MLT GlobeParasitology: Intestinal protozoa – page 3
69 Parasitology MCQs on Intestinal protozoa with answers and explanations.
An iodine wet mount of formed stool shows oval cysts 10–12 µm with four nuclei, longitudinal fibres (axonemes) and curved median bodies. The organism is:
Four nuclei with axonemes and median bodies in an oval cyst are typical of Giardia. E. histolytica cysts are round with up to four nuclei but lack axonemes and median bodies.
Humans become infected with Entamoeba histolytica by swallowing which stage?
The mature quadrinucleate cyst survives stomach acid and excysts in the intestine; trophozoites are destroyed outside the body and by gastric acid.
Which drug is the usual treatment for Trichomonas vaginalis infection?
Metronidazole (or tinidazole) is the standard treatment, and sexual partners are treated at the same time.
Giardia duodenalis trophozoites attach to the small-intestinal mucosa by means of a:
The concave ventral disc on the underside of the trophozoite acts like a suction cup on the duodenal epithelium.
A waterborne diarrhoea outbreak continues despite routine chlorination. Stool shows 4–6 µm oocysts staining red by modified ZN. The cause is:
Cryptosporidium oocysts resist chlorine and are modified acid-fast. Giardia cysts are larger and not acid-fast.
An iodine-stained stool mount shows a cyst with four nuclei and chromatoid bars that have smooth, rounded ends. Which organism is most likely?
Mature E. histolytica/dispar cysts have up to four nuclei and cigar-shaped chromatoid bodies with blunt ends. E. coli cysts have up to eight nuclei and splintered chromatoid bars; E. nana and Iodamoeba lack chromatoid bars.
Which protozoal infection is typically self-limiting in immunocompetent people but may become chronic and life-threatening in AIDS?
Cryptosporidium diarrhoea resolves within about two weeks in healthy hosts but can be severe and persistent with low CD4 counts. Untreated kala-azar, PAM and severe falciparum malaria are often fatal regardless.
Which intestinal amoeba is a non-pathogenic commensal that resembles a small E. histolytica?
E. hartmanni looks like E. histolytica but is smaller (cysts under 10 µm) and non-pathogenic, so micrometry is used to separate them.
Which of the following is a flagellate that lives in the human intestine?
Chilomastix mesnili is an intestinal commensal flagellate with a lemon-shaped cyst. T. vaginalis is urogenital, Endolimax is an amoeba, and Echinococcus is a tapeworm.
Which statement about Giardia duodenalis is FALSE?
Giardia causes malabsorption and diarrhoea, not haemolysis. The string test (Entero-Test) samples duodenal fluid for trophozoites.
Which intestinal protozoan exists only as a trophozoite and does not form cysts?
Pentatrichomonas (Trichomonas) hominis, like other trichomonads, has no cyst stage. Balantidium, Giardia and E. histolytica are transmitted as cysts.
Aspiration of a right-lobe liver abscess yields odourless reddish-brown 'anchovy sauce' pus. Which parasite is most likely?
Amoebic liver abscess contents look like anchovy sauce; trophozoites are found mainly in the abscess wall, so serology or antigen tests help. Fasciola and Schistosoma cause other hepatic lesions.
After a Giardia cyst is swallowed, excystation is triggered by gastric acid and completed so that trophozoites emerge in the:
Excystation starts in the acid stomach and finishes in the duodenum, where each cyst yields two trophozoites that attach to the mucosa. Encystation later occurs lower down.
Small acid-fast oocysts (4 to 6 micrometers) in a modified Ziehl-Neelsen stool smear of a patient with watery diarrhea indicate:
Cryptosporidium oocysts stain pink-red with modified acid-fast stain. Toxoplasma oocysts are shed by cats, not found in human stool; human toxoplasmosis is diagnosed by serology.
After exposure to Trichomonas vaginalis, symptoms usually appear within about:
The incubation period is about 4 days to 4 weeks, and many infections, especially in men, stay asymptomatic.
The mature cyst of Entamoeba histolytica usually measures about:
E. histolytica cysts are 10 to 20 micrometers with up to four nuclei. Cysts under 10 micrometers suggest E. hartmanni, so an ocular micrometer is needed.
Which feature of an amoebic trophozoite most reliably indicates Entamoeba histolytica rather than E. dispar on microscopy?
E. histolytica and E. dispar look identical, but only the invasive E. histolytica ingests red cells. Otherwise, antigen or PCR tests are needed to tell them apart.
Round oocysts 8–10 µm that stain variably with modified acid-fast stain and show blue autofluorescence under UV light are:
Cyclospora oocysts are about twice the size of Cryptosporidium, stain unevenly acid-fast, and autofluoresce. Cryptosporidium does not autofluoresce.
A large elongated (about 25 × 12 µm) acid-fast oocyst containing one or two sporoblasts is seen in stool. The organism is:
Cystoisospora belli produces large ellipsoidal oocysts with one or two sporoblasts. It is the only intestinal coccidian commonly associated with eosinophilia.
An Entamoeba cyst has 8 nuclei with eccentric karyosomes and splinter-like chromatoid bodies. It is:
Mature E. coli cysts have up to 8 nuclei and splinter-like chromatoid bodies. E. histolytica/dispar cysts have up to 4 nuclei and smooth, rounded chromatoid bars.