Medical Parasitology 2026/2027 | Medical Parasitology Study Guide, Parasitology
Exam Prep, Practice Questions with Answers & Explanations, Protozoa, Helminths,
Nematodes, Cestodes, Trematodes, Arthropods, Parasite Life Cycles, Host-Parasite
Relationships, Diagnosis, Treatment, Prevention & Epidemiology
Question 1: Which of the following protozoan parasites is the causative agent of
primary amoebic meningoencephalitis (PAM), a rapidly fatal infection acquired
by swimming in warm freshwater?
A. Entamoeba histolytica
B. Acanthamoeba species
C. Naegleria fowleri
D. Balamuthia mandrillaris
CORRECT ANSWER: C. Naegleria fowleri
Rationale: Naegleria fowleri is a free-living amoeboflagellate that enters the
central nervous system through the olfactory neuroepithelium, typically after
exposure to warm freshwater. It causes primary amoebic meningoencephalitis
(PAM), which is almost universally fatal. Entamoeba histolytica causes intestinal
and hepatic amoebiasis, while Acanthamoeba and Balamuthia cause
granulomatous amoebic encephalitis (GAE), a more chronic and subacute
condition.
Question 2: A 34-year-old male presents with intermittent bloody diarrhea,
right upper quadrant tenderness, and a history of recent travel to Mexico. Stool
microscopy reveals trophozoites with ingested red blood cells. Which of the
following is the most likely causative organism?
A. Giardia lamblia
B. Entamoeba coli
C. Entamoeba histolytica
D. Balantidium coli
CORRECT ANSWER: C. Entamoeba histolytica
Rationale: The presence of trophozoites with ingested red blood cells in stool is a
hallmark of Entamoeba histolytica, which causes invasive amoebiasis. The
patient's presentation of bloody diarrhea and right upper quadrant tenderness is
consistent with amoebic colitis and possible liver abscess. Giardia lamblia causes
non-bloody diarrhea and does not ingest erythrocytes. Entamoeba coli is a non-
,pathogenic commensal, and Balantidium coli is a ciliate that typically causes
dysentery in individuals with pig contact.
Question 3: Which of the following is the infective stage of Entamoeba
histolytica for humans?
A. Trophozoite
B. Cyst
C. Sporozoite
D. Promastigote
CORRECT ANSWER: B. Cyst
Rationale: The infective stage of Entamoeba histolytica is the mature
quadrinucleate cyst, which is ingested through contaminated food or water. The
cyst excysts in the small intestine to release trophozoites, which colonize the large
intestine. Trophozoites are the invasive and motile form but are not the infective
stage. Sporozoites are the infective stage of Plasmodium species, and
promastigotes are the infective stage of Leishmania species.
Question 4: A 28-year-old female presents with foul-smelling, greasy stools,
bloating, and flatulence after returning from a camping trip in the Rocky
Mountains. She reports drinking untreated stream water. Which of the
following is the most likely causative agent?
A. Cryptosporidium parvum
B. Giardia lamblia
C. Cyclospora cayetanensis
D. Entamoeba histolytica
CORRECT ANSWER: B. Giardia lamblia
Rationale: Giardia lamblia is the most common protozoan cause of waterborne
diarrhea in the United States and is classically associated with drinking untreated
surface water in mountainous regions. The hallmark symptoms include foul-
smelling, greasy stools, bloating, and flatulence due to malabsorption.
Cryptosporidium and Cyclospora also cause waterborne diarrhea but are less
associated with greasy stools and typically cause more profuse watery diarrhea.
,Question 5: Which of the following statements regarding the life cycle of
Toxoplasma gondii is correct?
A. Humans are the definitive host, and cats are intermediate hosts.
B. The definitive host is the cat, and humans serve as intermediate hosts.
C. The parasite is transmitted to humans exclusively through tick bites.
D. Oocysts are produced in human skeletal muscle tissue.
CORRECT ANSWER: B. The definitive host is the cat, and humans serve as
intermediate hosts.
Rationale: Toxoplasma gondii is an obligate intracellular protozoan for which the
definitive host is the domestic cat and other felids, where sexual reproduction
occurs, leading to oocyst shedding in feces. Humans and other warm-blooded
animals serve as intermediate hosts, in which asexual reproduction occurs,
leading to tissue cyst formation in muscle and brain. Transmission to humans
occurs through ingestion of oocysts from cat feces, consumption of undercooked
meat containing tissue cysts, or transplacentally.
Question 6: A 45-year-old male with AIDS (CD4 count 45 cells/µL) presents with
a severe, progressive headache, confusion, and focal neurological deficits. MRI
reveals multiple ring-enhancing lesions. Which of the following is the most likely
causative agent?
A. Plasmodium falciparum
B. Trypanosoma cruzi
C. Toxoplasma gondii
D. Naegleria fowleri
CORRECT ANSWER: C. Toxoplasma gondii
Rationale: In patients with advanced HIV/AIDS (CD4 < 100 cells/µL), reactivation
of latent Toxoplasma gondii infection is a leading cause of toxoplasmic
encephalitis, presenting with ring-enhancing lesions on neuroimaging.
Plasmodium falciparum causes cerebral malaria but is not associated with ring-
enhancing lesions. Trypanosoma cruzi causes Chagas disease, and Naegleria
fowleri causes primary amoebic meningoencephalitis, which presents acutely
rather than with multiple ring-enhancing lesions.
, Question 7: Which of the following is the primary mode of transmission of
Cryptosporidium parvum?
A. Airborne droplets
B. Fecal-oral route, often through contaminated water
C. Sexual contact
D. Vector-borne transmission by mosquitoes
CORRECT ANSWER: B. Fecal-oral route, often through contaminated water
Rationale: Cryptosporidium parvum is transmitted via the fecal-oral route, most
commonly through ingestion of contaminated water (drinking water and
recreational water). The oocysts are highly resistant to chlorination, leading to
waterborne outbreaks. It is not transmitted by airborne droplets, sexual contact,
or mosquito vectors.
Question 8: A 30-year-old male presents with a painless, indurated skin lesion
(chancriform lesion) on his leg after returning from a trip to the Middle East.
Weeks later, he develops hepatosplenomegaly and pancytopenia. Which of the
following is the most likely causative organism?
A. Leishmania donovani
B. Trypanosoma brucei
C. Plasmodium vivax
D. Leishmania tropica
CORRECT ANSWER: A. Leishmania donovani
Rationale: Leishmania donovani causes visceral leishmaniasis (kala-azar),
characterized by fever, hepatosplenomegaly, pancytopenia, and
hypergammaglobulinemia. The initial skin lesion at the site of the sandfly bite may
be a painless papule or nodule. Leishmania tropica causes cutaneous
leishmaniasis (oriental sore), which remains localized to the skin. Trypanosoma
brucei causes African sleeping sickness, and Plasmodium vivax causes malaria.
Question 9: Which of the following Plasmodium species is known to cause the
most severe form of malaria and is associated with cerebral malaria, severe
anemia, and acute respiratory distress syndrome?
Exam Prep, Practice Questions with Answers & Explanations, Protozoa, Helminths,
Nematodes, Cestodes, Trematodes, Arthropods, Parasite Life Cycles, Host-Parasite
Relationships, Diagnosis, Treatment, Prevention & Epidemiology
Question 1: Which of the following protozoan parasites is the causative agent of
primary amoebic meningoencephalitis (PAM), a rapidly fatal infection acquired
by swimming in warm freshwater?
A. Entamoeba histolytica
B. Acanthamoeba species
C. Naegleria fowleri
D. Balamuthia mandrillaris
CORRECT ANSWER: C. Naegleria fowleri
Rationale: Naegleria fowleri is a free-living amoeboflagellate that enters the
central nervous system through the olfactory neuroepithelium, typically after
exposure to warm freshwater. It causes primary amoebic meningoencephalitis
(PAM), which is almost universally fatal. Entamoeba histolytica causes intestinal
and hepatic amoebiasis, while Acanthamoeba and Balamuthia cause
granulomatous amoebic encephalitis (GAE), a more chronic and subacute
condition.
Question 2: A 34-year-old male presents with intermittent bloody diarrhea,
right upper quadrant tenderness, and a history of recent travel to Mexico. Stool
microscopy reveals trophozoites with ingested red blood cells. Which of the
following is the most likely causative organism?
A. Giardia lamblia
B. Entamoeba coli
C. Entamoeba histolytica
D. Balantidium coli
CORRECT ANSWER: C. Entamoeba histolytica
Rationale: The presence of trophozoites with ingested red blood cells in stool is a
hallmark of Entamoeba histolytica, which causes invasive amoebiasis. The
patient's presentation of bloody diarrhea and right upper quadrant tenderness is
consistent with amoebic colitis and possible liver abscess. Giardia lamblia causes
non-bloody diarrhea and does not ingest erythrocytes. Entamoeba coli is a non-
,pathogenic commensal, and Balantidium coli is a ciliate that typically causes
dysentery in individuals with pig contact.
Question 3: Which of the following is the infective stage of Entamoeba
histolytica for humans?
A. Trophozoite
B. Cyst
C. Sporozoite
D. Promastigote
CORRECT ANSWER: B. Cyst
Rationale: The infective stage of Entamoeba histolytica is the mature
quadrinucleate cyst, which is ingested through contaminated food or water. The
cyst excysts in the small intestine to release trophozoites, which colonize the large
intestine. Trophozoites are the invasive and motile form but are not the infective
stage. Sporozoites are the infective stage of Plasmodium species, and
promastigotes are the infective stage of Leishmania species.
Question 4: A 28-year-old female presents with foul-smelling, greasy stools,
bloating, and flatulence after returning from a camping trip in the Rocky
Mountains. She reports drinking untreated stream water. Which of the
following is the most likely causative agent?
A. Cryptosporidium parvum
B. Giardia lamblia
C. Cyclospora cayetanensis
D. Entamoeba histolytica
CORRECT ANSWER: B. Giardia lamblia
Rationale: Giardia lamblia is the most common protozoan cause of waterborne
diarrhea in the United States and is classically associated with drinking untreated
surface water in mountainous regions. The hallmark symptoms include foul-
smelling, greasy stools, bloating, and flatulence due to malabsorption.
Cryptosporidium and Cyclospora also cause waterborne diarrhea but are less
associated with greasy stools and typically cause more profuse watery diarrhea.
,Question 5: Which of the following statements regarding the life cycle of
Toxoplasma gondii is correct?
A. Humans are the definitive host, and cats are intermediate hosts.
B. The definitive host is the cat, and humans serve as intermediate hosts.
C. The parasite is transmitted to humans exclusively through tick bites.
D. Oocysts are produced in human skeletal muscle tissue.
CORRECT ANSWER: B. The definitive host is the cat, and humans serve as
intermediate hosts.
Rationale: Toxoplasma gondii is an obligate intracellular protozoan for which the
definitive host is the domestic cat and other felids, where sexual reproduction
occurs, leading to oocyst shedding in feces. Humans and other warm-blooded
animals serve as intermediate hosts, in which asexual reproduction occurs,
leading to tissue cyst formation in muscle and brain. Transmission to humans
occurs through ingestion of oocysts from cat feces, consumption of undercooked
meat containing tissue cysts, or transplacentally.
Question 6: A 45-year-old male with AIDS (CD4 count 45 cells/µL) presents with
a severe, progressive headache, confusion, and focal neurological deficits. MRI
reveals multiple ring-enhancing lesions. Which of the following is the most likely
causative agent?
A. Plasmodium falciparum
B. Trypanosoma cruzi
C. Toxoplasma gondii
D. Naegleria fowleri
CORRECT ANSWER: C. Toxoplasma gondii
Rationale: In patients with advanced HIV/AIDS (CD4 < 100 cells/µL), reactivation
of latent Toxoplasma gondii infection is a leading cause of toxoplasmic
encephalitis, presenting with ring-enhancing lesions on neuroimaging.
Plasmodium falciparum causes cerebral malaria but is not associated with ring-
enhancing lesions. Trypanosoma cruzi causes Chagas disease, and Naegleria
fowleri causes primary amoebic meningoencephalitis, which presents acutely
rather than with multiple ring-enhancing lesions.
, Question 7: Which of the following is the primary mode of transmission of
Cryptosporidium parvum?
A. Airborne droplets
B. Fecal-oral route, often through contaminated water
C. Sexual contact
D. Vector-borne transmission by mosquitoes
CORRECT ANSWER: B. Fecal-oral route, often through contaminated water
Rationale: Cryptosporidium parvum is transmitted via the fecal-oral route, most
commonly through ingestion of contaminated water (drinking water and
recreational water). The oocysts are highly resistant to chlorination, leading to
waterborne outbreaks. It is not transmitted by airborne droplets, sexual contact,
or mosquito vectors.
Question 8: A 30-year-old male presents with a painless, indurated skin lesion
(chancriform lesion) on his leg after returning from a trip to the Middle East.
Weeks later, he develops hepatosplenomegaly and pancytopenia. Which of the
following is the most likely causative organism?
A. Leishmania donovani
B. Trypanosoma brucei
C. Plasmodium vivax
D. Leishmania tropica
CORRECT ANSWER: A. Leishmania donovani
Rationale: Leishmania donovani causes visceral leishmaniasis (kala-azar),
characterized by fever, hepatosplenomegaly, pancytopenia, and
hypergammaglobulinemia. The initial skin lesion at the site of the sandfly bite may
be a painless papule or nodule. Leishmania tropica causes cutaneous
leishmaniasis (oriental sore), which remains localized to the skin. Trypanosoma
brucei causes African sleeping sickness, and Plasmodium vivax causes malaria.
Question 9: Which of the following Plasmodium species is known to cause the
most severe form of malaria and is associated with cerebral malaria, severe
anemia, and acute respiratory distress syndrome?
