MLT GlobeParasitology: Intestinal protozoa
69 Parasitology MCQs on Intestinal protozoa with answers and explanations.
A wet mount of a vaginal swab in saline is examined for jerky, flagellated motile organisms. This is the usual first test for:
T. vaginalis is detected by its motility in a fresh vaginal wet mount (or NAAT). Gonorrhoea needs culture or NAAT from endocervical/urethral samples.
Protozoa that move by extending temporary cytoplasmic projections (pseudopodia) are traditionally grouped as:
Amoebae (Sarcodina/Rhizopoda, now Amoebozoa) move with pseudopodia. Flagellates use flagella, ciliates use cilia, and apicomplexans lack obvious locomotor organelles in most stages.
Entamoeba coli lives in the human colon feeding on bacteria and debris, gaining food while causing the host no harm or benefit. This relationship is called:
In commensalism one partner benefits while the other is unaffected. In mutualism both benefit, and in parasitism the host is harmed.
Motile pear-shaped flagellates with jerky movement are seen in a urine sediment from a woman with vaginal discharge. The most likely organism is:
T. vaginalis inhabits the urogenital tract and commonly contaminates female urine. T. tenax lives in the mouth, P. hominis in the intestine, and Giardia in the small bowel.
Which protozoan exists only as a trophozoite, having no cyst stage in its life cycle?
T. vaginalis has no cyst; the trophozoite is transmitted directly by sexual contact. Balantidium, Endolimax and Giardia all form cysts that serve as infective stages.
In a fresh stool from a patient with bloody diarrhoea, an amoeboid trophozoite containing ingested red blood cells is seen. This finding points to:
Erythrophagocytosis is the most reliable microscopic marker of invasive E. histolytica (it distinguishes it from the look-alike E. dispar). The other organisms do not ingest red cells.
Which protozoan infection is classified as a sexually transmitted infection?
T. vaginalis is spread mainly by sexual intercourse and is the commonest non-viral STI. The others are faeco-oral infections, though amoebiasis and giardiasis can occasionally follow oral-anal sex.
A patient with advanced HIV has severe chronic watery diarrhoea; a modified acid-fast stain shows 4–6 µm red oocysts. Which parasite is most likely?
Cryptosporidium oocysts are small and acid-fast and cause life-threatening chronic diarrhoea in AIDS. The other parasites do not stain acid-fast.
Giardia duodenalis trophozoites show a 'falling leaf' motility in a saline mount. Which organelles drive this movement?
Giardia trophozoites have eight flagella in four pairs, giving characteristic tumbling motility. Cilia belong to Balantidium, pseudopodia to amoebae.
Which trichomonad is a non-pathogenic commensal of the human mouth?
T. tenax lives in the oral cavity and is considered non-pathogenic. T. vaginalis, Giardia and E. histolytica all cause disease.
Of the following flagellates found in humans, which is a recognised pathogen?
T. vaginalis causes vaginitis and urethritis. Chilomastix, Pentatrichomonas and T. tenax are commensals.
A pig farmer develops dysentery caused by a large ciliated protozoan. Which animal is the main reservoir of this infection?
Balantidium coli is carried commonly by pigs, and human infection follows contact with pig faeces. Cats are linked to Toxoplasma and dogs to Echinococcus.
Routine stool microscopy diagnoses Giardia duodenalis by finding:
Giardia is identified by its cysts (oval, 4 nuclei) or binucleate trophozoites; antigen tests and PCR are also used. It has no larval, egg or oocyst stage.
Which protozoan is the largest one known to infect humans?
Balantidium coli trophozoites may reach 50–200 µm and are the only ciliate pathogen of humans. The amoebae and Giardia are much smaller.
A traveller has bulky, pale, foul-smelling fatty stools (steatorrhoea) and bloating after drinking stream water. Which protozoan is most likely?
Giardia coats the duodenal and jejunal mucosa, causing malabsorption and steatorrhoea. E. histolytica causes bloody dysentery; the trichomonads do not cause malabsorption.
Entamoeba histolytica trophozoites move with directional progressive motility using:
Amoebae move by extending finger-like hyaline pseudopodia. Cilia are used by Balantidium and flagella by Giardia and trichomonads.
The resistant stage of a protozoan, enclosed in a thick protective wall that survives outside the host, is the:
Cysts have a tough wall that protects them in the environment and are usually the infective stage. Trophozoites are fragile feeding forms.
Which species of the genus Entamoeba causes invasive disease (dysentery, liver abscess) in humans?
E. histolytica is the pathogenic species. E. coli, E. hartmanni and the morphologically identical E. dispar are non-invasive commensals.
How many nuclei does the cyst of Entamoeba histolytica contain, from immature to mature?
E. histolytica cysts have 1–4 nuclei, the mature infective cyst being quadrinucleate. Eight nuclei suggest E. coli.
Which parasite has a direct life cycle, needing only a single (human) host?
E. histolytica passes directly from human to human via cysts. Trypanosoma, Leishmania and Plasmodium need an insect vector as a second host.