MLT GlobeDOH exam preparation (Abu Dhabi) – page 35
700 practice MCQs for the DOH medical laboratory exam. Level: Basic to intermediate.
A pregnant woman has positive Toxoplasma IgG and IgM. Which test best helps show the infection occurred more than 4 months ago?
High IgG avidity indicates infection at least 3–5 months earlier, because IgM can persist for months or years. Low avidity supports recent infection.
Undiluted CSF is counted in a Neubauer hemocytometer. A total of 45 WBCs is counted in all 9 large squares (each 1 mm² × 0.1 mm deep). The WBC count is:
Volume counted = 9 × 0.1 µL = 0.9 µL. Cells/µL = 45 ÷ 0.9 × dilution factor 1 = 50 cells/µL.
Pleural fluid pH is requested for a parapneumonic effusion. Which handling is correct?
Air exposure lets CO2 escape and raises pH, so the sample must be anaerobic and measured with a blood gas analyser. A pH below 7.2 suggests a complicated effusion needing drainage.
Yellow-brown spheres with spiky projections ('thorny apples') are seen in an old, alkaline urine specimen. They are most likely:
Ammonium biurate crystals form in alkaline, often old specimens and look like thorny apples. They convert to uric acid when acetic acid is added. Leucine spheres have concentric rings and no spikes.
Red cells treated with EDTA–glycine–acid (EGA) to remove IgG cannot be reliably typed for which antigens?
EGA removes IgG but destroys Kell system antigens, so Kell typing needs another method, e.g., chloroquine.
A donor received long-acting injectable HIV pre-exposure prophylaxis (PrEP). Under current FDA guidance, the donor is deferred for:
Injectable PrEP can delay antibody and NAT detection of HIV for a long time, so FDA requires a 2-year deferral. Oral PrEP or PEP needs a 3-month deferral.
Antibodies to Mia (GP.Mur) are an important cause of hemolytic transfusion reactions and HDFN in:
GP.Mur is common in Taiwan, southern China and Thailand, so anti-Mia is a routine concern there, and screening cells may include Mia.
At sea level (760 mmHg), breathing room air (FiO2 0.21), a patient has PaO2 70 mmHg and PaCO2 40 mmHg. Using PAO2 = FiO2 × (760 − 47) − PaCO2/0.8, the A–a gradient is about:
PAO2 = 0.21 × 713 − 40/0.8 = 149.7 − 50 = 99.7 mmHg. A–a gradient = 99.7 − 70 ≈ 30 mmHg, higher than normal for most adults, suggesting a gas exchange problem.
Tissue factor pathway inhibitor (TFPI) mainly acts by:
TFPI first binds factor Xa and then the TF–VIIa complex, switching off the initiation phase. Activated protein C with protein S degrades Va and VIIIa.
VWF multimer analysis shows loss of high- and intermediate-molecular-weight multimers, low VWF activity/antigen ratio, and no enhanced response to low-dose ristocetin. The vWD type is:
Type 2A lacks large multimers and has low function relative to antigen. Type 2B also loses large multimers but shows increased low-dose ristocetin response; type 2M has normal multimers.
Under the WHO 5th edition, AML with erythroid differentiation (pure erythroid leukemia) requires erythroid precursors of at least 80% of marrow cells with pronormoblasts of at least:
The WHO 5th edition defines this entity by ≥80% erythroid precursors with ≥30% pronormoblasts; it is strongly linked to biallelic TP53 changes. Myeloblast percentage is not used in this definition.
In the MHC class II pathway, the invariant chain (Ii) mainly serves to:
The invariant chain occupies the class II groove in the ER; in the endosome it is degraded to CLIP, which HLA-DM exchanges for exogenous peptide. Transport of cytosolic peptides into the ER is done by TAP.
Dendritic cells can present peptides from ingested (exogenous) antigens on MHC class I to activate CD8 T cells. This process is called:
Cross-presentation lets dendritic cells prime CD8 T cells against viruses or tumours that do not infect the dendritic cell itself. Normally exogenous antigen goes to MHC class II.
A child has haemolytic anaemia, thrombocytopenia and acute kidney injury without preceding diarrhoea. C3 is low and C4 is normal. Which defect is most likely?
Atypical haemolytic uraemic syndrome is often due to factor H defects, causing uncontrolled alternative pathway activation on endothelium (low C3, normal C4). C1 inhibitor deficiency causes angioedema with low C4.
A patient with glomerulonephritis has persistently very low C3 with normal C4. An autoantibody that stabilises the alternative pathway C3 convertase is found. This autoantibody is:
C3 nephritic factor binds C3bBb and protects it from decay, causing continuous C3 consumption; it is linked to C3 glomerulopathy. Anti-C1q antibodies relate to lupus nephritis and lower C4 as well.
A patient's HIV-1/2 antigen/antibody combination assay is reactive, but the HIV-1/HIV-2 antibody differentiation immunoassay is negative. The next step is:
A reactive screen with negative differentiation may be acute HIV-1 infection (antigen present before antibody) or a false-positive screen. An HIV-1 RNA test resolves this; reporting negative could miss acute infection.
A creatinine clearance is 100 mL/min in a patient with a body surface area of 1.20 m². Corrected to 1.73 m², the clearance is about:
Corrected clearance = 100 × 1.73/1.20 ≈ 144 mL/min/1.73 m². Multiplying by 1.20/1.73 instead gives 69.
Over one month, 9% of a laboratory's NALC-NaOH processed sputum cultures on solid media are overgrown by bacteria and fungi. The most likely cause is:
An acceptable contamination rate is about 2–5%. A higher rate suggests weak NaOH or too short exposure; a rate below 2% suggests over-harsh processing that also kills mycobacteria.
Which transport medium should not be used for stool when Vibrio infection is suspected?
Glycerol is toxic to vibrios, so buffered glycerol saline is unsuitable, although it can be used for Shigella. Cary-Blair is the preferred transport medium for Vibrio.
In a pregnancy with maternal diabetes, the amniotic L/S ratio is 2.1. Which additional finding best confirms fetal lung maturity?
In diabetic pregnancies the L/S ratio can reach 2.0 while the lungs remain immature; phosphatidylglycerol appears later and confirms maturity. The fern test identifies amniotic fluid, not lung maturity.