MLT GlobeQCHP exam preparation (Qatar) – page 31
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
The secretory component attached to secretory IgA is derived from:
Epithelial cells bind dimeric IgA with the polymeric Ig receptor and transport it across the cell; part of the receptor stays attached as secretory component. The J chain is made by plasma cells.
A flow cytometry report shows WBC 5.0 × 10⁹/L, lymphocytes 20%, and CD4+ T cells 25% of lymphocytes. The absolute CD4 count is:
5.0 × 10⁹/L × 0.20 = 1.0 × 10⁹/L lymphocytes; × 0.25 = 0.25 × 10⁹/L, which is 250 cells/µL. 1000 cells/µL is the absolute lymphocyte count.
Inherited deficiency of early classical pathway components (C1q, C2 or C4) is most strongly associated with:
Early classical components help clear immune complexes and apoptotic cells; without them, lupus-like autoimmunity develops (especially with C1q deficiency). Neisseria infections relate to terminal component deficiency.
An infant is diagnosed with severe combined immunodeficiency. Which vaccine must be avoided?
BCG is a live attenuated vaccine and can cause disseminated infection in severe immunodeficiency. Toxoid, inactivated and recombinant vaccines cannot replicate, although responses may be poor.
Paired sera for a viral infection show an acute-phase titre of 1:16 and a convalescent titre of 1:128 taken 2 weeks later and tested together. The best interpretation is:
A rise of four-fold or more (here eight-fold) between paired sera tested in the same run indicates recent infection. A one-dilution difference is within normal test variation.
Which weak attractive force arises from temporary fluctuations in electron clouds and works only when antigen and antibody are very close together?
Van der Waals forces come from transient dipoles between electron clouds and act only over very short distances, so a close fit is essential. Ionic bonds need oppositely charged groups.
In a haemagglutination inhibition test for rubella antibody, a positive result is shown by:
Rubella virus agglutinates certain red cells. Patient antibody neutralises the virus so red cells do not agglutinate; the inhibition of agglutination is the positive result.
In a complement fixation test, the patient's serum contains the antibody being tested for. The indicator system will show:
Antigen-antibody complexes fix (use up) the added complement, so none is left to lyse the antibody-coated sheep red cells. Haemolysis means complement was free, i.e., no antibody.
The classical tube test for antistreptolysin O (ASO) antibody is based on:
Patient ASO neutralises streptolysin O so that it can no longer lyse rabbit or human red cells; the highest dilution without haemolysis gives the titre. Latex methods exist, but the classical test is neutralisation.
A patient recovering from acute hepatitis B has HBsAg negative, anti-HBs negative and IgM anti-HBc positive. This stage is the:
In the window period HBsAg has cleared but anti-HBs is not yet detectable; IgM anti-HBc may be the only marker. Missing it would give a false-negative diagnosis.
A traveller has had fever for 2 days. Which test is most useful to diagnose dengue at this stage?
NS1 antigen circulates from the first day of fever, usually for about 5 days, before IgM appears. IgG rises later, especially in primary infection, so it is not useful this early.
Which assay identifies the exact HLA specificities of a transplant candidate's antibodies most precisely?
Single-antigen beads each carry one HLA antigen, so the fluorescence pattern shows the exact specificity of each antibody. A CDC crossmatch only shows whether antibody reacts with one donor's cells.
Blood for monitoring tacrolimus levels in a transplant recipient should be:
Tacrolimus is concentrated in red cells, so whole blood (EDTA) is measured, usually as a trough level just before the next dose. Serum or plasma would give falsely low values.
A baby had delayed umbilical cord separation and now has skin infections without pus. The WBC count shows marked neutrophilia. Flow cytometry would most likely show absent:
Leukocyte adhesion deficiency type I is caused by lack of the beta-2 integrin CD18, so neutrophils cannot leave vessels: high blood neutrophils but no pus. CD19 absence suggests agammaglobulinaemia.
In a healthy full-term infant, total serum IgG is normally lowest at about:
Maternal IgG falls after birth while the infant's own production is still low, giving a physiological nadir around 3–6 months. At birth IgG is high because of placental transfer.
Tetanospasmin causes muscle rigidity and spasms mainly by:
Tetanus toxin travels to the spinal cord and blocks release of the inhibitory transmitters glycine and GABA, so motor neurons fire without control, causing spastic paralysis.
On cycloserine-cefoxitin-fructose agar (CCFA), Clostridioides difficile typically forms:
C. difficile forms yellow, flat, ground-glass colonies on CCFA with a horse-stable (p-cresol) odour and yellow-green fluorescence under UV light.
A vaginal Gram stain is scored by the Nugent method and gives a score of 8. This result indicates:
Nugent scores are 0–3 normal, 4–6 intermediate and 7–10 bacterial vaginosis. High scores reflect loss of lactobacilli and many Gardnerella, Bacteroides/Prevotella and curved Mobiluncus-type rods.
An anaerobic culture from a lung abscess grows thin gram-negative rods with pointed ends (spindle shape) that are indole positive. The most likely organism is:
F. nucleatum appears as thin, fusiform rods with tapered ends, is indole positive and forms bread-crumb-like colonies. Bacteroides are pleomorphic rods with rounded ends; Veillonella are cocci.
Cystine-lactose-electrolyte-deficient (CLED) agar is useful for urine cultures because it:
The lack of electrolytes prevents Proteus from swarming, so other colonies can be counted. CLED supports both gram-positive and gram-negative uropathogens.