Parasitology Practice Questions

Q: The enteric cycle of Toxoplasma gondii occurs in which host?

Cat. ### Explanation **Correct Answer: B. Cat** *Toxoplasma gondii* is an obligate intracellular protozoan with a complex life cycle involving two distinct phases: the **Enteric (Sexual) cycle** and the **Exo-enteric (Asexual) cycle**. 1. **Why Cat is Correct:** Members of the family **Felidae** (domestic cats and their relatives) are the **definitive hosts**. The enteric cycle occurs exclusively in the epithelial cells of the cat's small intestine. Here, the parasite undergoes gametogony and zygote formation, resulting in the production of **oocysts**, which are then excreted in the feces. This is the only stage where environmental contamination via oocysts occurs. 2. **Why Other Options are Incorrect:** * **A, C, and D (Rat, Cow, Sheep):** These are **intermediate hosts**. In these animals (and humans), only the **exo-enteric cycle** occurs. The parasite exists as rapidly multiplying **tachyzoites** (acute phase) or slow-growing **bradyzoites** within tissue cysts (chronic phase). They do not produce oocysts; therefore, the enteric cycle cannot take place in them. ### NEET-PG High-Yield Pearls: * **Definitive Host:** Cat (Sexual cycle). * **Intermediate Host:** Humans, mammals, and birds (Asexual cycle). * **Infective Stages:** 1. **Oocysts** (from cat feces). 2. **Bradyzoites** (in undercooked meat/tissue cysts). 3. **Tachyzoites** (transplacental transmission). * **Diagnosis:** Sabin-Feldman Dye Test (Gold Standard), though rarely used now; IgM/IgG ELISA is standard. * **Congenital Triad:** Chorioretinitis, Hydrocephalus, and Intracranial calcifications. * **Treatment of Choice:** Pyrimethamine + Sulfadiazine.

Q: How many nuclei are present in the mature cyst of E. histolytica?

4. ### Explanation The life cycle of *Entamoeba histolytica* involves two stages: the **trophozoite** (motile, feeding stage) and the **cyst** (infective, resistant stage). **Why Option C is correct:** The process of encystation begins when a trophozoite undergoes dehydration in the colon. It first forms a **precyst**, which contains a single nucleus. This nucleus undergoes two successive mitotic divisions to form a **mature quadrinucleate cyst**. This 4-nucleated stage is the infective form for humans; it is resistant to gastric acid and environmental stressors. **Analysis of Incorrect Options:** * **Option A (1 nucleus):** This represents the early **uninucleate cyst**. While present in the life cycle, it is immature and not the diagnostic hallmark of a "mature" cyst. * **Option B (2 nuclei):** This represents the intermediate **binucleate cyst** stage. * **Option D (8 nuclei):** This is a classic "distractor" in NEET-PG. An 8-nucleated mature cyst is the characteristic feature of ***Entamoeba coli***, a non-pathogenic commensal. Differentiating between 4 nuclei (*E. histolytica*) and 8 nuclei (*E. coli*) is a frequent exam topic. **Clinical Pearls for NEET-PG:** * **Infective dose:** As few as 10–100 cysts can cause infection. * **Morphology:** Mature cysts also contain **chromidial bars** with rounded/blunt ends (cigar-shaped), composed of ribosomes. In contrast, *E. coli* chromidial bars have splintered/frayed ends. * **Excystation:** Occurs in the small intestine, where one quadrinucleate cyst gives rise to eight small metacystic trophozoites. * **Nuclear Morphology:** Look for a central **karyosome** and fine, uniform peripheral chromatin (the "bull's eye" appearance).

Q: A farmer's wife develops abdominal pain and diarrhea, followed several days later by fever, periorbital edema, eosinophilia, and myalgia. She does not remember eating anything unusual recently but notes that she makes her own pork sausage. Which of the following diagnostic techniques would be most helpful for this patient?

Muscle biopsy. ### Explanation The clinical presentation of abdominal pain followed by the triad of **periorbital edema, myalgia, and significant eosinophilia** in a patient consuming homemade pork sausage is classic for **Trichinellosis**, caused by the nematode *Trichinella spiralis*. **Why Muscle Biopsy is Correct:** The life cycle of *Trichinella* involves the ingestion of undercooked meat containing encysted larvae. After maturing in the small intestine (enteral phase), the female releases newborn larvae that migrate via the bloodstream to highly oxygenated skeletal muscles (parenteral phase). Here, they encyst within "nurse cells." A **muscle biopsy** (typically from the deltoid or gastrocnemius) performed 2–4 weeks after infection is the definitive diagnostic test to visualize these **encysted coiled larvae**. **Why Other Options are Incorrect:** * **Stool for ova and parasites:** Unlike most intestinal helminths, *Trichinella* eggs hatch within the female worm, and adults are rarely shed in feces. Therefore, stool examination is almost always negative and unreliable. * **Scotch tape test:** This is the diagnostic gold standard for *Enterobius vermicularis* (pinworm) to detect eggs deposited on the perianal skin. * **Gastric biopsy:** The parasite resides in the small intestine during the early phase and the muscles during the late phase; it does not involve the gastric mucosa. **High-Yield NEET-PG Pearls:** * **Source:** Undercooked pork (domestic cycle) or wild game like bear/boar (sylvatic cycle). * **Key Lab Finding:** Marked **Eosinophilia** (often >20%) and elevated **Creatine Kinase (CK)** due to muscle inflammation. * **Splinter hemorrhages:** Often seen under the fingernails alongside periorbital edema. * **Treatment:** Albendazole or Mebendazole (effective against intestinal adults); corticosteroids are added for severe systemic/myocardial symptoms.

Q: A 25-year-old patient presented with signs of meningoencephalitis following swimming in a nearby local pond one week back. The CSF on wet mount microscopy revealed motile unicellular microorganisms. What is the most likely organism?

Naegleria fowleri. ### **Explanation** **Correct Option: A. *Naegleria fowleri*** The clinical presentation describes a classic case of **Primary Amoebic Meningoencephalitis (PAM)**. *Naegleria fowleri* is a free-living thermophilic amoeba found in warm freshwater bodies (ponds, lakes). * **Mechanism:** The organism enters the nasal cavity during swimming, penetrates the **cribriform plate**, and migrates via the olfactory nerves to the brain. * **Diagnosis:** The presence of **motile trophozoites** (showing eruptive pseudopodia) in a wet mount of fresh CSF is the gold standard for rapid diagnosis. **Why other options are incorrect:** * **B. *Entamoeba histolytica*:** Primarily causes intestinal amoebiasis and liver abscesses. While it can rarely cause brain abscesses, it does not present as acute meningoencephalitis following swimming. * **C. *Giardia lamblia*:** An intestinal flagellate causing malabsorption and diarrhea; it has no CNS involvement. * **D. *Trypanosoma* species:** *T. brucei* causes African Sleeping Sickness, which is a chronic CNS infection transmitted by the Tsetse fly, not via water exposure. --- ### **High-Yield Clinical Pearls for NEET-PG** * **Drug of Choice:** **Amphotericin B** (often combined with Rifampicin or Miltefosine), though the prognosis remains extremely poor (mortality >95%). * **Differentiating Feature:** Unlike *Acanthamoeba* (which causes Granulomatous Amoebic Encephalitis in immunocompromised hosts), *Naegleria* causes **acute** infection in **previously healthy** individuals. * **Culture:** Can be grown on **Non-nutrient agar (NNA)** seeded with *E. coli*. * **CSF Findings:** Resembles pyogenic meningitis (high neutrophils, low sugar, high protein), but the presence of motile amoebae distinguishes it.

Q: Which tissue is most commonly affected in cysticercosis?

Muscle. **Explanation:** Cysticercosis is caused by the larval stage (*Cysticercus cellulosae*) of the pork tapeworm, **Taenia solium**. Humans become accidental intermediate hosts by ingesting eggs through contaminated food or water. **1. Why Muscle is Correct:** While Neurocysticercosis (NCC) is the most clinically significant and symptomatic presentation, **subcutaneous tissue and skeletal muscles** are statistically the **most common** sites of larval lodgment. In many cases, muscular involvement remains asymptomatic or presents as small, palpable, painless nodules, which is why it is often underdiagnosed compared to neurological involvement. **2. Analysis of Incorrect Options:** * **Brain (Option A):** This is the most common site for **symptomatic** cysticercosis and the leading cause of adult-onset seizures worldwide. However, in terms of absolute frequency of larval distribution in the body, it ranks second to muscle/subcutaneous tissue. * **Eye (Option B):** Ocular cysticercosis occurs in about 1–3% of cases. It can involve the subretinal space or vitreous, potentially causing blindness, but it is far less common than muscular or neural involvement. * **Liver (Option C):** The liver is the primary site for **Hydatid disease** (*Echinococcus granulosus*), not cysticercosis. *T. solium* larvae rarely involve the hepatic parenchyma. **High-Yield NEET-PG Pearls:** * **Definitive Host:** Human (harbors adult worm). * **Intermediate Host:** Pig (normal); Human (accidental). * **Mode of Infection for Cysticercosis:** Ingestion of eggs (fecal-oral) or autoinfection. (Note: Ingesting undercooked pork leads to intestinal Taeniasis, not cysticercosis). * **Diagnosis:** MRI is the gold standard for NCC (shows "scolex within a cyst"). * **Treatment:** Albendazole is the drug of choice (combined with steroids to prevent inflammatory response to dying larvae).

Q: What is the gold standard for the diagnosis of echinococcal cyst?

ELISA. **Explanation:** The diagnosis of **Echinococcal cyst (Hydatid disease)**, caused primarily by *Echinococcus granulosus*, relies on a combination of serology and imaging. **Why ELISA is the Correct Answer:** In the context of establishing a definitive diagnosis, **ELISA (Enzyme-Linked Immunosorbent Assay)** for IgG antibodies is considered the **gold standard** (specifically the screening test of choice with high sensitivity). While imaging identifies the lesion, serological confirmation is required to differentiate it from other cystic lesions (like simple cysts or abscesses) and to confirm the parasitic etiology. A positive ELISA is often followed by Western Blot for 100% specificity. **Analysis of Incorrect Options:** * **USG (Ultrasonography):** This is the **investigation of choice (IOC)** for screening and staging (Gharbi/WHO classification). It is excellent for visualizing daughter cysts and "water lily" signs but cannot provide a definitive serological diagnosis. * **CT Scan:** This is the best modality for detecting **wall calcification** and evaluating complications (rupture/infection) or cysts in extrahepatic locations. It is more sensitive than USG for calcification but is not the diagnostic gold standard. * **X-ray:** This may show incidental curvilinear calcification (eggshell calcification) in the right upper quadrant, but it lacks the sensitivity and specificity for a formal diagnosis. **Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** "Water lily sign" (detached endocyst membrane) seen on USG/CT. * **Treatment of Choice:** Surgical excision (modified PAIR technique or cystectomy) combined with **Albendazole**. * **Caution:** Diagnostic aspiration (FNA) is generally **contraindicated** due to the risk of anaphylactic shock from spillage of cyst fluid. * **Calcification:** A completely calcified cyst usually indicates a dead/inactive parasite.

Question 1

The enteric cycle of Toxoplasma gondii occurs in which host?

Accepted Answer

Cat

Answer

Rat

Answer

Cow

Answer

Sheep

Question 2

How many nuclei are present in the mature cyst of E. histolytica?

Accepted Answer

4

Answer

1

Answer

2

Answer

8

Question 3

A 62-year-old patient presents with hematuria. Investigations reveal renal calculi, calcifications in the wall of the urinary bladder, and a small contracted bladder. What is the most likely cause?

Accepted Answer

Schistosomiasis

Answer

Prostate calculi

Answer

Carcinoma of the bladder

Answer

Bladder diverticulitis

Question 4

A farmer's wife develops abdominal pain and diarrhea, followed several days later by fever, periorbital edema, eosinophilia, and myalgia. She does not remember eating anything unusual recently but notes that she makes her own pork sausage. Which of the following diagnostic techniques would be most helpful for this patient?

Accepted Answer

Muscle biopsy

Answer

Gastric biopsy

Answer

Scotch tape test

Answer

Stool for ova and parasites

Question 5

A 25-year-old patient presented with signs of meningoencephalitis following swimming in a nearby local pond one week back. The CSF on wet mount microscopy revealed motile unicellular microorganisms. What is the most likely organism?

Accepted Answer

Naegleria fowleri

Answer

Entamoeba histolytica

Answer

Giardia lamblia

Answer

Trypanosoma species

Question 6

Which tissue is most commonly affected in cysticercosis?

Accepted Answer

Muscle

Answer

Brain

Answer

Eye

Answer

Liver

Question 7

A 28-year-old man presents with vague muscle pain involving his right arm that developed several weeks after eating undercooked meat. A biopsy from the affected muscle reveals encysted forms of Trichinella spiralis. What type of white blood cell is most likely to be increased in numbers in the peripheral blood of this individual?

Accepted Answer

Eosinophil

Answer

Basophil

Answer

Macrophage

Answer

Neutrophil

Question 8

Dracunculosis infection occurs through which mode of transmission?

Accepted Answer

Ingestion of water containing cyclops infected with the parasite

Answer

Ingestion of water containing the parasite directly

Answer

Ingestion of contaminated fish

Answer

Penetration of skin by the parasite

Question 9

Sputum examination is not useful in the diagnosis of which of the following parasitic infections?

Accepted Answer

Trichuris trichura

Answer

Ancylostoma duodenale

Answer

Paragonimiasis

Answer

Strongyloides stercoralis

Question 10

What is the gold standard for the diagnosis of echinococcal cyst?

Accepted Answer

ELISA

Answer

X-ray

Answer

CT scan

Answer

USG

Parasitology — MCQs

Parasitology — MCQs

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