Parasitology Practice Questions

Q: A patient presents with complaints of lower abdominal pain, fever, and repeated stools containing little fecal matter, blood, and mucus. Microscopy of the stool shows a paucity of WBCs and a positive test for heme. Which is the likely causative organism?

Entamoeba. ### Explanation The clinical presentation of lower abdominal pain, fever, and frequent small-volume stools containing blood and mucus is classic for **Amoebic Dysentery**, caused by *Entamoeba histolytica*. **Why Entamoeba is correct:** The defining feature in this question is the **"paucity of WBCs"** (pus cells) in the stool. *E. histolytica* produces a cytotoxin that lyses leukocytes (leukocytolysis). Therefore, despite being an invasive infection, the stool microscopy shows few inflammatory cells, RBCs in clumps (due to erythrophagocytosis), and a positive heme test. This contrasts with Bacillary Dysentery (e.g., *Shigella*), which presents with numerous pus cells. **Why other options are incorrect:** * **Giardia:** Causes malabsorptive "steatorrhea" (foul-smelling, fatty stools). It is non-invasive, so it does **not** cause blood or mucus in the stool. * **Staphylococcus & Clostridium perfringens:** These typically cause **food poisoning** characterized by watery diarrhea and vomiting due to preformed toxins or enterotoxins. They do not typically cause a dysenteric picture with blood and mucus. **NEET-PG High-Yield Pearls:** * **Stool Microscopy:** Look for **Trophozoites with ingested RBCs** (pathognomonic for *E. histolytica*). * **Flask-shaped ulcers:** The characteristic lesion formed in the colon. * **Charcot-Leyden crystals:** Often found in the stool due to eosinophil breakdown. * **Treatment:** Metronidazole or Tinidazole (Tissue amoebicides) followed by Diloxanide furoate (Luminal amoebicide) to clear the carrier state.

Q: The prepatent period in lymphatic filariasis is defined as the time interval between inoculation of infective larvae and which of the following?

First appearance of detectable microfilaria. ### Explanation **Concept Overview:** In parasitology, the **prepatent period** is the time interval between the entry of the infective stage of a parasite into the host and the earliest time at which its presence can be demonstrated (usually via the detection of eggs, larvae, or microfilariae in clinical samples). **Why Option B is Correct:** In Lymphatic Filariasis, the infective stage is the **L3 larvae** (inoculated by the mosquito). These larvae migrate to the lymphatics and mature into adult worms. The prepatent period ends when these adult worms mate and produce **microfilariae (mf)** that can be detected in the peripheral blood. For *Wuchereria bancrofti*, this period typically lasts **8 to 12 months**. **Analysis of Incorrect Options:** * **Option A & C (Blockage/Lymphoedema):** These represent the **Incubation Period** (time between infection and clinical symptoms) or the chronic stage of the disease. The prepatent period is a biological timeline, whereas these are clinical milestones. * **Option D (Development of adult worm):** While the development of the adult worm occurs during the prepatent period, the period only "ends" once the worm begins producing detectable offspring (microfilariae). **High-Yield NEET-PG Pearls:** * **Infective Stage:** L3 Larvae (transmitted by *Culex quinquefasciatus* for Bancroftian filariasis). * **Diagnostic Stage:** Microfilaria (detected via peripheral blood smear, ideally collected between 10 PM – 2 AM due to nocturnal periodicity). * **Incubation Period:** Usually 8–16 months (longer than the prepatent period). * **Drug of Choice:** Diethylcarbamazine (DEC) 6mg/kg for 12 days. * **National Target:** The Global Programme to Eliminate Lymphatic Filariasis (GPELF) aims for **elimination**, not eradication, primarily through Mass Drug Administration (MDA).

Q: Which of the following diseases is transmitted by egg ingestion?

Hydatidosis. **Explanation:** The correct answer is **Hydatidosis** (Cystic Echinococcosis). This disease is caused by the larval stage of the dog tapeworm, *Echinococcus granulosus*. Humans serve as accidental intermediate hosts and acquire the infection by **ingesting embryonated eggs** passed in the feces of definitive hosts (dogs). Once ingested, the eggs hatch in the small intestine, releasing oncospheres that migrate to the liver and lungs to form hydatid cysts. **Analysis of Incorrect Options:** * **Taeniasis (A):** Caused by *Taenia saginata* or *Taenia solium*. It is transmitted by the **ingestion of larvae** (Cysticercus bovis/cellulosae) in undercooked beef or pork. (Note: Ingesting *T. solium* eggs causes Cysticercosis, not Taeniasis). * **Trichinosis (B):** Caused by *Trichinella spiralis*. It is transmitted by the **ingestion of encysted larvae** in undercooked meat (usually pork). * **Strongyloidosis (D):** Caused by *Strongyloides stercoralis*. Transmission occurs via **filariform larvae penetrating the skin** from contaminated soil, not by egg ingestion. **High-Yield Clinical Pearls for NEET-PG:** * **Modes of Transmission Mnemonic:** Remember **"EAT"** for organisms transmitted by egg ingestion: **E**nterobius, **A**scaris, **T**richuris, and *Echinococcus*. * **Hydatid Cyst:** Characterized by "Water lily sign" on imaging (detached endocyst) and managed via the **PAIR** technique (Puncture, Aspiration, Injection, Re-aspiration). * **Casoni’s Test:** An immediate hypersensitivity skin test used for diagnosis (though largely replaced by serology).

Q: All are seen in the cyst of E. Histolytica except?

Refractile nucleus. ### Explanation The correct answer is **D. Refractile nucleus**. In the life cycle of *Entamoeba histolytica*, the cyst is the infective stage. In a **fresh, unstained wet mount preparation**, the nuclei of the cyst are **not visible** (non-refractile). They only become visible when stained with iodine or hematoxylin. In contrast, the nucleus of the **trophozoite** stage is often described as having a delicate, refractile nuclear membrane in some clinical contexts, but specifically for the cyst, the "refractile" property is not a characteristic feature. **Analysis of Options:** * **A. Glycogen mass:** Immature cysts (unicucleated and binucleated) contain a dense mass of glycogen which serves as a food reserve. This stains mahogany brown with iodine. It disappears as the cyst matures into the quadrinucleate stage. * **B. Chromatid bars:** These are oblong, cigar-shaped structures composed of ribonucleoproteins. They are characteristic of *E. histolytica* cysts and have rounded ends (unlike *E. coli* which has splintered ends). They also disappear in the mature stage. * **C. Eccentric nucleus:** While the nucleus of *E. histolytica* typically features a **central karyosome**, the nuclear positioning within the cytoplasm of a developing cyst can appear eccentric, especially when pushed aside by a large glycogen vacuole. **NEET-PG High-Yield Pearls:** * **Infective Stage:** Mature quadrinucleate cyst. * **Diagnostic Stage:** Trophozoite (in acute dysentery) or Cyst (in chronic cases/carriers). * **Chromatid Bars:** "Cigar-shaped" in *E. histolytica*; "Splinter-like/Frayed" in *E. coli*. * **Nuclear Morphology:** Small, central karyosome with fine, uniform peripheral chromatin (described as a "cartwheel appearance"). * **Staining:** Iodine is used to visualize nuclei and glycogen; Trichrome or Iron-hematoxylin is used for permanent smears.

Q: A patient with AIDS returned from Haiti with acute diarrhea. The stool revealed an oval organism (8 to 9 um in diameter) that was acid-fast and fluoresced blue under ultraviolet light. What is the most likely identification of this organism?

Cyclospora. ### Explanation The clinical presentation and laboratory findings point specifically to **Cyclospora cayetanensis**, a coccidian parasite that is a common cause of traveler’s diarrhea and opportunistic infections in immunocompromised patients (like those with AIDS). **Why Cyclospora is correct:** 1. **Size:** The organism measures **8–10 µm**, which is the classic "double the size" rule compared to *Cryptosporidium*. 2. **Acid-fast staining:** It is **variably acid-fast**, meaning some oocysts stain red while others appear as "ghosts." 3. **Autofluorescence:** This is the pathognomonic feature for NEET-PG. Under UV light (330–365 nm), *Cyclospora* oocysts exhibit **intense blue fluorescence** due to the presence of phenolic compounds in the oocyst wall. 4. **Epidemiology:** It is frequently associated with travel to tropical regions like Haiti or the consumption of contaminated imported produce (e.g., raspberries, basil). **Why other options are incorrect:** * **Cryptosporidium:** While also acid-fast and common in AIDS, it is significantly smaller (**4–6 µm**) and **does not fluoresce** under UV light. * **Giardia:** This is a flagellated protozoan, not a coccidian. It is **not acid-fast** and has a characteristic pear shape with "falling leaf" motility. * **Enterocytozoon (Microsporidia):** These are much smaller (**1–2 µm**) and require special stains like Modified Trichrome or Calcofluor White; they are not typically identified by standard acid-fast staining. **High-Yield Pearls for NEET-PG:** * **Size Comparison:** *Cryptosporidium* (4–6 µm) < *Cyclospora* (8–10 µm) < *Isospora/Cystoisospora* (25–30 µm). * **Fluorescence:** Only *Cyclospora* (Blue) and *Isospora* (Green/Blue) show autofluorescence. * **Treatment:** Unlike *Cryptosporidium* (Nitazoxanide), *Cyclospora* is treated with **Trimethoprim-Sulfamethoxazole (TMP-SMX)**.

Q: Schistosomiasis infects humans at which of the following stages of its life cycle?

Cercariae. **Explanation:** The correct answer is **Cercariae**. Schistosomiasis (Bilharzia) is unique among trematodes because it infects humans via **direct skin penetration** rather than ingestion. 1. **Why Cercariae is correct:** The infective stage for humans is the **fork-tailed cercaria**. These are released from the intermediate host (freshwater snails) into water. When a human comes into contact with contaminated water, the cercariae penetrate the intact skin using proteolytic enzymes. During penetration, they lose their tails to become **schistosomulae**, which then migrate through the venous circulation to the liver and eventually the vesical or mesenteric venous plexuses. 2. **Why other options are incorrect:** * **Egg:** Eggs are the **diagnostic stage** excreted in human feces or urine. They are not infective to humans; instead, they must reach water to hatch into miracidia. * **Miracidia:** This is the infective stage for the **intermediate host (snail)**. Miracidia hatch from eggs in water and seek out specific snail species (e.g., *Biomphalaria*, *Bulinus*) to continue the life cycle. **High-Yield NEET-PG Pearls:** * **Intermediate Hosts:** *S. haematobium* (Bulinus snail), *S. mansoni* (Biomphalaria snail), *S. japonicum* (Oncomelania snail). * **Clinical Presentation:** Initial infection often presents as **"Swimmer’s Itch"** (dermatitis at the site of cercarial entry). Chronic *S. haematobium* is a major risk factor for **Squamous Cell Carcinoma of the urinary bladder**. * **Morphology:** Schistosomes are non-hermaphroditic (dioecious) "blood flukes" where the female resides in the **gynecophoric canal** of the male.

Q: T. trichuria maintains its position in the intestinal tract by:

Anchorage with its anterior portion. ### Explanation **Correct Answer: B. Anchorage with its anterior portion** *Trichuris trichiura* (Whipworm) has a characteristic morphology where the **anterior three-fifths** of the body is thin and hair-like, while the posterior two-fifths is thick and bulbous. To maintain its position in the large intestine (primarily the cecum), the worm uses its thin, needle-like anterior end to **thread itself into the mucosal epithelium**. It creates a tunnel within the superficial layers of the mucosa, effectively "stitching" itself into the intestinal wall. This mechanical anchorage prevents the worm from being expelled by peristalsis. **Analysis of Incorrect Options:** * **A. Penetration into tissues:** While the anterior end enters the mucosa, it does not deeply penetrate into the systemic tissues or organs like *Strongyloides* or *Trichinella*. It remains localized to the superficial mucosal layers. * **C. Retention in the folds of the mucosa:** This is a passive mechanism. *T. trichiura* uses an active mechanical anchorage rather than simply hiding in the crypts or folds. * **D. Attachment with its cutting plate:** This is the mechanism used by **Hookworms** (*Ancylostoma duodenale*), which possess teeth or cutting plates to attach to the villi. *Trichuris* lacks these structures. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Known as the "Whipworm" due to its whip-like shape. * **Egg Morphology:** Characteristic **Barrel-shaped** (lemon-shaped) eggs with **bipolar mucus plugs**. * **Clinical Presentation:** Heavy infections in children can lead to **Rectal Prolapse** due to increased peristalsis and straining. * **Anemia:** Unlike hookworms, *Trichuris* causes blood loss primarily through mucosal oozing at the site of attachment, leading to Iron Deficiency Anemia in heavy loads. * **Treatment:** Albendazole or Mebendazole are the drugs of choice.

Q: A 6-year-old boy presented with intermittent abdominal cramps and loose stools. Stool examination revealed ova approximately 100 micrometers in size. Which of the following is NOT a causative organism for these findings?

Opisthorchis viverrini. **Explanation:** The core of this question lies in the **morphology and size of helminth eggs**. In parasitology, egg size is a critical diagnostic feature. **Why Opisthorchis viverrini is the correct answer:** *Opisthorchis viverrini* (Southeast Asian liver fluke) produces some of the **smallest** eggs among human helminths, measuring approximately **25–30 μm x 10–15 μm**. They are operculated, flask-shaped, and contain a mature miracidium. Since the question specifies an egg size of **100 μm**, *Opisthorchis* is excluded as a causative agent. **Analysis of Incorrect Options (Large-sized eggs):** * **Fasciola gigantica:** Known for producing very large, operculated, unembryonated eggs, typically measuring **160–190 μm**. * **Echinostoma ilocanum:** This intestinal fluke produces large, operculated eggs measuring approximately **80–120 μm**. * **Gastrodiscoides hominis:** An amphistome fluke where the eggs are large, operculated, and measure about **150 μm x 60–90 μm**. **Clinical Pearls for NEET-PG:** * **Large Eggs (>100 μm):** *Fasciola hepatica/gigantica*, *Fasciolopsis buski*, *Gastrodiscoides hominis*, and *Schistosoma* species. * **Small Eggs (<30 μm):** *Opisthorchis* species, *Clonorchis sinensis*, and *Heterophyes heterophyes*. * **Bile-stained eggs:** Remember the mnemonic **"ABC"** – *Ascaris*, *Bile-stained* (most flukes), and *Capillaria*. * **Opisthorchis/Clonorchis** are high-yield because they are strongly associated with **Cholangiocarcinoma** (bile duct cancer).

Q: Which of the following is characterized by sheathed microfilariae?

Wuchereria bancrofti. **Explanation:** The presence or absence of a **sheath** (a delicate, translucent covering) and the arrangement of nuclei in the tail are the primary morphological features used to differentiate microfilariae in peripheral blood smears. **1. Why Wuchereria bancrofti is correct:** *Wuchereria bancrofti* is a classic example of a **sheathed** microfilaria. A key diagnostic feature is that its nuclei do not extend to the tip of the tail (the tail tip is "empty"). This distinguishes it from other sheathed species. **2. Analysis of Options:** * **Brugia malayi:** This is also a **sheathed** microfilaria. However, in the context of single-choice questions where *W. bancrofti* is the primary representative of lymphatic filariasis, it is often the preferred answer. *Note: If this were a multiple-select question, both A and D would be correct.* * **Loa loa:** This is also **sheathed**, but it is primarily associated with African eye worm and Calabar swellings. Its nuclei extend to the very tip of the tail. * **Mansonella perstans:** This species is **unsheathed**. Other unsheathed species include *Mansonella ozzardi* and *Onchocerca volvulus*. **3. NEET-PG Clinical Pearls:** * **Sheathed Microfilariae:** *Wuchereria bancrofti, Brugia malayi, Loa loa.* * **Unsheathed Microfilariae:** *Mansonella* species, *Onchocerca volvulus.* * **Tail Tip Nuclei (High Yield):** * *W. bancrofti:* No nuclei at the tip. * *B. malayi:* Two distinct terminal nuclei. * *Loa loa:* Continuous row of nuclei to the tip. * **Diurnal vs. Nocturnal:** *W. bancrofti* shows **nocturnal periodicity** (10 PM – 2 AM), which is why blood collection is recommended at night. * **Drug of Choice:** Diethylcarbamazine (DEC) is the mainstay of treatment for *W. bancrofti*.

Question 1

A patient presents with complaints of lower abdominal pain, fever, and repeated stools containing little fecal matter, blood, and mucus. Microscopy of the stool shows a paucity of WBCs and a positive test for heme. Which is the likely causative organism?

Accepted Answer

Entamoeba

Answer

Giardia

Answer

Staphylococcus

Answer

Clostridium perfringens

Question 2

The prepatent period in lymphatic filariasis is defined as the time interval between inoculation of infective larvae and which of the following?

Accepted Answer

First appearance of detectable microfilaria

Answer

Blockage of lymphatics

Answer

Development of lymphoedema

Answer

Development of adult worm

Question 3

Which of the following statements is true regarding Acanthamoeba keratitis?

Accepted Answer

Acanthamoeba does not depend upon a human host for the completion of its life-cycle.

Answer

For the isolation of the causative agent, corneal scraping should be cultured on a nutrient agar plate.

Answer

The causative agent, Acanthamoeba is a helminth whose normal habitat is soil.

Answer

Keratitis due to Acanthamoeba is not seen in the immunocompromised patient.

Question 4

Which of the following diseases is transmitted by egg ingestion?

Accepted Answer

Hydatidosis

Answer

Taeniasis

Answer

Trichinosis

Answer

Strongyloidosis

Question 5

All are seen in the cyst of E. Histolytica except?

Accepted Answer

Refractile nucleus

Answer

Glycogen mass

Answer

Chromatid bars

Answer

Eccentric nucleus

Question 6

A patient with AIDS returned from Haiti with acute diarrhea. The stool revealed an oval organism (8 to 9 um in diameter) that was acid-fast and fluoresced blue under ultraviolet light. What is the most likely identification of this organism?

Accepted Answer

Cyclospora

Answer

Giardia

Answer

Cryptosporidium

Answer

Enterocytozoon

Question 7

Schistosomiasis infects humans at which of the following stages of its life cycle?

Accepted Answer

Cercariae

Answer

Egg

Answer

Miracidia

Answer

All of the above

Question 8

T. trichuria maintains its position in the intestinal tract by:

Accepted Answer

Anchorage with its anterior portion

Answer

Penetration into tissues

Answer

Retention in the folds of the mucosa

Answer

Attachment with its cutting plate

Question 9

A 6-year-old boy presented with intermittent abdominal cramps and loose stools. Stool examination revealed ova approximately 100 micrometers in size. Which of the following is NOT a causative organism for these findings?

Accepted Answer

Opisthorchis viverrini

Answer

Fasciola gigantica

Answer

Echinostoma ilocanum

Answer

Gastrodiscoides hominis

Question 10

Which of the following is characterized by sheathed microfilariae?

Accepted Answer

Wuchereria bancrofti

Answer

Loa loa

Answer

Mansonella perstans

Answer

Brugia malayi

Parasitology — MCQs

Parasitology — MCQs

On this page

Practice by Chapter

Want unlimited practice?