MLT GlobeDOH exam preparation (Abu Dhabi) – page 29
700 practice MCQs for the DOH medical laboratory exam. Level: Basic to intermediate.
Anti-SS-A (Ro) antibodies in a pregnant woman carry a risk of which condition in the newborn?
Maternal IgG anti-Ro/SS-A crosses the placenta and can damage the fetal cardiac conduction system, causing congenital heart block and neonatal lupus rash.
A patient with myositis, interstitial lung disease and 'mechanic's hands' most likely has which antibody?
Anti-Jo-1 (histidyl-tRNA synthetase) defines the antisynthetase syndrome: myositis, interstitial lung disease, arthritis and mechanic's hands. Its ANA pattern is cytoplasmic.
The preferred initial serologic screening test for celiac disease in a patient with normal total IgA is:
IgA anti-tTG is sensitive and specific and is the recommended first test. Total IgA should be checked because IgA deficiency causes false-negative IgA-based tests.
Natural killer (NK) cells are best identified by which marker combination?
NK cells lack CD3 (no T-cell receptor) but express CD16 and CD56. CD3+ CD56+ cells are NKT cells, a different T-cell subset.
A 25-year-old has recurrent attacks of non-itchy swelling of the face and bowel wall without urticaria. C4 is low between attacks. Which protein is most likely deficient?
Hereditary angioedema is caused by C1 inhibitor deficiency, leading to uncontrolled C1 activity (low C4) and excess bradykinin. Urticaria is typically absent.
Interferon-gamma's most important effect in cell-mediated immunity is to:
Interferon-gamma, produced by Th1 cells and NK cells, activates macrophages and increases MHC expression. This is key in defense against intracellular organisms such as mycobacteria.
Regulatory T cells that maintain self-tolerance are typically characterised by:
Regulatory T cells express CD4, high CD25 (IL-2 receptor alpha) and the transcription factor FOXP3. Loss of FOXP3 causes the severe autoimmune syndrome IPEX.
A one-step sandwich immunoassay gives a falsely low result for an extremely high hCG level. The best troubleshooting step is to:
Very high antigen saturates both capture and detection antibodies (high-dose hook effect). Diluting the sample brings it into the measuring range and reveals the true high value.
In the reverse syphilis screening algorithm, the treponemal EIA is reactive but the RPR is non-reactive. The next step is to:
A discordant EIA+/RPR− result needs a second treponemal test. If TP-PA is reactive it suggests past treated or untreated syphilis; if non-reactive the EIA is likely false-positive.
In a complement-dependent cytotoxicity (CDC) crossmatch, the test uses:
Recipient serum is incubated with donor lymphocytes and complement. Cell death shows donor-specific cytotoxic antibody, a contraindication to transplant.
Which material is commonly used to check the wavelength accuracy of a spectrophotometer?
Holmium oxide (or didymium) glass has sharp absorbance peaks at known wavelengths, used to verify wavelength calibration. Neutral density filters check photometric (absorbance) accuracy.
In a fluorometer, how does the measured emitted light compare with the excitation light?
Fluorescent molecules lose some energy before emitting, so emitted light has a longer wavelength (Stokes shift). The detector is placed at 90° to reduce interference from the excitation beam.
In a time-of-flight (TOF) mass analyzer, such as in MALDI-TOF identification of bacteria, which ions reach the detector first?
All ions receive similar kinetic energy, so lighter ions (lower m/z) travel faster through the flight tube and arrive first. Neutral molecules are not accelerated or detected.
A laboratory will grow and manipulate cultures of Mycobacterium tuberculosis. The minimum biosafety level required is:
Propagation and manipulation of M. tuberculosis cultures require BSL-3 because of aerosol transmission risk. Preparing smears from clinical specimens may be done under BSL-2 practices in a biosafety cabinet.
Which biological safety cabinet protects the worker and the environment but NOT the product from contamination?
A Class I cabinet draws unfiltered room air across the work surface, so the product is not protected. Class II cabinets supply HEPA-filtered downflow air that protects the product as well.
Picric acid used in histology must be stored:
Dry picric acid is shock-sensitive and can form explosive picrates with metals. It is kept wet with water and stored in non-metal containers.
According to the CDC sequence for removing PPE, which item is usually removed first?
CDC guidance removes the most contaminated item, gloves, first, then goggles/face shield, gown and finally the mask or respirator, followed by hand hygiene.
Only a coagulation (sodium citrate) tube is ordered, and a butterfly (winged) set is used. The phlebotomist should first:
The air in the tubing would cause underfilling of the citrate tube. A discard tube (plain or citrate) fills the dead space so the 9:1 blood-to-anticoagulant ratio is kept.
A patient is asked to repeatedly clench and pump the fist during venipuncture. Which analyte is most likely to be falsely increased?
Muscle contraction releases potassium from muscle cells into local blood, falsely raising serum potassium. Fist clenching should be avoided during collection.
A test is evaluated in 200 subjects: true positives 45, false positives 15, false negatives 5, true negatives 135. The positive predictive value is:
PPV = TP ÷ (TP + FP) = 45 ÷ 60 = 75%. 90% is the sensitivity (45 ÷ 50) and 90% is also the specificity (135 ÷ 150); 96% is the NPV.