MLT GlobeParasitology: Lab diagnosis & methods – page 2
37 Parasitology MCQs on Lab diagnosis & methods with answers and explanations.
Why must a fresh liquid stool for trophozoite detection be examined, or preserved, within about 30 minutes of passage?
Trophozoites in liquid stool are fragile and lose motility and morphology quickly. Formed stools contain mainly cysts and can wait longer.
In the formalin-ethyl acetate sedimentation method, the parasites are recovered from the:
Ethyl acetate extracts fats and debris into a top plug, while parasites sediment by centrifugation. The sediment is examined as wet mounts.
Which stain is most widely used for permanent smears to identify intestinal protozoa?
Trichrome (or iron haematoxylin) stained permanent smears show nuclear detail needed to identify protozoa. Iodine is used for temporary wet mounts only.
A calibrated ocular micrometer is essential in parasitology mainly because:
Many protozoa and eggs look alike but differ in size, e.g. E. histolytica versus E. hartmanni. The micrometer must be calibrated for each objective of each microscope.
Why is a saline wet mount examined along with an iodine mount of fresh stool?
Iodine kills trophozoites and stops movement; saline keeps them alive so their motility can be seen.
A thick blood film for malaria is not fixed with methanol before Giemsa staining because:
Unfixed red cells lyse (dehaemoglobinise) in the aqueous stain, leaving parasites and white cells visible.
Stool for ova and parasite examination should be collected before the patient receives:
Barium, antacids and some drugs obscure or distort parasites for 1–2 weeks, so specimens should be taken first.
Why is Lugol's iodine added to a stool wet mount when looking for Entamoeba cysts?
Iodine stains glycogen and nuclear chromatin, so the number and structure of nuclei can be counted for species identification. Chromatoid bodies are seen better in saline mounts.
Standard flotation concentration of stool is least suitable for detecting:
Operculated fluke eggs and schistosome eggs are heavy or collapse in high-density solutions, so sedimentation is preferred. Protozoan cysts and hookworm eggs float well.
Protozoa that ingest solid food particles by phagocytosis into food vacuoles show which type of nutrition?
Holozoic nutrition means taking in particulate food, as amoebae do with bacteria and red cells. Saprozoic nutrition means absorbing dissolved nutrients across the surface.
On a thick film, 400 asexual parasites are counted against 200 WBCs. The patient's WBC count is 8 × 10^9/L (8000/µL). The parasite density is:
Parasites/µL = (parasites counted ÷ WBCs counted) × WBC/µL = (400 ÷ 200) × 8000 = 16,000. When the WBC count is unknown, 8000/µL is often assumed.
A returning traveller has fever, but the first blood films are negative for malaria. The best next step is:
One negative set does not rule out malaria; films should be repeated (with rapid antigen testing) until three sets are negative. Serology does not diagnose acute malaria.
In a two-vial stool collection system, the vial containing 10% formalin is mainly used for:
Formalin preserves eggs, larvae and cysts well for concentration methods. The second vial (PVA or a zinc/SAF substitute) is used to prepare permanent stained smears.
Which method is most sensitive for detecting Strongyloides larvae in stool?
Larvae crawl across nutrient agar carrying bacteria and leave visible tracks, making agar plate culture more sensitive than direct examination.
For zinc sulphate flotation concentration of stool, the zinc sulphate solution is adjusted to a specific gravity of about:
At about 1.18 (1.20 for formalin-fixed stool), cysts and light eggs float while most debris sinks.
The Kato-Katz thick smear technique is mainly used to:
Kato-Katz uses a template to give a fixed amount of stool, allowing eggs per gram to be calculated for intensity surveys.
A malaria rapid test based on HRP2 is negative, but films show P. falciparum. A known cause of this false-negative result is:
Some P. falciparum strains lack the hrp2/hrp3 genes and produce no HRP2 antigen, so HRP2-based RDTs miss them. Very high parasitemia (prozone) can also cause false negatives.