Parasitology Practice Questions

Q: Napier's aldehyde test is used in the diagnosis of which condition?

Kala azar. **Explanation:** **Napier’s Aldehyde Test** (also known as the Formol-gel test) is a non-specific biochemical test used for the presumptive diagnosis of **Kala-azar (Visceral Leishmaniasis)**. **Why Kala-azar is correct:** The underlying principle of this test is the detection of **hypergammaglobulinemia** (a massive increase in IgG levels). In chronic Kala-azar, there is a profound polyclonal B-cell activation leading to excessive serum globulins. When 1–2 drops of 40% formaldehyde are added to 1 mL of the patient's serum, the serum undergoes gelification and becomes opaque (resembling the white of a boiled egg) within 2 to 20 minutes. A positive result typically appears only after the disease has been present for at least 3 months. **Why other options are incorrect:** * **Malaria:** Diagnosis is primarily made via peripheral blood smears (thick and thin) or Rapid Diagnostic Tests (RDTs) detecting HRP-2 or LDH antigens. * **Typhus Fever:** Diagnosed using the **Weil-Felix test** (a heterophile agglutination test) or molecular methods like PCR. * **Enteric Fever:** Diagnosed via the **Widal test** (detecting O and H antibodies) or gold-standard blood/bone marrow cultures. **High-Yield Clinical Pearls for NEET-PG:** * **Chopra’s Antimony Test:** Another non-specific test for Kala-azar using urea stibamine (positive result = profuse flocculent precipitate). * **Specific Diagnosis:** The gold standard for Kala-azar is the demonstration of **LD bodies** (amastigotes) in splenic or bone marrow aspirates. * **rK39 Immunochromatographic Strip Test:** The most common rapid diagnostic test used in the field today due to its high sensitivity and specificity. * **Montenegro Skin Test:** Negative in active Kala-azar but becomes positive after recovery (indicates delayed hypersensitivity).

Q: Xenodiagnosis is used in the diagnosis of a disease that is transmitted by the bite of which of the following vectors?

Reduviid bug. **Explanation:** **Xenodiagnosis** is a specialized diagnostic technique where a laboratory-bred, pathogen-free vector is allowed to feed on a patient suspected of having a specific infection. The vector is later dissected to check for the multiplication of the parasite. 1. **Why Reduviid bug is correct:** Xenodiagnosis is the classic diagnostic method for **Chagas disease** (American Trypanosomiasis), caused by *Trypanosoma cruzi*. The vector for this disease is the **Reduviid bug** (also known as the Triatomine or "kissing" bug). In chronic stages of the disease, when parasitemia is too low to be detected by standard blood smears, uninfected Reduviid bugs are allowed to bite the patient. If *T. cruzi* is present, it multiplies in the bug's hindgut and is detected in its feces after 2–4 weeks. 2. **Why other options are incorrect:** * **Body louse:** Transmits Epidemic typhus (*Rickettsia prowazekii*) and Relapsing fever. Diagnosis is usually via serology or PCR. * **Deer tick (*Ixodes*):** Transmits Lyme disease and Babesiosis. Diagnosis relies on clinical presentation (Erythema migrans) and serology (ELISA/Western Blot). * **Mosquito:** Transmits Malaria, Filariasis, and various viral fevers. Diagnosis is typically via peripheral blood smears (Malaria) or antigen detection. **High-Yield Clinical Pearls for NEET-PG:** * **Chagas Disease Triad:** Cardiomyopathy (apical aneurysm), Megaesophagus, and Megacolon. * **Romaña’s sign:** Unilateral painless periorbital swelling (portal of entry through the conjunctiva). * **C-shaped Trypomastigotes:** Characteristic morphology of *T. cruzi* on peripheral smears. * **NNN Medium:** The specialized culture medium used for both *Leishmania* and *Trypanosoma*.

Q: 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?

Oocyst. **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.

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

Naegleria fowleri. **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.

Q: Which of the following organisms does NOT cause arthritis?

Trichuris trichiura. **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.

Q: 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?

Cryptosporidium parvum. ### 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.

Q: Which stage of the malaria parasite is found in the gametocytes?

Gametocytes. **Explanation:** The life cycle of *Plasmodium* involves two distinct phases: the asexual cycle in humans (schizogony) and the sexual cycle in the female *Anopheles* mosquito (sporogony). **Why Gametocytes is correct:** Gametocytes are the **sexual stages** of the malaria parasite that develop within human red blood cells (RBCs). While most merozoites continue the asexual cycle, a small proportion differentiates into male (microgametocytes) and female (macrogametocytes) forms. These are the **infective stages for the mosquito vector**. When a mosquito bites an infected human, it ingests these gametocytes, which then undergo fertilization in the mosquito's midgut. **Why other options are incorrect:** * **Trophozoite:** This is the active, feeding stage within the RBC. It is characterized by the "ring form" (early trophozoite) and is responsible for the clinical symptoms of malaria but does not represent the sexual stage. * **Merozoites:** These are the products of schizogony. They are released when an infected RBC or hepatocyte ruptures and are responsible for invading new erythrocytes. * **Schizonts:** This is the mature asexual stage where the parasite undergoes nuclear division (multiple fission) before bursting to release merozoites. **High-Yield Clinical Pearls for NEET-PG:** * **Infective stage for Humans:** Sporozoites (injected by mosquito). * **Infective stage for Mosquito:** Gametocytes (ingested from human). * **Morphology:** *P. falciparum* gametocytes are uniquely **crescent or banana-shaped**, whereas other species are spherical. * **Primaquine:** This is the drug of choice for its **gametocidal** action (especially against *P. falciparum*), helping to prevent the transmission of malaria to the mosquito vector.

Question 1

A 3-year-old girl presents with acute abdominal pain, a 5-day history of vomiting and abdominal distension, and absence of stool passage. She has been vomiting bilious material for the past day. Physical examination reveals a lethargic child with a firm, tender abdomen and peritoneal signs. Intraoperative findings include resection of approximately 20 cm of markedly distended small intestine containing a tightly knotted ball of nematodes (15-35 cm in length, tapered ends without hooks). The patient's parasite has a life cycle stage where larvae hatch in the duodenum, penetrate the small intestinal wall, and migrate. The passage of these migrating larvae most commonly produces symptomatic disease in which of the following organs?

Accepted Answer

Lungs

Answer

Heart

Answer

Liver

Answer

Pancreas

Question 2

Napier's aldehyde test is used in the diagnosis of which condition?

Accepted Answer

Kala azar

Answer

Malaria

Answer

Typhus fever

Answer

Enteric fever

Question 3

Giardia lambia infection is common in which of the following conditions?

Accepted Answer

Selective IgA deficiency

Answer

Selective IgM deficiency

Answer

Common variable hypogammaglobulinemia

Answer

Severe combined immunodeficiency

Question 4

Xenodiagnosis is used in the diagnosis of a disease that is transmitted by the bite of which of the following vectors?

Accepted Answer

Reduviid bug

Answer

Body louse

Answer

Deer tick

Answer

Mosquito

Question 5

Which of the following statements concerning malaria is TRUE?

Accepted Answer

Red cells negative for the Duffy blood group antigen are resistant to Plasmodium vivax.

Answer

Malaria caused by each of the four plasmodial species can relapse after initial illness.

Answer

Renal impairment is a grave prognostic sign in falciparum malaria.

Answer

Plasmodium malariae can cause immune-mediated nephropathy.

Question 6

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?

Accepted Answer

Oocyst

Answer

Egg

Answer

Cyst

Answer

Merozoite

Question 7

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

Accepted Answer

Naegleria fowleri

Answer

Entamoeba histolytica

Answer

Escherichia coli

Answer

Babesia coli

Question 8

Which of the following organisms does NOT cause arthritis?

Accepted Answer

Trichuris trichiura

Answer

Wuchereria bancrofti

Answer

Echinococcus granulosus

Answer

Dracunculus medinensis

Question 9

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?

Accepted Answer

Cryptosporidium parvum

Answer

Toxoplasma gondii

Answer

Coccidioides immitis

Answer

Sarcocystis

Question 10

Which stage of the malaria parasite is found in the gametocytes?

Accepted Answer

Gametocytes

Answer

Trophozoite

Answer

Merozoites

Answer

Schizonts

Parasitology — MCQs

Parasitology — MCQs

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