MLT GlobeDOH exam preparation (Abu Dhabi) – page 31
700 practice MCQs for the DOH medical laboratory exam. Level: Basic to intermediate.
Plasma separated from a whole blood donation and shipped for fractionation into plasma derivatives, rather than transfusion, is called:
Recovered plasma comes from whole blood donations and goes to fractionation. Source plasma is collected by plasmapheresis specifically for fractionation.
For laboratory classification of antiphospholipid syndrome, a positive lupus anticoagulant, anticardiolipin or anti-beta-2 glycoprotein I result must be:
Antiphospholipid antibodies can appear transiently with infection or drugs, so persistence is required on two occasions at least 12 weeks apart. A positive ANA is not part of the criteria.
A 55-year-old man has nephrotic syndrome. Serum antibodies to the phospholipase A2 receptor (PLA2R) are positive. The most likely diagnosis is:
Anti-PLA2R antibodies are found in most cases of primary membranous nephropathy and help separate it from secondary forms. Anti-GBM disease is marked by anti-GBM antibodies with linear staining.
CTLA-4, expressed on activated and regulatory T cells, mainly:
CTLA-4 competes with CD28 for B7 with higher affinity and delivers an inhibitory signal, limiting immune responses. The IL-2 receptor alpha chain is CD25.
Which of these immunoglobulin classes has the shortest half-life in serum?
Free serum IgE has a half-life of only about 2 days, although IgE bound to mast cells lasts much longer. IgG lasts about 21–23 days.
In the lectin pathway, mannose-binding lectin binds microbial carbohydrates and activates which enzymes to cleave C4 and C2?
MBL-associated serine proteases (MASPs) act like C1r/C1s and cleave C4 and C2 without antibody. Factor B and factor D belong to the alternative pathway.
The role of properdin in the alternative complement pathway is to:
Properdin (factor P) binds C3bBb and extends its half-life several-fold, amplifying C3 cleavage. Cleavage of factor B is done by factor D.
Complement receptor 1 (CR1, CD35) on human red cells mainly helps to:
C3b-coated immune complexes bind CR1 on red cells and are stripped off by macrophages in the liver and spleen. CR1 also acts as a cofactor for factor I.
Plain Haemophilus influenzae type b polysaccharide vaccine works poorly in infants, but the conjugate vaccine works well because protein conjugation:
Polysaccharides are T-independent antigens that young infants respond to poorly. Linking them to a protein carrier lets helper T cells help polysaccharide-specific B cells, producing IgG and memory.
Thymus-independent antigens, such as repeating bacterial polysaccharides, typically induce:
These antigens cross-link many B-cell receptors without T-cell help, so the response is mostly IgM, with weak affinity maturation and little memory. Strong IgG memory requires T-cell help.
In Ouchterlony double diffusion, the precipitin lines from two neighbouring antigen wells join, but one line extends past the junction as a spur. This indicates:
A spur means one antigen has all the epitopes of the other plus extra ones, so antibodies to the extra epitopes pass the junction: partial identity. Complete identity gives a smooth fused arc without a spur.
A TSH result by two-site sandwich immunoassay is unexpectedly high and falls to normal after the sample is treated with a heterophile blocking reagent. The interference was most likely due to:
Heterophile or human anti-mouse antibodies can link the capture and labelled antibodies without analyte, causing false-high sandwich results that correct with blocking reagents. Biotin causes falsely low sandwich results.
Latex particles coated with antibody are used to detect bacterial antigen in a sample. This technique is:
In reverse passive agglutination, carrier particles carry antibody and detect soluble antigen. Passive agglutination uses antigen-coated carriers to detect patient antibody.
Coagglutination tests use Staphylococcus aureus cells as carriers because of their:
Protein A binds IgG by its Fc portion, leaving Fab sites facing outward to agglutinate with antigen. Clumping factor binds fibrinogen, not antibody.
A blood donor has a reactive anti-HCV immunoassay, but HCV RNA is not detected. The most likely interpretation is:
Antibody without detectable RNA suggests cleared past infection or a false-positive screen; a second antibody assay can separate these. Window-period infection would show RNA without antibody.
A 4-month-old boy has severe infections and very low T and NK cells with normal B-cell numbers. The most likely defect is in the:
X-linked SCID from IL2RG mutation (T−B+NK−) blocks IL-7 and IL-15 signalling needed for T and NK development. Bruton tyrosine kinase defects cause absent B cells with normal T cells.
A 28-year-old has recurrent sinus and lung infections. IgG and IgA are low, B-cell numbers are normal and antibody response to vaccines is poor. The most likely diagnosis is:
CVID usually presents after childhood with low IgG plus low IgA or IgM, poor vaccine responses and normal or slightly low B-cell numbers. X-linked agammaglobulinaemia presents in infancy with absent B cells.
A child has eczema, recurrent 'cold' staphylococcal skin abscesses, pneumonias with lung cysts and serum IgE above 2000 IU/mL. The most likely diagnosis is:
Autosomal dominant hyper-IgE (Job) syndrome from STAT3 mutation gives very high IgE, eczema, cold abscesses and pneumatoceles. Wiskott-Aldrich syndrome also has eczema but with thrombocytopenia and small platelets.
The Clark electrode used to measure pO₂ works on which principle?
The Clark electrode applies a fixed voltage; oxygen is reduced at the platinum cathode and the current is proportional to pO₂. Potentiometric glass electrodes measure pH and pCO₂.
A blood urea nitrogen (BUN) result is 28 mg/dL. Each urea molecule contains 2 nitrogen atoms (28 g of nitrogen per mole of urea). What is the result as urea in mmol/L?
28 mg/dL = 280 mg/L; each mole of urea has 28 g of nitrogen, so 280 ÷ 28 = 10 mmol/L. Dividing by 56 would give 5 mmol/L in error.