MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 24
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
In immunohistochemistry on formalin-fixed tissue, heat-induced epitope retrieval (e.g., citrate buffer pH 6) is performed mainly to:
Formalin cross-links hide epitopes. Heating in buffer breaks these links and unmasks antigens so the primary antibody can bind.
In an HRP–DAB immunohistochemistry method, sections are treated with dilute hydrogen peroxide before the primary antibody in order to:
Red cells and granulocytes contain peroxidase activity that would convert DAB to brown product and give false positives. Hydrogen peroxide inactivates it beforehand.
On ethanol-fixed neutrophils, a perinuclear (p-ANCA) immunofluorescence pattern most often corresponds to antibodies against:
p-ANCA usually reflects anti-myeloperoxidase, seen in microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis. c-ANCA with anti-proteinase 3 is typical of granulomatosis with polyangiitis.
Goodpasture (anti-GBM) disease shows linear IgG staining along the glomerular basement membrane. This is classified as:
Antibody binding directly to a fixed tissue antigen (type IV collagen) is type II hypersensitivity and gives linear staining. Immune complex disease (type III) gives granular staining.
Which statement about MHC class II molecules is correct?
Class II (HLA-DR, -DQ, -DP) is expressed mainly on B cells, macrophages and dendritic cells. Beta-2 microglobulin is part of class I, and HLA-A, -B, -C are class I genes.
Which cytokine is the main stimulus for hepatic production of C-reactive protein and other acute-phase proteins?
IL-6 (with IL-1 and TNF-alpha) acts on hepatocytes to increase CRP, fibrinogen and serum amyloid A. IL-4 and IL-13 are Th2 cytokines linked to IgE production.
After successful treatment of early syphilis, which pattern is expected?
Non-treponemal titres fall at least fourfold after effective therapy and are used to follow treatment. Treponemal antibodies usually remain for life.
The ASO titre is most useful for diagnosing which condition?
ASO rises 1–3 weeks after group A streptococcal infection and supports diagnosis of rheumatic fever or glomerulonephritis. After skin infection ASO often stays low; anti-DNase B is better.
High-sensitivity CRP (hs-CRP) is mainly used to:
hs-CRP measures low CRP levels (below about 10 mg/L); values above 3 mg/L suggest higher cardiovascular risk. It should not be measured during acute illness.
A monospot (heterophile antibody) test is negative in a 3-year-old with clinical infectious mononucleosis. The most appropriate next test is:
Heterophile antibodies are often absent in young children and early in illness. EBV VCA IgM positive with EBNA negative indicates acute infection.
Compared with the CDC crossmatch, the flow cytometric crossmatch:
Flow crossmatch uses fluorescent anti-human IgG to detect antibody bound to donor T and B cells, including antibodies that do not fix complement. It is more sensitive than CDC.
A child with recurrent catalase-positive infections (such as Staphylococcus aureus and Aspergillus) has an abnormal dihydrorhodamine (DHR) flow test. The diagnosis is:
Chronic granulomatous disease is a defect of NADPH oxidase, so neutrophils cannot produce an oxidative burst. The DHR test replaced the nitroblue tetrazolium (NBT) test.
Which laboratory pattern best fits hyper-IgM syndrome due to CD40 ligand deficiency?
CD40L on T cells is needed for B-cell class switching. Without it, B cells make IgM but little IgG, IgA or IgE.
Absolute CD4 count is calculated as WBC × % lymphocytes × % CD4+ lymphocytes. For WBC 5.0 × 10⁹/L, lymphocytes 20% and CD4+ 15% of lymphocytes, the CD4 count is:
5000/µL × 0.20 = 1000 lymphocytes/µL; 1000 × 0.15 = 150 CD4 cells/µL. A value below 200 cells/µL defines AIDS in an HIV-infected person.
On egg yolk agar, half of the plate is covered with antitoxin. An isolate shows an opaque zone only on the half without antitoxin. This Nagler reaction identifies which toxin?
C. perfringens alpha toxin is a lecithinase that makes an opaque zone on egg yolk agar; specific antitoxin blocks it. The other toxins do not act on egg yolk lecithin.
A Gram stain of an expectorated sputum shows more than 25 squamous epithelial cells and fewer than 10 neutrophils per low-power field. What is the correct action?
Many squamous cells with few neutrophils show the sample is mostly saliva from the mouth. Culture would grow oral flora and mislead, so the specimen should be rejected and recollected.
Four blood culture bottles (two sets) are collected from a patient with fever. Only one aerobic bottle grows coagulase-negative staphylococci after 3 days. This result most likely represents:
Coagulase-negative staphylococci in a single bottle out of several sets usually come from skin. True bacteraemia normally gives growth in more than one set, often with a shorter time to positivity.
Why does CLSI state that disk diffusion should not be used to test vancomycin against Staphylococcus aureus?
Vancomycin is a large molecule that diffuses poorly, so VISA strains give zones similar to susceptible strains. An MIC method is required for S. aureus and vancomycin.
Why is disk diffusion not recommended for colistin susceptibility testing of gram-negative rods?
Colistin is a large cationic molecule that diffuses poorly, so zones are small and do not separate susceptible from resistant strains. CLSI recommends broth microdilution, broth disk elution or agar test methods.
A stool isolate forms red colonies without black centres on XLD agar. It is non-motile, lysine decarboxylase negative and H2S negative. The most likely genus is:
Shigella does not ferment xylose, lactose or sucrose, so colonies stay red, and it lacks H2S, motility and lysine decarboxylase. Salmonella usually gives red colonies with black centres.