Parasitology Practice Questions

Q: A stool examination reveals acid-fast organisms with oocytes measuring 5 micrometers, causing diarrhea in an HIV-positive patient. What is the causative organism?

Cryptosporidium. ### Explanation The correct answer is **Cryptosporidium parvum**. This is a classic presentation of opportunistic diarrhea in immunocompromised patients, particularly those with HIV/AIDS. #### Why Cryptosporidium is Correct: 1. **Size:** The oocysts of *Cryptosporidium* are characteristically small, measuring **4–6 µm** (average 5 µm). This is the most defining morphological feature in the question. 2. **Staining:** It is an **acid-fast** organism. On Modified Ziehl-Neelsen (kinyoun) stain, they appear as bright red, spherical bodies against a blue/green background. 3. **Clinical Context:** It is a leading cause of chronic, voluminous, watery diarrhea in HIV patients with CD4 counts <200 cells/mm³. #### Why Other Options are Incorrect: * **Isospora belli (Cystoisospora):** While also acid-fast and common in HIV, its oocysts are much larger (**25–30 µm**) and typically **ellipsoidal/oval** in shape. * **Microsporidia:** These are the smallest (1–3 µm). Crucially, they are **not** typically identified by standard acid-fast staining; they require specialized stains like Chromotrope 2R or Calcofluor White. * **Blastocystis hominii:** This is a polymorphic protozoan (often seen in vacuolar form). It is **not acid-fast** and its role as a primary pathogen is often debated. #### NEET-PG High-Yield Pearls: * **Morphology Trick:** Remember the "Rule of Sizes" for acid-fast oocysts: *Cryptosporidium* (5 µm) < *Cyclospora* (10 µm) < *Isospora* (25 µm). * **Diagnosis:** Stool examination using **Modified Acid-Fast stain** is the gold standard. * **Treatment:** In HIV patients, the most effective treatment is **HAART** (to restore CD4 count). **Nitazoxanide** is the drug of choice in immunocompetent patients. * **Transmission:** Frequently associated with waterborne outbreaks due to its resistance to chlorination.

Q: Which of the following is NOT transmitted by Aedes mosquitoes?

Malaria. **Explanation:** The correct answer is **Malaria**. This question tests the fundamental knowledge of vector-borne diseases, specifically distinguishing between viral and protozoal transmission cycles. **1. Why Malaria is the Correct Answer:** Malaria is caused by protozoan parasites of the genus *Plasmodium*. It is exclusively transmitted by the bite of an infected **female Anopheles mosquito**, not *Aedes*. The life cycle of the malaria parasite involves a sexual cycle in the Anopheles mosquito (definitive host) and an asexual cycle in humans (intermediate host). **2. Why the other options are incorrect:** * **Chikungunya, Zika, and Dengue** are all viral infections (Arboviruses) primarily transmitted by **Aedes aegypti** and **Aedes albopictus**. * These mosquitoes are "day-biters" and typically breed in clean, stagnant water (e.g., flower pots, discarded tires). **Clinical Pearls for NEET-PG:** * **Aedes Mosquito:** Known as the "Tiger Mosquito" due to white stripes on its body. It is the vector for Dengue, Chikungunya, Zika, Yellow Fever, and Rift Valley Fever. * **Dengue:** Look for "break-bone fever" and retro-orbital pain in clinical vignettes. * **Zika:** High-yield association with **Microcephaly** in newborns and **Guillain-Barré Syndrome** in adults. * **Chikungunya:** Characterized by severe, often persistent, debilitating joint pain (arthritis). * **Malaria Vector:** *Anopheles stephensi* is the primary urban malaria vector in India, while *Anopheles culicifacies* is the major rural vector.

Q: Cutaneous larva migrans is caused by which organism?

Ancylostoma braziliense. **Explanation:** **Cutaneous Larva Migrans (CLM)**, also known as "creeping eruption," is a zoonotic infection caused by the larvae of animal hookworms. 1. **Why Ancylostoma braziliense is correct:** The most common cause of CLM is **Ancylostoma braziliense** (the hookworm of cats and dogs). When humans come into contact with soil contaminated by animal feces, the filariform larvae penetrate the skin. Because humans are accidental, non-definitive hosts, the larvae lack the enzymes (collagenases) necessary to penetrate the basement membrane and reach the circulation. Consequently, they remain trapped in the epidermis, migrating aimlessly and creating characteristic **serpiginous, erythematous, pruritic tracks.** 2. **Why the other options are incorrect:** * **Anisakis simplex:** Causes **Anisakiasis**, a gastrointestinal infection acquired by consuming raw or undercooked seafood containing the larvae. * **Toxocara species:** Causes **Visceral Larva Migrans (VLM)** and **Ocular Larva Migrans (OLM)**. Unlike CLM, these larvae migrate through deeper internal organs (liver, lungs, eyes) because they are ingested rather than penetrating the skin. * **Necator americanus:** This is a human hookworm. While it can cause a transient "ground itch" at the site of entry, it successfully penetrates the dermis to complete its life cycle in the human intestine, rather than remaining confined to the skin as a migrating larva. **High-Yield NEET-PG Pearls:** * **Clinical Presentation:** Intensely itchy, snake-like tracks, most commonly on the feet (sand-box exposure). * **Treatment of Choice:** Topical or oral **Albendazole** or **Ivermectin**. * **Key Distinction:** CLM = *Ancylostoma braziliense* (Skin); VLM = *Toxocara canis* (Viscera). * **Larva Currens:** A similar but much faster-moving eruption caused by *Strongyloides stercoralis*.

Q: A HIV-positive individual has diarrheal episodes. Stool examination reveals oocysts of size 8-10 microns. What is the most likely diagnosis?

Cyclospora. ### **Explanation** The clinical presentation of chronic diarrhea in an HIV-positive patient points toward opportunistic coccidian parasites. The definitive diagnostic feature in this question is the **size of the oocyst (8–10 µm).** **1. Why Cyclospora is Correct:** * **Cyclospora cayetanensis** produces spherical oocysts that measure **8–10 µm** in diameter. * They are acid-fast (variable) and typically appear as "wrinkled" spheres. * In HIV patients, they cause prolonged, watery diarrhea similar to Cryptosporidium but are distinguished primarily by their larger size. **2. Why Other Options are Incorrect:** * **Cryptosporidium parvum:** This is the most common cause of diarrhea in AIDS patients, but its oocysts are significantly smaller, measuring **4–6 µm**. * **Isospora (Cystoisospora) belli:** These oocysts are much larger (**25–30 µm**) and have a characteristic elliptical/oval shape, unlike the spherical shape of Cyclospora. * **Cryptococcus:** This is a fungus, not a coccidian parasite. While it can cause disseminated disease in HIV, it typically presents as meningitis and is identified by India Ink staining of CSF, not as oocysts in stool. **3. High-Yield Clinical Pearls for NEET-PG:** * **Size Comparison (The "Rule of 5s"):** * *Cryptosporidium:* ~5 µm (Smallest) * *Cyclospora:* ~10 µm (Double the size of Cryptosporidium) * *Isospora:* ~25–30 µm (Largest, oval) * **Autofluorescence:** *Cyclospora* oocysts exhibit blue-green autofluorescence under UV microscopy, a key diagnostic feature. * **Staining:** All three (Cryptosporidium, Cyclospora, Isospora) are **Modified Acid-Fast positive**. * **Treatment:** While *Cryptosporidium* is difficult to treat (Nitazoxanide), both *Cyclospora* and *Isospora* respond well to **Cotrimoxazole (TMP-SMX)**.

Q: Erythrocytic schizogony of Plasmodium liberates which stage?

Merozoites. **Explanation:** The life cycle of *Plasmodium* involves two distinct phases of asexual reproduction (schizogony) in the human host: the **Exo-erythrocytic stage** (liver) and the **Erythrocytic stage** (RBCs). In the erythrocytic stage, a merozoite infects a red blood cell, matures into a ring-form **trophozoite**, and then develops into a **schizont**. When the schizont matures, the RBC ruptures, liberating a new generation of **merozoites** into the bloodstream. These released merozoites then infect fresh RBCs, repeating the cycle. The clinical paroxysm (fever and chills) in malaria coincides with this synchronous rupture of RBCs and the release of merozoites and metabolic byproducts. **Analysis of Options:** * **A. Sporozoites:** These are the infective forms of the parasite. They are inoculated into the human host by the bite of an infected female *Anopheles* mosquito. * **B. Trophozoites:** This is the active, feeding stage of the parasite *inside* the RBC. It does not represent the stage liberated upon schizogony; rather, it matures into the schizont. * **D. Phanerozoites:** This term refers to the exo-erythrocytic stages found in the tissues (liver) of birds/animals in certain species. In humans, the equivalent is the pre-erythrocytic schizont. **High-Yield Clinical Pearls for NEET-PG:** * **Hypnozoites:** These are dormant liver stages found only in *P. vivax* and *P. ovale*, responsible for **relapses**. * **Recrudescence:** Seen in *P. falciparum* and *P. malariae* due to the persistence of low-level parasitemia in the blood (not liver). * **Schüffner’s dots:** Seen in RBCs infected with *P. vivax* and *P. ovale*. * **Maurer’s clefts:** Seen in RBCs infected with *P. falciparum*.

Q: In malaria, where does the sexual cycle occur?

Gametocytes to sporozoites. **Explanation:** The life cycle of *Plasmodium* involves two hosts: humans (intermediate host) and the female *Anopheles* mosquito (definitive host). **1. Why Option B is Correct:** The **sexual cycle (Sporogony)** occurs entirely within the mosquito. It begins when the mosquito ingests male and female **gametocytes** from an infected human. In the mosquito's gut, these mature into gametes, undergo fertilization to form a zygote (ookinete), and eventually produce thousands of **sporozoites** (the infective stage for humans). Thus, the transition from gametocytes to sporozoites represents the completion of the sexual phase. **2. Analysis of Incorrect Options:** * **Option A (Sporozoites to gametocytes):** This represents the **asexual cycle (Schizogony)** which occurs in humans. It includes the exo-erythrocytic (liver) and erythrocytic (RBC) stages. * **Option C (Occurs in humans):** Humans host the asexual cycle only. The sexual cycle requires the cooler body temperature and specific physiological environment of the mosquito. * **Option D (Responsible for relapse):** Relapse is caused by **hypnozoites** (dormant liver stages), specifically seen in *P. vivax* and *P. ovale*. This is a feature of the asexual cycle, not the sexual cycle. **Clinical Pearls for NEET-PG:** * **Definitive Host:** Female *Anopheles* mosquito (where sexual fusion occurs). * **Intermediate Host:** Humans (where asexual multiplication occurs). * **Infective form to humans:** Sporozoites (found in mosquito salivary glands). * **Infective form to mosquito:** Gametocytes (found in human peripheral blood). * **Exflagellation:** This process refers to the formation of male microgametes in the mosquito's midgut—a high-yield term often associated with the sexual cycle.

Q: A 20-year-old male patient presents with fever, intermittent abdominal pain, and dysuria. He has a history of swimming in a freshwater lake 10 days prior. Urine microscopy findings are provided. Which of the following is the causative agent?

Schistosoma hematobium. **Explanation:** The clinical presentation of fever, abdominal pain, and **dysuria** following freshwater exposure (swimming) is classic for **Schistosomiasis** (Snail fever). The key diagnostic clue is the site of involvement: **urinary symptoms (dysuria)** point directly toward **Schistosoma haematobium**. **1. Why Schistosoma haematobium is correct:** * **Habitat:** Unlike other species, *S. haematobium* adult worms reside in the **vesical and pelvic venous plexuses**. * **Pathogenesis:** Eggs penetrate the bladder wall to be excreted in urine, causing inflammation, hematuria, and dysuria. * **Morphology:** It is characterized by eggs with a **terminal spine**, which would be visible on urine microscopy. **2. Why other options are incorrect:** * **S. mansoni & S. japonicum:** These species primarily inhabit the **inferior and superior mesenteric veins**, respectively. They cause intestinal and hepatic schistosomiasis (bloody diarrhea, portal hypertension) rather than urinary symptoms. *S. mansoni* eggs have a **lateral spine**, while *S. japonicum* eggs are rounded with a **rudimentary lateral knob**. * **S. mekongi:** Similar to *S. japonicum*, it causes intestinal disease and is restricted to the Mekong River basin in Southeast Asia. **Clinical Pearls for NEET-PG:** * **Intermediate Host:** Freshwater snails (*Bulinus* species for *S. haematobium*). * **Infective Stage:** **Cercaria** (enters via skin penetration). * **Diagnostic Stage:** Eggs in urine (terminal spine) or stool. * **Chronic Complication:** *S. haematobium* is a known risk factor for **Squamous Cell Carcinoma of the bladder**. * **Drug of Choice:** Praziquantel.

Q: Ova in stool are not of diagnostic significance in which of the following helminthic infections?

Strongyloides. ### Explanation The diagnostic hallmark of **Strongyloides stercoralis** (Threadworm) infection is the presence of **rhabditiform larvae** in the stool, not eggs (ova). **Why Strongyloides is the correct answer:** In the life cycle of *Strongyloides stercoralis*, the adult female lives in the submucosa of the small intestine and lays eggs. These eggs hatch almost immediately within the intestinal mucosa. Therefore, by the time the parasite is excreted in the feces, it is already in the **larval stage**. Finding eggs in the stool is extremely rare and usually only occurs in cases of severe hyperinfection or heavy purgation. **Analysis of Incorrect Options:** * **Ankylostoma duodenale (Hookworm):** Diagnosis is primarily made by identifying characteristic **non-bile stained, segmented eggs** (usually at the 4-8 cell stage) in the stool. * **Enterobius vermicularis (Pinworm):** While the NIH swab/Scotch tape test is the gold standard (detecting eggs on the perianal skin), the diagnostic stage is the **planoconvex egg**. These are occasionally seen in routine stool exams, though less frequently than other helminths. * **Trichuris trichiura (Whipworm):** Diagnosis is confirmed by finding the characteristic **barrel-shaped eggs** with bipolar mucus plugs in the stool. **High-Yield Clinical Pearls for NEET-PG:** * **Strongyloides** is unique because it can cause **autoinfection**, where rhabditiform larvae transform into filariform larvae within the gut and re-penetrate the intestinal wall or perianal skin. * **Hyperinfection syndrome** is a life-threatening complication seen in immunocompromised patients (especially those on steroids). * **Larva Currens:** A rapidly moving, serpiginous cutaneous eruption is pathognomonic for Strongyloides. * **Drug of Choice:** Ivermectin is the preferred treatment for Strongyloidiasis.

Q: A 30-year-old female stored her contact lenses in tap water and presented with severe retinitis and deterioration of vision. Culture of the tap water and vitreous fluid would most likely reveal which of the following microorganisms?

Acanthamoeba. **Explanation:** The clinical presentation of a contact lens wearer using tap water for storage who develops severe ocular involvement is a classic "high-yield" scenario for **Acanthamoeba keratitis**. **Why Acanthamoeba is correct:** Acanthamoeba is a free-living amoeba found in soil and water (including tap water). In contact lens users, poor hygiene—specifically using non-sterile tap water or homemade saline—allows the organism to contaminate the lenses. It causes a painful, sight-threatening keratitis characterized by a pathognomonic **ring-shaped corneal infiltrate**. Diagnosis is confirmed by demonstrating trophozoites or double-walled cysts on corneal scrapings or culture on **non-nutrient agar (NNA) seeded with E. coli**. **Why other options are incorrect:** * **Naegleria fowleri:** Known as the "brain-eating amoeba," it causes Primary Amoebic Meningoencephalitis (PAM) after swimming in warm freshwater. It does not typically cause ocular infections. * **Pneumocystis jirovecii:** An opportunistic fungus that primarily causes interstitial pneumonia in immunocompromised patients (e.g., HIV/AIDS). It is not associated with contact lens use or keratitis. * **Babesia:** An intraerythrocytic protozoan transmitted by *Ixodes* ticks, causing a malaria-like illness (Babesiosis). It does not cause eye infections. **Clinical Pearls for NEET-PG:** * **Stain of choice:** Calcofluor white (binds to chitin in the cyst wall). * **Culture medium:** Non-nutrient agar with *E. coli* (look for "trailing" or "tracks" made by the amoeba). * **Pathognomonic sign:** Radial keratoneuritis (inflammation of corneal nerves) leading to exquisite pain out of proportion to clinical findings. * **Treatment:** Topical biguanides (PHMB) or chlorhexidine.

Question 1

A stool examination reveals acid-fast organisms with oocytes measuring 5 micrometers, causing diarrhea in an HIV-positive patient. What is the causative organism?

Accepted Answer

Cryptosporidium

Answer

Isospora belli

Answer

Microsporidia

Answer

Blastocystis hominii

Question 2

Which of the following is NOT transmitted by Aedes mosquitoes?

Accepted Answer

Malaria

Answer

Chikungunya

Answer

Zika

Answer

Dengue

Question 3

Cutaneous larva migrans is caused by which organism?

Accepted Answer

Ancylostoma braziliense

Answer

Anisakis simplex

Answer

Toxocara species

Answer

Necator americanus

Question 4

A HIV-positive individual has diarrheal episodes. Stool examination reveals oocysts of size 8-10 microns. What is the most likely diagnosis?

Accepted Answer

Cyclospora

Answer

Cryptococcus

Answer

Cryptosporidium parvum

Answer

Isospora

Question 5

Erythrocytic schizogony of Plasmodium liberates which stage?

Accepted Answer

Merozoites

Answer

Sporozoites

Answer

Trophozoites

Answer

Phanerozoites

Question 6

In malaria, where does the sexual cycle occur?

Accepted Answer

Gametocytes to sporozoites

Answer

Sporozoites to gametocytes

Answer

Occurs in humans

Answer

Responsible for relapse

Question 7

A 20-year-old male patient presents with fever, intermittent abdominal pain, and dysuria. He has a history of swimming in a freshwater lake 10 days prior. Urine microscopy findings are provided. Which of the following is the causative agent?

Accepted Answer

Schistosoma hematobium

Answer

Schistosoma mansoni

Answer

Schistosoma japonicum

Answer

Schistosoma mekongi

Question 8

Which of the following statements is FALSE regarding filariasis?

Accepted Answer

Humoral immunity plays a dominant role

Answer

Morbidity increases with age in endemic areas

Answer

It is usually unilateral

Answer

Man is the only host for filariasis

Question 9

Ova in stool are not of diagnostic significance in which of the following helminthic infections?

Accepted Answer

Strongyloides

Answer

Ankylostoma

Answer

Enterobius

Answer

Trichuris

Question 10

A 30-year-old female stored her contact lenses in tap water and presented with severe retinitis and deterioration of vision. Culture of the tap water and vitreous fluid would most likely reveal which of the following microorganisms?

Accepted Answer

Acanthamoeba

Answer

Naegleria

Answer

Pneumocystis

Answer

Babesia

Parasitology — MCQs

Parasitology — MCQs

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