MLT GlobeDHA exam preparation (Dubai) – page 20
700 practice MCQs for the DHA medical laboratory exam. Level: Basic to intermediate.
One autosomal recessive form of SCID results from deficiency of which enzyme?
ADA deficiency causes toxic metabolites that kill lymphocytes.
The Paul–Bunnell test for infectious mononucleosis detects heterophile antibodies by:
EBV-induced heterophile IgM antibodies agglutinate sheep (or horse) erythrocytes. The Davidsohn absorption step improves specificity.
Immunizing a rabbit with human albumin produces antibodies of different specificities because albumin:
Large protein antigens have multiple epitopes, each stimulating different B-cell clones, producing a polyclonal response.
Which statement about the T-cell receptor is FALSE?
Classes I and II describe MHC molecules; TCRs are classified as alpha-beta or gamma-delta.
Clonal selection theory states that:
Each lymphocyte has a single pre-existing receptor specificity; antigen selects and expands matching clones.
Raised specific antibody in a newborn's cord blood suggests congenital infection when the antibody class is:
IgM does not cross the placenta, so it must come from the fetus. IgG may be maternal.
Secretory IgA resists degradation on mucosal surfaces because it:
Polymeric Ig receptor transports IgA across epithelium and a part remains as secretory component protecting it.
Which is required in the alternative pathway but not in the classical pathway?
Factor B (with factor D and properdin) forms the alternative C3 convertase. C3 and later components are shared.
The alternative complement pathway is triggered by:
Microbial surfaces stabilize C3b, triggering the alternative pathway. Antigen–antibody complexes trigger the classical pathway.
In type II (cytotoxic) hypersensitivity, what event starts the tissue damage?
Type II injury begins when IgG or IgM binds antigens on cells or tissue, triggering complement lysis, opsonisation or ADCC. Mast-cell IgE is type I; circulating complexes are type III.
Tumour-specific transplantation antigens (TSTAs) are best described as molecules that are:
TSTAs arise from mutated or viral proteins unique to the tumour, so they are absent from normal cells. Oncofetal molecules shared with embryonic tissue are tumour-associated antigens.
Alpha-fetoprotein and CEA are tumour-associated antigens. What property distinguishes such antigens from tumour-specific ones?
Tumour-associated (oncofetal) antigens are normal proteins re-expressed or overexpressed by tumours; they occur in fetal tissue and at low levels in adults, which limits their specificity.
Lymphokine-activated killer (LAK) cells used in cancer immunotherapy are generated mainly by culturing which cells with IL-2?
LAK cells are predominantly NK cells (with some T cells) stimulated by high-dose IL-2, which then lyse tumour cells without MHC restriction.
A tumour avoids destruction by cytotoxic T cells. Which change in the tumour most directly allows this escape?
Cytotoxic T cells need antigen presented on MHC class I. Antigen loss (or MHC I loss) removes the target; increasing either would make killing more likely.
A kidney graft turns cyanotic and fails within minutes of reperfusion. This is most likely:
Preformed anti-ABO or anti-HLA antibodies fix complement on graft endothelium immediately. Acute rejection occurs over days to months; chronic over months to years.
After eye trauma, antibodies to lens proteins appear. These proteins are 'sequestered antigens', meaning they:
Sequestered antigens (lens, sperm, some CNS proteins) are anatomically isolated, so tolerance never develops; release after injury can trigger autoimmunity, e.g. sympathetic ophthalmia.
Coagglutination reagents use killed bacteria whose protein A binds the Fc portion of IgG. Which organism is used?
S. aureus Cowan I is rich in protein A, which binds IgG Fc and leaves the Fab arms free to agglutinate with antigen.
Red cells coated with IgG antibodies that cannot bridge cells on their own are made to agglutinate by adding anti-human globulin. Which technique is this?
Incomplete (non-agglutinating) IgG sensitises red cells without visible clumping; anti-human globulin cross-links the bound IgG, producing agglutination.
Atypical lymphocytes with folded, 'cerebriform' nuclei in the blood of a patient with a generalized red skin rash suggest:
Sézary cells are malignant CD4+ T cells with convoluted cerebriform nuclei. Sézary syndrome is the leukemic form of cutaneous T-cell lymphoma, with erythroderma and lymphadenopathy.
A jaundiced patient has markedly raised ALP and GGT with only mildly raised ALT and AST. This pattern suggests:
ALP and GGT are made by bile duct cells and rise most in cholestasis. In hepatitis the transaminases rise much more than ALP; in bone disease ALP rises but GGT stays normal.