MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 6
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
Coating of bacteria with IgG or C3b so that phagocytes ingest them more easily is called:
Opsonins such as IgG and C3b bind Fc and complement receptors on phagocytes, increasing phagocytosis. Chemotaxis is directed cell migration and diapedesis is movement through vessel walls.
In a sandwich (double-antibody) ELISA for an antigen, the enzyme label is attached to:
The antigen is captured by solid-phase antibody and then detected by an enzyme-labelled antibody to a different epitope. Colour is proportional to antigen amount.
Poorly washed ELISA wells most often cause:
Inadequate washing leaves unbound enzyme conjugate in the well, which reacts with substrate and gives falsely high colour. Washing removes unbound reactants between steps.
In flow cytometry, forward scatter mainly reflects a cell's:
Forward scatter light is proportional to cell size, while side scatter reflects internal complexity or granularity. Fluorescence measures antigen expression.
Chemiluminescent immunoassays generate their signal through:
Chemiluminescent labels (such as acridinium esters or luminol) emit light when oxidised, which is measured by a luminometer. Nephelometry, in contrast, measures light scattered by immune complexes.
Why are siblings the most likely HLA-identical donors?
Each child inherits one parental haplotype from each parent as a block. Two siblings therefore have a 25% chance of sharing both haplotypes.
Chocolate agar supports the growth of Haemophilus and Neisseria because heating the blood:
Gentle heating lyses red cells, releasing hemin and NAD, and inactivates NADases in the blood. This makes chocolate agar enriched, not selective.
A medium that contains an indicator so that organisms can be told apart by a biochemical reaction, without inhibiting their growth, is called:
A differential medium shows different colony reactions (for example lactose fermentation) through an indicator. A selective medium inhibits some organisms, and an enriched medium adds nutrients for fastidious ones.
The expected IMViC pattern (indole, methyl red, Voges-Proskauer, citrate) for Escherichia coli is:
E. coli is indole and methyl red positive, VP and citrate negative. The reverse pattern (– – + +) is typical of Klebsiella and Enterobacter.
After growth on Loeffler serum medium, methylene blue staining of Corynebacterium diphtheriae shows:
Loeffler medium promotes polyphosphate storage granules that stain red-purple (metachromatic) with methylene blue. Terminal spores suggest Clostridium, and safety-pin staining suggests Yersinia or Burkholderia.
A coagulase-negative staphylococcus from the urine of a sexually active 22-year-old woman is resistant to a 5 µg novobiocin disk. It is most likely:
S. saprophyticus is the novobiocin-resistant CoNS that causes UTI in young women. S. epidermidis and most other clinically important CoNS are novobiocin susceptible.
Lancefield grouping of beta-hemolytic streptococci is based on:
Lancefield groups (A, B, C, F, G) are defined by the cell wall carbohydrate (C substance). M protein is used for typing within group A, not for grouping.
For diagnosis of pulmonary tuberculosis by smear and culture, the usual recommendation is to collect:
Three good-quality sputa, 8–24 hours apart and including an early-morning sample, increase detection. Pooled 24-hour collections are rejected because of overgrowth and dilution, and saliva is not sputum.
A gardener develops nodules that follow the lymphatic vessels up the arm after a thorn injury. Culture at 25 °C shows thin hyphae with conidia arranged like flower petals. The organism is:
Lymphocutaneous spread after a plant injury with a mold showing rosette ('flowerette') conidia is typical of Sporothrix. It converts to cigar-shaped yeasts at 37 °C.
Hair infected with Microsporum canis viewed under a Wood's lamp typically shows:
Most Microsporum species causing ectothrix infection fluoresce yellow-green under Wood's light. Trichophyton tonsurans (endothrix) does not fluoresce; coral-red fluorescence suggests erythrasma.
A diabetic patient in ketoacidosis has necrotic tissue in the nasal cavity. Histology shows broad, ribbon-like, rarely septate hyphae with right-angle branching. The likely order is:
Rhino-orbital-cerebral mucormycosis in ketoacidosis shows wide, pauciseptate, ribbon-like hyphae branching at wide angles. Aspergillus hyphae are narrow, regularly septate and branch at acute angles.
Pneumocystis jirovecii in bronchoalveolar lavage is best demonstrated by:
Pneumocystis cannot be cultured routinely; cysts are shown with immunofluorescence, methenamine silver or calcofluor, and PCR is also used. Culture on SDA gives no growth.
Which incubation conditions are recommended for isolating Campylobacter jejuni from stool?
C. jejuni is thermophilic and microaerophilic; about 5% O2, 10% CO2 at 42 °C on selective media favours it and suppresses normal stool flora. It does not grow in ambient air or strict anaerobic conditions.
A young man develops rapidly fatal meningoencephalitis a few days after swimming in a warm freshwater lake. Motile amoebae are seen in a wet mount of CSF. The organism is:
Primary amoebic meningoencephalitis from Naegleria follows freshwater exposure through the nose. CSF should be examined warm and not refrigerated. Acanthamoeba causes slower granulomatous encephalitis or keratitis.
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.