’s Dots:** Seen in *P. vivax* and *P. ovale*. * **Ziemann’s Dots:** Seen in *P. malariae*. * **RBC Age Preference:** *P. vivax/ovale* prefer young RBCs (reticulocytes); *P. malariae* prefers old RBCs; *P. falciparum* is "indiscriminate" (infects RBCs of all ages), leading to severe anemia and high parasite load. * **Gold Standard Diagnosis:** Microscopy (Thick smear for detection, Thin smear for species identification).

Question 555: What is the infective form of the malarial parasite transmitted through blood transfusion?

A. Merozoite
B. Trophozoite (Correct Answer)
C. Sporozoite
D. Schizont

Explanation: ### Explanation The correct answer is **B. Trophozoite**. **1. Why Trophozoite is Correct:** In a natural infection via a female *Anopheles* mosquito, the infective form is the **sporozoite**. However, in **Transfusion-Induced Malaria**, the parasite is transmitted directly from the donor’s blood. Since the hepatic (pre-erythrocytic) cycle is bypassed, the blood contains asexual erythrocytic stages. Among these, the **trophozoite** (specifically the ring form) is the most common and persistent stage found in the donor's red blood cells that initiates infection in the recipient. **2. Analysis of Incorrect Options:** * **C. Sporozoite:** This is the infective form for **natural infection** via mosquito bite. Sporozoites are found in the mosquito's salivary glands and are absent in human blood during a transfusion. * **A. Merozoite & D. Schizont:** While these asexual stages are present in the blood, they are transient or represent a later stage of the erythrocytic cycle. The trophozoite is the primary diagnostic and infective stage circulating in the donor's blood. **3. NEET-PG High-Yield Pearls:** * **Incubation Period:** Transfusion malaria has a **shorter incubation period** because the pre-erythrocytic (liver) stage is bypassed. * **Relapse:** There is **no risk of relapse** (no hypnozoites) in transfusion malaria, even in *P. vivax* or *P. ovale*, because the liver stage never occurs. * **Drug of Choice:** Radical treatment with Primaquine is **not required** for transfusion malaria; only blood schizonticides (like Chloroquine) are needed. * **Storage:** Malarial parasites can survive in blood stored at 4°C for up to 2–3 weeks.

Question 556: Primary amoebic meningoencephalitis is caused by which organism?

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

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. It enters the body through the nasal mucosa while swimming or diving, penetrates the cribriform plate, and migrates along the olfactory nerves to the brain. * **Clinical Course:** It causes acute, fulminant hemorrhagic necrotizing meningoencephalitis in previously healthy individuals, leading to death within 7–10 days. 2. **Why other options are incorrect:** * **Entamoeba histolytica:** Causes intestinal amoebiasis and amoebic liver abscess. While it can rarely cause brain abscesses, it does not cause PAM. * **Escherichia coli:** A common cause of bacterial meningitis, particularly in neonates, but it is a bacterium, not an amoeba. * **Balamuthia mandrillaris:** Along with *Acanthamoeba*, this causes **Granulomatous Amoebic Encephalitis (GAE)**, which is a subacute or chronic infection typically seen in immunocompromised hosts, unlike the acute PAM caused by *Naegleria*. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Finding:** Wet mount of CSF shows **actively motile trophozoites** (slug-like movement). Note: Cysts are never seen in brain tissue or CSF in *Naegleria* infections. * **Drug of Choice:** Amphotericin B (often used in combination with Miltefosine). * **Key Differentiator:** PAM occurs in healthy individuals with a history of swimming; GAE occurs in immunocompromised individuals via skin lesions or inhalation.

Question 557: Which parasite penetrates the skin for entry into the body?

A. Ankylostoma duodenale (Correct Answer)
B. Strongyloides
C. Round worm
D. Trichuris trichura

Explanation: The correct answer is **Ankylostoma duodenale**. ### **Explanation** The primary mode of infection for certain helminths is the **percutaneous route**, where the infective larval stage (Filariform larva) actively penetrates the intact skin of a human host, usually through the feet when walking barefoot on contaminated soil. 1. **Ankylostoma duodenale (Hookworm):** The filariform larva penetrates the skin, enters the venous circulation, travels to the lungs, ascends the trachea, is swallowed, and finally matures in the small intestine. **Strongyloides stercoralis** also shares this mechanism; however, in the context of this specific question, *Ankylostoma* is the classic representative of skin-penetrating nematodes. 2. **Roundworm (Ascaris lumbricoides):** Infection occurs via the **fecal-oral route** through the ingestion of embryonated eggs in contaminated food or water. There is no skin penetration. 3. **Trichuris trichiura (Whipworm):** Infection occurs strictly via the **ingestion** of embryonated eggs. It does not have a tissue migratory phase or a skin-penetration phase. ### **NEET-PG High-Yield Pearls** * **The "Skin Penetrators" Mnemonic:** Remember **"S-A-N-D"** for parasites that enter through the skin: * **S:** *Strongyloides stercoralis* * **A:** *Ankylostoma duodenale* & *A. braziliense* (Cutaneous Larva Migrans) * **N:** *Necator americanus* * **D:** *Dermatobia hominis* (Botfly) / **Schistosomes** (Cercariae). * **Ground Itch:** This is the allergic reaction/pruritic dermatitis seen at the site of entry of Hookworm larvae. * **Loeffler’s Syndrome:** Characterized by transient pulmonary infiltrates and eosinophilia; it occurs during the lung migration phase of *Ankylostoma*, *Necator*, and *Ascaris*. Note: *Trichuris* does **not** cause Loeffler’s as it lacks a migratory cycle.

Question 558: Which statement is true regarding the trophozoites of E. histolytica?

A. Has an eccentric karyosome
B. Nuclear membrane is without chromatin
C. Shows erythrophagocytosis (Correct Answer)
D. Presence of bacteria inside the cell

Explanation: **Explanation:** *Entamoeba histolytica* is the causative agent of amoebiasis. Distinguishing it from the morphologically identical but non-pathogenic *E. dispar* is a common NEET-PG focus. **1. Why Option C is Correct:** The presence of **ingested Red Blood Cells (erythrophagocytosis)** within the cytoplasm of a trophozoite is the **pathognomonic** feature of *E. histolytica*. It indicates invasive disease, as the parasite actively destroys host tissue and consumes erythrocytes. This feature is never seen in *E. dispar* or *E. coli*. **2. Why Other Options are Incorrect:** * **Option A:** *E. histolytica* has a **central** karyosome (nucleolus). An **eccentric** (off-center) karyosome is a characteristic feature of *Entamoeba coli*, a non-pathogenic commensal. * **Option B:** The nuclear membrane of *E. histolytica* is lined with **fine, uniformly distributed peripheral chromatin**. A membrane "without chromatin" is not characteristic of this genus. * **Option D:** The presence of ingested bacteria is typical of **Entamoeba coli**. *E. histolytica* trophozoites in invasive disease usually contain RBCs but rarely contain bacteria, whereas commensal amoebae feed on intestinal flora. **High-Yield Clinical Pearls for NEET-PG:** * **Motility:** Trophozoites show unidirectional, purposeful movement using finger-like **pseudopodia**. * **Nuclear Structure:** Described as having a "Cartwheel appearance" due to the central karyosome and delicate chromatin. * **Stain of Choice:** Iron-hematoxylin or Trichrome stain. * **Quadrinucleate Cyst:** The infective stage; contains up to 4 nuclei, rounded chromatoid bars (cigar-shaped), and a glycogen mass.

Question 559: All of the following statements about toxoplasmosis are true except?

A. May spread by organ transplantation
B. Maternal infection acquired after 6 months has high risk of transmission
C. Oocyst in freshly passed cat's faeces is infective
D. Arthralgia, sore throat and abdominal pain are the most common manifestations (Correct Answer)

Explanation: **Explanation:** The correct answer is **D** because it is a false statement. In immunocompetent individuals, *Toxoplasma gondii* infection is most commonly **asymptomatic** (80–90% of cases). When symptoms do occur, the most common clinical presentation is **painless cervical lymphadenopathy**, often accompanied by a self-limiting flu-like illness (fever, malaise, and myalgia). Arthralgia and abdominal pain are not characteristic features. **Analysis of other options:** * **Option A (True):** Toxoplasmosis can be transmitted via organ transplantation (especially heart and liver) if the donor is seropositive and the recipient is seronegative, or through blood transfusions containing tachyzoites. * **Option B (True):** The risk of congenital transmission increases with gestational age. If maternal infection is acquired in the **third trimester** (after 6 months), the transmission rate is highest (60–80%), though the severity of fetal damage is lower compared to first-trimester infections. * **Option C (True):** This is a high-yield distinction. Oocysts passed in cat feces are **not** immediately infective. They must undergo **sporulation** in the environment, which typically takes **1 to 5 days**. Therefore, freshly passed feces are not an immediate source of infection. **High-Yield Clinical Pearls for NEET-PG:** * **Definitive Host:** Cat (sexual cycle occurs here). * **Intermediate Host:** Humans and other mammals (asexual cycle). * **Infective Stages:** Sporulated oocysts (from soil/water), Bradyzoites (in undercooked meat), and Tachyzoites (transplacental). * **Diagnosis:** Sabin-Feldman Dye Test (Gold Standard). * **Congenital Triad:** Hydrocephalus, Chorioretinitis, and Intracranial calcifications. * **Treatment:** Pyrimethamine + Sulfadiazine (Spiramycin is used for pregnant women to prevent transmission).

Question 560: Lymphatic obstruction occurs with which of the following parasites?

A. Filarial worm (Correct Answer)
B. Hook worm
C. Guinea worm
D. Pin worm

Explanation: **Explanation:** **Correct Option: A. Filarial worm (*Wuchereria bancrofti* / *Brugia malayi*)** Lymphatic filariasis is primarily caused by *Wuchereria bancrofti*. The adult worms reside in the **afferent lymphatic vessels** and lymph nodes. The pathogenesis involves a combination of mechanical obstruction by the adult worms and a profound inflammatory response (Th2-mediated) to the parasite and its endosymbiont, *Wolbachia*. Over time, chronic inflammation leads to **lymphangiosclerosis** and fibrosis, resulting in permanent lymphatic obstruction, lymphoedema, and eventually **Elephantiasis**. **Incorrect Options:** * **B. Hookworm (*Ancylostoma duodenale*):** These reside in the small intestine and feed on blood. They are primarily associated with **Iron Deficiency Anemia** and ground itch, not lymphatic pathology. * **C. Guinea worm (*Dracunculus medinensis*):** The adult female migrates to the subcutaneous tissues (usually of the lower limbs) to create a blister and release larvae. It causes local ulcers and secondary infections but does not obstruct lymphatics. * **D. Pinworm (*Enterobius vermicularis*):** These inhabit the cecum and appendix. The hallmark clinical feature is **perianal pruritus** (nocturnal itching) as the female migrates to the perianal skin to lay eggs. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Culex quinquefasciatus* is the most common vector for *W. bancrofti* in India. * **Nocturnal Periodicity:** Microfilariae appear in the peripheral blood between **10 PM and 2 AM**. * **Tropical Pulmonary Eosinophilia (TPE):** A hypersensitivity reaction to filarial antigens characterized by nocturnal cough, wheezing, and high eosinophil counts. * **Drug of Choice:** Diethylcarbamazine (DEC). Note: DEC is contraindicated in Onchocerciasis due to the Mazzotti reaction. * **Diagnostic Gold Standard:** Demonstration of microfilariae in a peripheral blood smear (thick film).

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