Parasitic life cycles US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Parasitic life cycles. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Parasitic life cycles US Medical PG Question 1: A 31-year-old man comes to the physician because of a 2-day history of abdominal pain and diarrhea. He reports that his stools are streaked with blood and mucus. He returned from a vacation in the Philippines 3 weeks ago. His vital signs are within normal limits. Abdominal examination shows hyperactive bowel sounds. A photomicrograph of a trichrome-stained wet mount of a stool specimen is shown. Which of the following organisms is the most likely cause of this patient's symptoms?
- A. Campylobacter jejuni
- B. Giardia lamblia
- C. Cryptosporidium parvum
- D. Shigella dysenteriae
- E. Entamoeba histolytica (Correct Answer)
Parasitic life cycles Explanation: ***Entamoeba histolytica***
- The patient's symptoms of **bloody, mucoid diarrhea** (dysentery) after travel to the **Philippines** are highly suggestive of **amebic dysentery** caused by *Entamoeba histolytica*.
- A **trichrome-stained wet mount** of stool revealing trophozoites or cysts of *Entamoeba histolytica* in this clinical context would confirm the diagnosis.
*Campylobacter jejuni*
- While *Campylobacter jejuni* can cause **bloody diarrhea** and is a common cause of **traveler's diarrhea**, it is a **bacterial infection**, and the question implies a parasitic etiology with the mention of a trichrome-stained wet mount.
- Infection is typically associated with consumption of undercooked poultry or contaminated water.
*Giardia lamblia*
- *Giardia lamblia* causes **giardiasis**, which typically presents with **non-bloody, watery diarrhea**, **steatorrhea**, abdominal cramps, and bloating.
- It does not usually cause **dysentery** or bloody stools.
*Cryptosporidium parvum*
- *Cryptosporidium parvum* causes **cryptosporidiosis**, characterized by **profuse, watery diarrhea** and abdominal cramps.
- While it can be severe in immunocompromised individuals, it typically does not cause **bloody or mucoid stools** (dysentery).
*Shigella dysenteriae*
- *Shigella dysenteriae* causes **shigellosis**, a severe form of dysentery with **bloody, mucoid stools**, fever, and tenesmus.
- While it fits the clinical picture of dysentery, the diagnostic method mentioned (trichrome-stained wet mount) is primarily used for identifying **parasitic organisms**, not bacteria like *Shigella*.
Parasitic life cycles US Medical PG Question 2: A 2-year-old girl is brought to the doctor by her mother with persistent scratching of her perianal region. The patient’s mother says that symptoms started 3 days ago and have progressively worsened until she is nearly continuously scratching even in public places. She says that the scratching is worse at night and disturbs her sleep. An anal swab and staining with lactophenol cotton blue reveal findings in the image (see image). Which of the following is the organism most likely responsible for this patient’s condition?
- A. Wuchereria bancrofti
- B. Enterobius vermicularis (Correct Answer)
- C. Ancylostoma duodenale
- D. Taenia saginata
- E. Ascaris lumbricoides
Parasitic life cycles Explanation: ***Enterobius vermicularis***
* The classic symptom of **E. vermicularis** (pinworm) infection is **perianal pruritus**, especially at night, caused by the female worms migrating to the perianal region to lay eggs.
* The image likely shows **pinworm eggs**, which are small, oval, and have one flattened side, visible with **lactophenol cotton blue staining** from an anal swab.
*Wuchereria bancrofti*
* **W. bancrofti** causes **lymphatic filariasis**, leading to **lymphedema** and **elephantiasis**, and is transmitted by mosquitoes.
* It does not cause perianal scratching and its microfilariae are found in blood, not perianal swabs.
*Ancylostoma duodenale*
* **A. duodenale** (hookworm) causes **iron-deficiency anemia** and **gastrointestinal symptoms** due to blood loss from the intestines.
* Hookworm eggs are typically recovered from **stool samples** and do not cause perianal itching as a primary symptom.
*Taenia saginata*
* **T. saginata** (beef tapeworm) infection is often **asymptomatic** or causes mild **abdominal discomfort** and **weight loss**.
* Diagnosis is made by finding **proglottids** or eggs in stool; perianal itching is not a characteristic feature.
*Ascaris lumbricoides*
* **A. lumbricoides** (roundworm) causes **pulmonary symptoms** during larval migration and **intestinal obstruction** or malnutrition in heavy infections.
* Its eggs are found in **stool samples**, and it does not typically cause perianal pruritus.
Parasitic life cycles US Medical PG Question 3: A 13-year-old boy is brought to a physician with severe fevers and headaches for 3 days. The pain is constant and mainly behind the eyes. He has myalgias, nausea, vomiting, and a rash for one day. Last week, during an academic winter break, he traveled on a tour with his family to several countries, including Brazil, Panama, and Peru. They spent many evenings outdoors without any protection against insect bites. There is no history of contact with pets, serious illness, or use of medications. The temperature is 40.0℃ (104.0℉); the pulse is 110/min; the respiratory rate is 18/min, and the blood pressure is 110/60 mm Hg. A maculopapular rash is seen over the trunk and extremities. Several tender lymph nodes are palpated in the neck on both sides. A peripheral blood smear shows no organisms. Which of the following is most likely responsible for this patient’s presentation?
- A. Chagas disease
- B. Zika virus
- C. Babesiosis
- D. Malaria
- E. Dengue fever (Correct Answer)
Parasitic life cycles Explanation: ***Dengue fever***
- This patient's symptoms (fever, **retro-orbital headache**, myalgias, nausea, vomiting, rash, and travel history to endemic areas like **Brazil, Panama, and Peru**) are classic for dengue fever. The **high fever (40°C)** and rash are also highly suggestive.
- Exposure to mosquito bites in tropical regions, typical of travel during an academic break, is a common mode of transmission for this **flavivirus**.
*Chagas disease*
- Chagas disease, caused by **Trypanosoma cruzi**, is typically transmitted by the **reduviid bug** (kissing bug).
- Acute symptoms can include **fever**, **Romana's sign** (unilateral periorbital swelling), and sometimes a chagoma, but the widespread **maculopapular rash** and severe retro-orbital headache are less characteristic.
*Zika virus*
- Zika virus infection can present with **fever**, **rash**, **arthralgia**, and **conjunctivitis**.
- While the travel history fits, the **severe retro-orbital headache**, high fever, and myalgias are more prominent in dengue fever; Zika symptoms are generally milder in adults.
*Babesiosis*
- Babesiosis is a **tick-borne** illness caused by **Babesia parasites**, often presenting with **fever**, **fatigue**, chills, and **hemolytic anemia**.
- There is no mention of tick exposure, and the characteristic rash and retro-orbital headache are not typical features of babesiosis.
*Malaria*
- Malaria, caused by **Plasmodium parasites** transmitted by **Anopheles mosquitoes**, presents with cyclical fevers, chills, sweats, and fatigue.
- While the travel history to endemic areas is relevant, the **retro-orbital headache** and **maculopapular rash** as described are not typical for uncomplicated malaria; malaria is also detected on a peripheral blood smear, which was negative here.
Parasitic life cycles US Medical PG Question 4: A 32-year-old woman presents to your office with abdominal pain and bloating over the last month. She also complains of intermittent, copious, non-bloody diarrhea over the same time. Last month, she had a cough that has since improved but has not completely resolved. She has no sick contacts and has not left the country recently. She denies any myalgias, itching, or rashes. Physical and laboratory evaluations are unremarkable. Examination of her stool reveals the causative organism. This organism is most likely transmitted to the human host through which of the following routes?
- A. Insect bite
- B. Penetration of skin (Correct Answer)
- C. Sexual contact
- D. Inhalation
- E. Animal bite
Parasitic life cycles Explanation: ***Penetration of skin***
- The symptoms of **abdominal pain**, **bloating**, **intermittent copious non-bloody diarrhea**, and a recent **cough** are highly suggestive of a **hookworm infection**.
- Hookworm larvae (filariform larvae) primarily penetrate the skin, usually through bare feet, as their mode of entry into the human host.
*Insect bite*
- Although some parasitic infections are transmitted by insect bites (e.g., malaria, Chagas disease), hookworms are not transmitted this way.
- **Insect-borne diseases** typically present with different clinical manifestations or geographical associations.
*Sexual contact*
- **Sexually transmitted infections** involve direct contact of mucous membranes or body fluids during sexual activity.
- Hookworm infection transmission through sexual contact is not a recognized route.
*Inhalation*
- **Inhalation** is a route of transmission for respiratory pathogens (e.g., influenza, tuberculosis) or certain fungal infections, but not for hookworms.
- While hookworm larvae migrate through the lungs, the initial infection pathway is not via inhalation.
*Animal bite*
- **Animal bites** transmit diseases like rabies or certain bacterial infections, but not parasitic hookworms.
- Hookworm infection does not result from direct contact with an animal's saliva or puncture wound.
Parasitic life cycles US Medical PG Question 5: A 31-year-old male traveler in Thailand experiences fever, headache, and excessive sweating every 48 hours. Peripheral blood smear shows trophozoites and schizonts indicative of Plasmodia infection. The patient is given chloroquine and primaquine. Primaquine targets which of the following Plasmodia forms:
- A. Schizont
- B. Hypnozoite (Correct Answer)
- C. Trophozoite
- D. Merozoite
- E. Sporozoite
Parasitic life cycles Explanation: ***Hypnozoite***
- **Primaquine** is a **radical cure** for malaria caused by *Plasmodium vivax* and *Plasmodium ovale* because it targets the dormant **hypnozoite** forms in the liver.
- The presence of **hypnozoites** leads to relapses, as they can reactivate and re-initiate the erythrocytic cycle.
*Schizont*
- **Schizonts** are merozoite-producing forms in red blood cells (**erythrocytic schizonts**) or liver cells (**hepatic schizonts**).
- While chloroquine targets **erythrocytic schizonts**, primaquine's primary unique action is against the dormant liver stages.
*Trophozoite*
- **Trophozoites** are the feeding and growing stages of the parasite within red blood cells, which mature into schizonts.
- **Chloroquine** is highly effective against **erythrocytic trophozoites** and schizonts, resolving acute malarial symptoms.
*Merozoite*
- **Merozoites** are released from ruptured schizonts and infect new red blood cells during the erythrocytic cycle.
- No specific antimalarial drug solely targets **merozoites** as a primary form; they are an infective stage for red blood cells.
*Sporozoite*
- **Sporozoites** are the forms injected by infected mosquitoes, which then travel to the liver and infect hepatocytes.
- While some drugs like atovaquone have activity against sporozoites, primaquine is specifically indicated for destroying the **hypnozoite** stage, preventing relapses.
Parasitic life cycles US Medical PG Question 6: A 52-year-old man presents with a 5-week history of multiple cutaneous ulcers on his left forearm and neck, which he first noticed after returning from a 2-month stay in rural Peru. He does not recall any trauma or arthropod bites. The lesions began as non-pruritic erythematous papules that became enlarged, ulcerated, and crusted. There is no history of fever or abdominal pain. He has been sexually active with a single partner since their marriage at 24 years of age. The physical examination reveals erythematous, crusted plaques with central ulceration and a raised border. There is no fluctuance, drainage, or sporotrichoid spread. A punch biopsy was performed, which revealed an ulcerated lesion with a mixed inflammatory infiltrate. Amastigotes within dermal macrophages are seen on Giemsa staining. What is the most likely diagnosis?
- A. Syphilis
- B. Histoplasmosis
- C. Cutaneous tuberculosis
- D. Cutaneous leishmaniasis (Correct Answer)
- E. Ecthyma
Parasitic life cycles Explanation: ***Cutaneous leishmaniasis***
- The presence of **amastigotes within dermal macrophages** on **Giemsa staining** is pathognomonic for **leishmaniasis**.
- The patient's travel history to **rural Peru**, along with the characteristic cutaneous lesions (erythematous papules progressing to enlarged, ulcerated, and crusted plaques), is highly suggestive of **cutaneous leishmaniasis**.
*Syphilis*
- **Syphilis** lesions can be ulcerative but are typically associated with **treponemes** (visible with darkfield microscopy or immunohistochemistry), not amastigotes.
- The patient's sexual history with a single partner since marriage at 24 makes primary or secondary syphilis less likely in the absence of other risk factors.
*Histoplasmosis*
- **Histoplasmosis** is a fungal infection that can cause cutaneous lesions, especially in immunocompromised individuals.
- Histopathology would reveal **yeast forms** and not amastigotes within macrophages.
*Cutaneous tuberculosis*
- **Cutaneous tuberculosis** can present with various skin lesions, including ulcers and plaques, but histopathology would show **granulomas** with **caseous necrosis** and **acid-fast bacilli**, not amastigotes.
- The absence of typical **tuberculosis** symptoms (e.g., fever, night sweats, weight loss) also makes this less likely.
*Ecthyma*
- **Ecthyma** is a skin infection often caused by **Streptococcus pyogenes** or **Staphylococcus aureus**, characterized by **punched-out ulcers** with adherent crusts.
- While it presents with ulcers, the biopsy finding of **amastigotes within macrophages** rules out bacterial ecthyma.
Parasitic life cycles US Medical PG Question 7: A 68-year-old man presents to his physician for symptoms of chronic weight loss, abdominal bloating, and loose stools. He notes that he has also been bothered by a chronic cough. The patient’s laboratory work-up includes a WBC differential, which is remarkable for an eosinophil count of 9%. Stool samples are obtained, with ova and parasite examination revealing roundworm larvae in the stool and no eggs. Which of the following parasitic worms is the cause of this patient’s condition?
- A. Taenia saginata
- B. Taenia solium
- C. Strongyloides stercoralis (Correct Answer)
- D. Necator americanus
- E. Ascaris lumbricoides
Parasitic life cycles Explanation: ***Strongyloides stercoralis***
- The presence of **larvae (rhabditiform)** in the stool, **pulmonary symptoms** (chronic cough), **gastrointestinal symptoms** (weight loss, bloating, loose stools), and **eosinophilia** are classic findings for *Strongyloides stercoralis* infection.
- Unlike most other intestinal nematodes, *Strongyloides* can establish an **autoinfection cycle**, meaning larvae can reinfect the host, leading to persistent and potentially severe infections even in immunocompetent individuals, without the need for external re-exposure or eggs in stool.
*Taenia saginata*
- This is a **tapeworm (cestode)** that causes taeniasis and is acquired by consuming undercooked beef.
- Diagnosis is typically made by finding **proglottids** or **eggs** in the stool, not larvae, and pulmonary symptoms are not characteristic.
*Taenia solium*
- This is another **tapeworm (cestode)**, acquired by consuming undercooked pork; it can cause taeniasis (intestinal infection) and cysticercosis (tissue infection).
- Similar to *T. saginata*, diagnosis involves finding **proglottids** or **eggs** in stool for intestinal infection, and it does not typically present with lung involvement or larvae in stool.
*Necator americanus*
- This is a **hookworm** that causes iron-deficiency anemia due to blood loss in the intestines.
- While it can cause some pulmonary symptoms as larvae migrate through the lungs, and gastrointestinal symptoms, the diagnostic hallmark is finding **eggs** in the stool, not larvae.
*Ascaris lumbricoides*
- This is the **giant roundworm**; infections are common and often asymptomatic, but heavy worm burdens can cause intestinal obstruction or malnutrition.
- While **pulmonary symptoms (Loeffler's syndrome)** can occur during larval migration, and eosinophilia is present, the diagnosis is confirmed by finding characteristic **mammillated eggs** in the stool, not larvae.
Parasitic life cycles US Medical PG Question 8: A 30-year-old man presents to the physician after he discovered a raised, red, string-shaped lesion beneath the skin on his right foot. The lesion seems to move from one location to another over the dorsum of his foot from day to day. He says that the lesion is extremely itchy and has not responded to over the counter topical treatment. He and his wife recently returned from a honeymoon in southern Thailand, where they frequented the tropical beaches. The physician diagnoses him with a parasitic infection and prescribes albendazole for the patient. With which of the following organisms is the patient most likely infected?
- A. Ancylostoma braziliense (Correct Answer)
- B. Dracunculus medinensis
- C. Necator americanus
- D. Strongyloides stercoralis
- E. Wuchereria bancrofti
Parasitic life cycles Explanation: ***Ancylostoma braziliense***
- This clinical presentation of a **pruritic, migratory, serpiginous rash** on the foot after exposure to contaminated sand (tropical beach in Thailand) is classic for **cutaneous larva migrans**, caused by hookworm larvae, predominantly *Ancylostoma braziliense*.
- The larvae penetrate the skin but cannot complete their life cycle in humans, instead migrating subcutaneously, causing the characteristic **"creeping eruption"**.
*Dracunculus medinensis*
- This parasite causes **dracunculiasis**, where the adult female worm migrates to the skin, creating a painful blister, often on the lower limbs, from which it emerges.
- It is acquired by ingesting **copepods** (water fleas) containing larvae, not by direct contact with contaminated sand, and the lesion typically ulcerates rather than migrating repeatedly.
*Necator americanus*
- This is a human hookworm that causes **iron deficiency anemia** and can lead to **cutaneous larva currens** from larval penetration, which is a rapidly advancing linear lesion, but it typically progresses to systemic infection where the worms reside in the small intestine.
- While it can cause an itchy rash at the site of penetration (ground itch), it does not cause the **chronic, migratory, serpiginous eruption** characteristic of cutaneous larva migrans.
*Strongyloides stercoralis*
- This parasite can cause **larva currens** (a rapidly moving linear skin eruption) and systemic complications, particularly in immunocompromised individuals.
- While it can cause skin lesions, the typical description is of a much faster-moving lesion that usually spreads from the anus and is less serpiginous and persistent in one area compared to the classic presentation of cutaneous larva migrans.
*Wuchereria bancrofti*
- This nematode causes **lymphatic filariasis** (elephantiasis), characterized by lymphedema, hydrocele, and chyluria, and is transmitted by **mosquito bites**.
- It does not cause cutaneous migratory lesions on the foot; its pathology relates to chronic lymphatic obstruction by adult worms.
Parasitic life cycles US Medical PG Question 9: A 31-year-old man comes to the physician because of severe muscle pain and fever for 4 days. He likes to go hunting and consumed bear meat 1 month ago. Examination shows periorbital edema and generalized muscle tenderness. His leukocyte count is 12,000/mm3 with 19% eosinophils. The release of major basic protein in response to this patient’s infection is most likely a result of which of the following?
- A. Increased expression of MHC class II molecules
- B. Interaction between Th1 cells and macrophages
- C. Antibody-dependent cell-mediated cytotoxicity (Correct Answer)
- D. Increased expression of MHC class I molecules
- E. Immune complex-dependent complement activation
Parasitic life cycles Explanation: ***Antibody-dependent cell-mediated cytotoxicity***
- The patient's symptoms (fever, muscle pain, periorbital edema, eosinophilia) after consuming undercooked bear meat are highly suggestive of **trichinellosis**, a parasitic infection.
- **Eosinophils** are crucial in the immune response against parasites. They release cytotoxic substances like **major basic protein** through antibody-dependent cell-mediated cytotoxicity (ADCC), where antibodies (IgE or IgG) bind to the parasite, and eosinophils then bind to the Fc region of these antibodies to mediate parasite killing.
*Increased expression of MHC class II molecules*
- **MHC class II molecules** are primarily expressed on antigen-presenting cells (APCs) and are involved in presenting extracellular antigens to **CD4+ T cells**, initiating a helper T cell response.
- While important for activating the immune system, increased MHC class II expression doesn't directly explain the release of major basic protein by eosinophils in response to parasitic infection.
*Interaction between Th1 cells and macrophages*
- **Th1 cells** primarily activate **macrophages** to kill intracellular pathogens and are involved in cell-mediated immunity.
- This pathway is less prominent in directly killing large extracellular parasites like *Trichinella*, where eosinophil-mediated ADCC is key.
*Increased expression of MHC class I molecules*
- **MHC class I molecules** are expressed on almost all nucleated cells and present endogenous antigens to **CD8+ cytotoxic T cells**, primarily targeting intracellular viral infections and tumor cells.
- This mechanism is not directly involved in the eosinophil effector function against extracellular parasitic infections.
*Immune complex-dependent complement activation*
- **Immune complexes** (antigen-antibody complexes) can activate the complement system, leading to immune complex-mediated tissue damage or pathogen clearance.
- While complement can be involved in parasitic infections, it does not directly lead to the release of **major basic protein** by eosinophils, which is a specific cytotoxic mechanism.
Parasitic life cycles US Medical PG Question 10: A 38-year-old man comes to the physician because of a 2-week history of abdominal pain and an itchy rash on his buttocks. He also has fever, nausea, and diarrhea with mucoid stools. One week ago, the patient returned from Indonesia, where he went for vacation. Physical examination shows erythematous, serpiginous lesions located in the perianal region and the posterior thighs. His leukocyte count is 9,000/mm3 with 25% eosinophils. Further evaluation is most likely to show which of the following findings?
- A. Rhabditiform larvae on stool microscopy (Correct Answer)
- B. Entamoeba histolytica antibodies on stool immunoassay
- C. Branching septate hyphae on KOH preparation
- D. Oocysts on acid-fast stool stain
- E. Giardia lamblia antibodies on stool immunoassay
Parasitic life cycles Explanation: ***Rhabditiform larvae on stool microscopy***
- The patient's symptoms, including **abdominal pain**, **diarrhea with mucoid stools**, **itchy rash (larva currens)**, and **eosinophilia** after traveling to Indonesia, are highly suggestive of **Strongyloidiasis**.
- **Rhabditiform larvae** are typically found in stool samples during the diagnostic phase of strongyloidiasis, as adult worms live in the small intestine and release these larvae.
*Entamoeba histolytica antibodies on stool immunoassay*
- While *Entamoeba histolytica* can cause **dysentery**, **abdominal pain**, and fever, it does not cause an **itchy migratory rash** or significant **eosinophilia**.
- Diagnosis typically involves detecting **trophozoites or cysts** in stool or specific **antigen detection**, not usually antibodies in stool.
*Branching septate hyphae on KOH preparation*
- **Branching, septate hyphae** are characteristic of **fungal infections**, such as dermatophytosis, which primarily affect the skin.
- This finding would not explain the systemic symptoms like **abdominal pain**, **diarrhea**, **fever**, and eosinophilia described in the patient.
*Oocysts on acid-fast stool stain*
- **Oocysts detected by acid-fast stain** are indicative of parasitic infections like **Cryptosporidiosis** or **Cyclosporiasis**, which cause **watery diarrhea** and abdominal cramps.
- These infections do not typically present with the **pruritic serpiginous rash** (larva currens) or the high level of **eosinophilia** seen in this patient.
*Giardia lamblia antibodies on stool immunoassay*
- *Giardia lamblia* causes **giardiasis**, characterized by **diarrhea**, **abdominal cramps**, **bloating**, and **malabsorption**.
- However, giardiasis typically does not cause **eosinophilia** or an **itchy migratory rash**, which are key features in this case.
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