MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 31
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
A complete MHC class I molecule on the cell surface consists of:
Class I is an MHC-encoded α chain non-covalently bound to β2-microglobulin, which is encoded on chromosome 15. The α/β pair of MHC-encoded chains describes class II.
Mycobacterial lipid and glycolipid antigens are presented to T cells by which molecules?
CD1 molecules are MHC-like proteins that present lipid antigens to certain T cells, including NKT cells. Classical MHC molecules present peptides.
In normal human serum, the approximate ratio of kappa-bearing to lambda-bearing immunoglobulin molecules is:
About two-thirds of human immunoglobulins carry kappa and one-third lambda light chains. A marked change in this ratio suggests a clonal B-cell population.
Digestion of IgG with pepsin produces:
Pepsin cuts below the hinge disulfide bonds, leaving a bivalent F(ab')2 that can still agglutinate or precipitate, while Fc is degraded. Two Fab plus Fc is the papain product.
Which immunoglobulin class is produced in the largest total amount each day, mostly in mucosal tissues?
Mucosal plasma cells secrete several grams of IgA daily, more than all other classes combined, but much is lost into secretions. IgG is highest in serum only because of its long half-life.
What is the theoretical antigen-binding valence of a pentameric IgM molecule?
Each of the five monomers has two antigen-binding sites, giving a valence of 10. In practice, steric hindrance often allows only about 5 sites to bind large antigens.
Cleavage of cell-bound C3b to inactive iC3b is carried out by which plasma protease, working with cofactors such as factor H?
Factor I cleaves C3b and C4b only in the presence of a cofactor (factor H, C4-binding protein, CR1 or MCP). Factor D cleaves factor B.
The CH50 assay measures the reciprocal of the serum dilution that:
CH50 tests the whole classical pathway (C1–C9) using sheep red cells coated with rabbit antibody. Unsensitised rabbit red cells are used for the alternative pathway (AH50).
A patient with systemic lupus erythematosus has an active flare with nephritis. Which complement pattern is most expected?
Immune complexes activate the classical pathway, consuming both C4 and C3; levels often rise again with remission. High levels are seen as acute-phase responses, not in active lupus nephritis.
Activated T cells that are repeatedly stimulated can be removed by activation-induced cell death. This is mainly mediated by:
Fas–FasL interaction triggers apoptosis of chronically activated lymphocytes, a peripheral tolerance mechanism; Fas defects cause autoimmune lymphoproliferative syndrome. CD28–B7 is a costimulatory, activating signal.
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.
An antibody binds a native protein but not the same protein after denaturation. The epitope it recognises is most likely:
Conformational epitopes are formed by amino acids brought together by folding, and they are lost when the protein unfolds. Linear epitopes are continuous sequences often still recognised after denaturation.
In the current CDC laboratory algorithm for HIV diagnosis, the initial screening test is:
The algorithm starts with a fourth-generation (or later) antigen/antibody immunoassay that detects p24 antigen and HIV-1/2 antibodies, shortening the window period. The Western blot is no longer part of the algorithm.
A 17-year-old with fever, sore throat and lymphadenopathy has EBV serology: VCA IgM positive, VCA IgG positive, EBNA IgG negative. This pattern indicates:
VCA IgM with VCA IgG and absent EBNA antibody fits acute primary infection; anti-EBNA appears only weeks to months later. Past infection shows VCA IgG and EBNA IgG without VCA IgM.
In the FTA-ABS test, the patient's serum is first mixed with a sorbent made from:
The sorbent from non-pathogenic Reiter treponemes removes cross-reacting group antibodies, improving specificity for T. pallidum. Cardiolipin antigen is used in non-treponemal tests such as RPR.
Which test on cerebrospinal fluid is most specific for diagnosing neurosyphilis?
A reactive CSF VDRL is highly specific for neurosyphilis (if the CSF is not blood-contaminated), although it lacks sensitivity. CSF FTA-ABS is sensitive and useful to exclude disease but less specific.
A jaundiced patient has HBsAg positive, HBeAg positive, IgM anti-HBc positive and anti-HBs negative. This indicates:
HBsAg with IgM anti-HBc indicates acute infection, and HBeAg shows active viral replication with high infectivity. Vaccination gives anti-HBs alone.
Cellular blood components for a stem cell transplant recipient are irradiated. The main purpose is to:
Gamma or X-ray irradiation (at least 25 Gy to the centre of the bag) stops donor T lymphocytes from proliferating, preventing TA-GVHD. Leukoreduction, not irradiation, reduces febrile reactions and CMV transmission.
Cyclosporine and tacrolimus suppress rejection mainly by:
Calcineurin inhibitors prevent NFAT activation in T cells, reducing transcription of IL-2 and T-cell proliferation. CD20 depletion is the action of rituximab.
A patient with recurrent pneumonia has normal total IgG. Which test best assesses specific antibody function?
Measuring serotype-specific antibodies before and 4–6 weeks after polysaccharide vaccination tests the ability to make functional antibody. Normal total IgG does not exclude specific antibody deficiency.