MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 12
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
Before collecting a blood culture, the venipuncture site is scrubbed with chlorhexidine in alcohol. What is the most important next step?
The antiseptic needs contact time and must dry (about 30 seconds for chlorhexidine) to reduce skin flora. Re-palpating the cleaned site with an ungloved finger recontaminates it.
Which medium does CLSI recommend for disk diffusion testing of Haemophilus influenzae?
HTM is Mueller-Hinton base with haematin, NAD and yeast extract, and it is transparent for reading zones. Chocolate agar is not standardised for AST.
Which virulence factor allows uropathogenic E. coli to bind uroepithelial cells and ascend to cause pyelonephritis?
P fimbriae (pyelonephritis-associated pili) bind to receptors on kidney and ureter epithelium. The K1 capsule is linked to neonatal meningitis, not upper urinary tract adhesion.
Most cases of E. coli neonatal meningitis are caused by strains that carry which antigen?
The K1 polysialic acid capsule resists phagocytosis and complement and is found in most E. coli strains that cause neonatal meningitis. O157:H7 is linked to hemorrhagic colitis.
In the rapid spot indole test using p-dimethylaminocinnamaldehyde (DMACA) reagent, a positive result is shown by:
DMACA reacts with indole to give a blue to blue-green colour within seconds. A red ring at the top of a broth is the positive reaction with Kovac's reagent in the tube test.
Shigella flexneri belongs to which Shigella serogroup?
Shigella serogroups are A (S. dysenteriae), B (S. flexneri), C (S. boydii) and D (S. sonnei).
A wet mount of vaginal discharge (pH 5.0, fishy odour with KOH) shows squamous cells so heavily covered with small coccobacilli that their borders are hidden. The main organism involved is:
These are clue cells, typical of bacterial vaginosis, in which Gardnerella vaginalis and anaerobes replace lactobacilli. Lactobacilli keep vaginal pH low and do not form clue cells.
In the Schaeffer-Fulton spore stain, endospores appear:
Malachite green is driven into spores with heat, and safranin counterstains the vegetative cells. Spores appear green inside pink-red cells. In a Gram stain, spores appear only as unstained areas.
Necrotizing pneumonia in a previously healthy young adult after influenza, caused by community-associated MRSA, is most strongly linked to which toxin?
Panton-Valentine leukocidin, a pore-forming toxin that destroys leukocytes, is carried by many community-associated MRSA strains and is linked to necrotizing pneumonia and skin abscesses.
The sandpaper-like rash and strawberry tongue of scarlet fever are caused by:
Scarlet fever is caused by streptococcal pyrogenic (erythrogenic) exotoxins, such as SpeA and SpeC, which are encoded by bacteriophages. Streptolysin O is a hemolysin, not the rash toxin.
For nucleic acid amplification testing (NAAT) for gonorrhoea and chlamydia in men, the preferred non-invasive specimen is:
The first 20–30 mL of voided urine contains organisms washed from the urethra and is the recommended NAAT specimen in men. Midstream urine dilutes urethral material.
A CSF specimen for suspected meningococcal meningitis arrives in the laboratory at night. How should it be held until processing?
N. meningitidis and H. influenzae are sensitive to cold, so CSF for bacterial culture must never be refrigerated. It should be kept at room temperature or incubated at 35 °C.
Plates for Neisseria gonorrhoeae culture should be incubated at:
N. gonorrhoeae is capnophilic and sensitive to drying and temperature changes. It grows best at 35–37 °C in 3–7% CO2 with high humidity.
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.
Viral transport medium usually contains protein (such as albumin or gelatin) and antimicrobial agents. The antimicrobials are added to:
Antibiotics and antifungals stop contaminating flora from damaging the specimen or cell cultures, while the protein stabilises the virus. They do not inactivate the virus.
Which dietary factor increases the absorption of non-heme iron?
Ascorbic acid keeps iron in the ferrous (Fe2+) form, which is absorbed more easily. Tannins, phytates and calcium all reduce non-heme iron absorption.
Hereditary hemochromatosis, a common inherited cause of iron overload, is most often due to mutation of which gene?
Most cases of hereditary hemochromatosis are caused by the HFE C282Y mutation, which leads to excess iron absorption. JAK2 relates to myeloproliferative neoplasms and HBB to beta-globin disorders.
The common morphological classification of anemia into microcytic, normocytic and macrocytic types is based on which index?
The MCV (mean cell volume) gives average red cell size and is used to sort anemias into microcytic, normocytic and macrocytic groups.
A normal biconcave red cell is described by which term?
A discocyte is the normal biconcave disc. Codocytes are target cells, dacrocytes are teardrop cells and echinocytes are burr cells.
In a healthy adult, lymphocytes usually make up about what percentage of the white cells?
Lymphocytes form about 20–40% of adult white cells. Neutrophils are about 50–70%, monocytes a few percent and basophils under 1%.