MLT GlobeQCHP exam preparation (Qatar) – page 12
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
Which Shigella produces Shiga toxin and causes the most severe dysentery, sometimes followed by hemolytic uremic syndrome?
S. dysenteriae type 1 carries the stx gene for Shiga toxin, which damages the colon and kidney endothelium. S. sonnei causes milder illness and is the main species in many high-income countries.
Chronic carriers of Salmonella Typhi most often harbour the organism in the:
Chronic typhoid carriage occurs mainly in the gallbladder, especially when gallstones are present, and organisms are shed in the stool for long periods.
After incubation in MR-VP broth, which reagents are added to perform the Voges-Proskauer test?
The VP test detects acetoin; alpha-naphthol and 40% KOH (Barritt's reagents) are added, and a red colour is positive. Methyl red is used for the separate MR test of mixed acid fermentation.
Which Enterobacterales genus is classically linked to osteomyelitis in patients with sickle cell disease?
Patients with sickle cell disease have bone infarcts and reduced splenic function, which make them prone to Salmonella osteomyelitis.
In the tube indole test, a red ring after adding Kovac's reagent shows that the organism broke down:
Tryptophanase splits tryptophan into indole, pyruvate and ammonia; indole reacts with p-dimethylaminobenzaldehyde in Kovac's reagent to form a red colour.
In a patient without prosthetic material, one of two blood culture sets grows a coagulase-negative staphylococcus in the aerobic bottle only after 3 days. The most likely interpretation is:
Coagulase-negative staphylococci in a single set, with long time to positivity and no risk devices, usually reflect skin contamination during collection. True bacteremia typically gives multiple positive sets.
Which viridans group streptococcus is most associated with dental caries because it makes sticky glucans (dextran) from sucrose?
S. mutans converts sucrose into adhesive glucans that form dental plaque and produces acid that damages enamel. S. anginosus is linked mainly to abscesses.
On a Gram stain of sputum, Mycobacterium tuberculosis typically appears as:
The lipid-rich wall resists crystal violet penetration, so mycobacteria stain weakly gram-positive, beaded, or not at all. An acid-fast stain is needed to show them.
Sabouraud dextrose agar favours the growth of fungi over most bacteria mainly because of its:
The low pH and high sugar content suppress many bacteria while fungi grow well. Cycloheximide is added only in modified formulas and inhibits saprophytic moulds, not bacteria.
On a Gram stain of stool from a patient with Campylobacter enteritis, the organisms typically appear as:
Campylobacter are thin, curved, S-shaped or seagull-wing rods. They stain faintly with safranin, so carbol fuchsin or basic fuchsin counterstain improves detection.
Several people develop itchy red papules and pustules on the trunk 1–2 days after using a poorly chlorinated hot tub. The most likely organism is:
Hot tub folliculitis is caused by P. aeruginosa, which survives in warm water with low chlorine and infects hair follicles.
In late summer, a 6-year-old has fever, headache and neck stiffness. CSF shows 120 WBC/µL, mostly lymphocytes, and normal glucose. Which test is most likely to identify the cause?
Enteroviruses cause most summer-autumn aseptic meningitis in children, and CSF RT-PCR is rapid and sensitive. India ink is for Cryptococcus.
Many passengers on a cruise ship develop sudden vomiting and watery diarrhoea lasting 1–2 days. Which laboratory test is preferred to confirm the likely viral cause?
Norovirus causes most outbreaks of acute gastroenteritis in adults and cannot be grown in routine cell culture; RT-PCR on stool is the test of choice. Rotavirus mainly affects young children.
A person who has followed a strict vegan diet for many years is at greatest risk of deficiency of which vitamin?
Vitamin B12 comes almost only from animal foods, so long-term strict vegans can become deficient. Folate, vitamin C and vitamin K are plentiful in plant foods.
On a Wright-stained smear, spherocytes are recognized as red cells that are:
Spherocytes are round, stain densely and have no central pallor because they have lost their biconcave shape.
A red cell with a central dark spot of hemoglobin, surrounded by a pale ring and an outer rim of hemoglobin, is a:
This bull's-eye appearance defines the target cell (codocyte), which is seen in liver disease, hemoglobinopathies and thalassemia.
On a blood smear, a normal red cell is roughly the same size as:
A normal red cell (about 7–8 µm) is similar in size to the nucleus of a small lymphocyte, a handy check when judging micro- or macrocytosis.
Red cells are described as microcytic when the adult MCV is below about:
The adult MCV reference range is roughly 80–100 fL. Below 80 fL is microcytic and above 100 fL is macrocytic.
The main function of neutrophils is:
Neutrophils are the first-line phagocytes against bacteria. Antibodies are made by plasma cells, and histamine comes from basophils and mast cells.
Helper T lymphocytes carry which surface marker?
Helper T cells are CD4-positive. Cytotoxic T cells are CD8-positive, and CD19 and CD20 are B-cell markers.