Parasitology Practice Questions

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.

Q: This is a schematic diagram depicting the body structure of which of these helminths?

Brugia malayi. ***Brugia malayi*** - Microfilariae have a **sheath** and **two discrete nuclei at the tail tip**, which are key distinguishing features visible in microscopic examination. - Causes **lymphatic filariasis** in Southeast Asia and is transmitted by **Mansonia** and **Anopheles** mosquitoes. *Onchocerca volvulus* - Microfilariae are **unsheathed** and have **no nuclei in the tail**, making them easily distinguishable from sheathed filariae. - Causes **river blindness (onchocerciasis)** and is transmitted by **Simulium** blackflies, primarily in Africa. *Loa loa* - Microfilariae have a **sheath** but show **nuclei extending to the tail tip** in a continuous pattern, unlike the discrete tail nuclei of Brugia. - Causes **loiasis** with characteristic **calabar swellings** and **eye worm** migration, endemic to Central and West Africa. *Wuchereria bancrofti* - Microfilariae have a **sheath** but **no nuclei in the tail tip**, distinguishing it from Brugia malayi's discrete tail nuclei. - Most common cause of **lymphatic filariasis** globally, transmitted by **Culex**, **Aedes**, and **Anopheles** mosquitoes.

Q: An immigrant from the Far East develops malaise, fever, and rigors, followed by upper right quadrant abdominal pain, vomiting, jaundice, and itching. His urine is dark and his feces are pale. Infestation with which of the following parasites is most strongly suggested by this patient's presentation?

Clonorchis sinensis. **Explanation:** The patient presents with **obstructive jaundice** (dark urine, pale stools, itching, and jaundice) and symptoms of **acute cholangitis** (fever, rigors, and RUQ pain). Given the patient's origin from the **Far East**, the most likely diagnosis is infestation with **Clonorchis sinensis** (Chinese Liver Fluke). 1. **Why Clonorchis sinensis is correct:** This parasite is endemic to East Asia. Humans are infected by consuming undercooked freshwater fish containing metacercariae. The larvae migrate to the **biliary tree**, where adult flukes cause mechanical obstruction, inflammation, and hyperplasia of the biliary epithelium. This leads to biliary stasis, secondary bacterial cholangitis, and pigment stone formation. 2. **Why other options are incorrect:** * **Enterobius vermicularis:** Causes perianal pruritus (pinworm), not biliary obstruction or jaundice. * **Plasmodium ovale:** Causes malaria with cyclical fever and hemolysis. While it can cause jaundice (pre-hepatic), it does not cause RUQ pain typical of biliary obstruction or pale stools. * **Taenia solium:** Causes intestinal taeniasis or cysticercosis (neurocysticercosis). It does not typically involve the biliary tract. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Association:** Chronic *Clonorchis sinensis* infection is a major risk factor for **Cholangiocarcinoma** (bile duct cancer). * **Diagnosis:** Identification of characteristic **operculated eggs** with "shoulders" and a small knob at the posterior pole in stool or bile. * **Treatment:** **Praziquantel** is the drug of choice. * **Intermediate Hosts:** 1st—Snail; 2nd—Freshwater (Cyprinid) fish.

Q: Autoinfection is a mode of transmission in which of the following parasites?

Cysticercosis. **Explanation:** **Autoinfection** occurs when an individual serves as both the reservoir and the host, leading to a cycle of infection without an external environmental stage. **Why Cysticercosis is Correct:** Cysticercosis is caused by the larval stage of *Taenia solium* (Pork tapeworm). While humans are the definitive hosts for the adult worm, they can become intermediate hosts for the larvae through **internal or external autoinfection**. * **External:** Fecal-oral contamination (ingesting eggs from one’s own stool). * **Internal:** Reverse peristalsis carries gravid proglottids from the intestine back to the stomach, where eggs hatch, penetrate the mucosa, and disseminate to tissues (brain, muscles). **Analysis of Incorrect Options:** * **A. Trichinella spiralis:** Transmission occurs via the ingestion of undercooked meat containing encysted larvae. There is no autoinfection cycle; the larvae must be ingested from an external source. * **C. Ancylostoma duodenale:** Transmission occurs via the penetration of the skin by filariform larvae found in the soil. * **D. Ascaris lumbricoides:** Infection occurs by ingesting embryonated eggs from contaminated soil. Eggs require a period of maturation (3 weeks) outside the host to become infective, making autoinfection impossible. **NEET-PG High-Yield Pearls:** * **Other parasites showing autoinfection:** *Strongyloides stercoralis* (most common cause of hyperinfection), *Enterobius vermicularis* (Pinworm), *Hymenolepis nana* (Dwarf tapeworm), and *Cryptosporidium hominis*. * **Neurocysticercosis (NCC):** The most common cause of adult-onset seizures in India; "Starry sky" appearance on CT/MRI is a classic finding. * **Strongyloides:** Unique because the rhabditiform larvae can transform into infective filariform larvae *within* the host's gut.

Question 1

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 2

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 3

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 4

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 5

Which of the following organisms does NOT cause arthritis?

Accepted Answer

Trichuris trichiura

Answer

Wuchereria bancrofti

Answer

Echinococcus granulosus

Answer

Dracunculus medinensis

Question 6

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 7

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

Accepted Answer

Gametocytes

Answer

Trophozoite

Answer

Merozoites

Answer

Schizonts

Question 8

This is a schematic diagram depicting the body structure of which of these helminths?

Accepted Answer

Brugia malayi

Answer

Onchocerca volvulus

Answer

Loa loa

Answer

Wuchereria bancrofti

Question 9

An immigrant from the Far East develops malaise, fever, and rigors, followed by upper right quadrant abdominal pain, vomiting, jaundice, and itching. His urine is dark and his feces are pale. Infestation with which of the following parasites is most strongly suggested by this patient's presentation?

Accepted Answer

Clonorchis sinensis

Answer

Enterobius vermicularis

Answer

Plasmodium ovale

Answer

Taenia solium

Question 10

Autoinfection is a mode of transmission in which of the following parasites?

Accepted Answer

Cysticercosis

Answer

Trichinella

Answer

Ankylostoma

Answer

Ascaris

Parasitology — MCQs

Parasitology — MCQs

On this page

Practice by Chapter

Want unlimited practice?