MLT GlobeQCHP exam preparation (Qatar) – page 37
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
After total thyroidectomy for papillary thyroid cancer, serum thyroglobulin is undetectable. Why must anti-thyroglobulin antibodies also be measured?
Thyroglobulin antibodies are present in many patients and interfere with sandwich immunoassays, usually giving falsely low results that could hide recurrence. Medullary cancer is followed with calcitonin.
In impedance counters, a 'sweep flow' behind the red cell aperture is used to prevent:
Cells that have passed the aperture can recirculate near it and create small extra pulses that mimic platelets. A steady sweep flow carries them away; coincidence is corrected mathematically, not by sweep flow.
Hb Constant Spring, common in Southeast Asia, is an alpha-chain variant caused by:
A mutation in the alpha2 termination codon adds 31 amino acids, producing an unstable chain made in small amounts, so it behaves like an alpha-thalassemia allele. Delta-beta fusion describes Hb Lepore.
A young adult with painful crises has MCV 66 fL, normal ferritin and HPLC showing HbS 72%, HbA 16%, HbA2 5.5% and HbF 6.5%. The most likely diagnosis is:
HbS greater than HbA, microcytosis and raised HbA2 indicate one beta-S gene with a beta+ thalassemia gene that makes a little HbA. In sickle cell trait HbA exceeds HbS and indices are normal; HbSS has no HbA.
An uncentrifuged citrated sample is refrigerated overnight before PT testing. The PT is unexpectedly short. The likely reason is:
Storage at 2–8 °C can activate factor VII and damage labile factors, so whole blood for PT should be kept at room temperature. Factor VIII loss would affect the aPTT, not shorten the PT.
The free (active) form of protein S is reduced in inflammation mainly because more protein S is bound to:
About 60% of protein S is bound to C4b-binding protein, an acute-phase protein; only free protein S acts as the cofactor for activated protein C. Free protein S antigen is the preferred assay.
A patient has a bleeding tendency, normal platelet aggregation and normal counts, but platelets fail to expose phosphatidylserine for coagulation. This defect is:
Scott syndrome is a defect in platelet procoagulant activity (phospholipid scrambling, ANO6/TMEM16F); aggregation tests are normal. Glanzmann and Hermansky-Pudlak show abnormal aggregation.
Under the WHO 5th edition, a patient with MDS-type dysplasia and 12% marrow blasts is best classified as:
MDS-IB2 has 10–19% marrow blasts (or 5–19% in blood, or Auer rods); MDS-IB1 has 5–9% marrow or 2–4% blood blasts. AML usually needs ≥20% blasts unless a defining genetic change is present.
A healthy 75-year-old has normal blood counts, but sequencing finds a DNMT3A mutation at 8% variant allele frequency. This is best called:
CHIP is a somatic myeloid driver mutation (VAF ≥2%) without cytopenia or a diagnosed hematologic neoplasm. MDS requires cytopenia and morphologic dysplasia or defining genetics.
In a manual differential, the technologist counted 100 cells and found 2 eosinophils. To improve the precision of the differential, the best approach is to:
Differential counts follow a binomial distribution, so counting more cells reduces imprecision, especially for low-frequency cells. The feathered edge gives distorted and unevenly distributed cells.
The Fontana–Masson method shows melanin as black because melanin is argentaffin, meaning it:
Argentaffin substances such as melanin reduce silver solutions by themselves. Argyrophilic structures, such as reticulin, bind silver but need an external reducing agent to show it.
A healthy 30-year-old has a HEp-2 ANA with a dense fine speckled nucleoplasm and strongly stained metaphase chromatin. Specific ENA tests are negative. This pattern (anti-DFS70) most likely indicates:
Isolated anti-DFS70 antibodies are common in healthy people and, when alone, make a systemic autoimmune rheumatic disease less likely. SLE-related antibodies such as anti-dsDNA give a homogeneous pattern.
Naive CD4 T cells differentiate into Th17 cells mainly in the presence of which cytokine combination?
TGF-β with IL-6 (maintained by IL-23) induces the transcription factor RORγt and the Th17 programme. TGF-β without IL-6 favours regulatory T cells instead.
Staphylococcal toxic shock syndrome toxin-1 activates up to 20% of all T cells because it:
Superantigens bridge MHC class II and certain Vβ chains without processing, stimulating many T cells and causing a cytokine storm. A normal peptide activates only rare specific clones.
A pregnant woman has positive Toxoplasma IgG and IgM. A high IgG avidity result on the same sample mainly indicates that:
IgG avidity rises with affinity maturation; high avidity means the infection was acquired months earlier (usually more than about 4 months). IgM can persist for many months, so it does not prove recent infection.
An infant has repeated severe pneumonia and bacteraemia from encapsulated bacteria. CH50 and AH50 are both zero, and C3 is undetectable. The most likely diagnosis is:
C3 is needed by all pathways for opsonisation and MAC formation, so its absence abolishes both CH50 and AH50 and causes severe pyogenic infections. C9 deficiency gives only low (not zero) lytic activity and mild disease.
A child has chronic mucocutaneous candidiasis, hypoparathyroidism and adrenal insufficiency. A mutation in which gene, needed for thymic expression of tissue antigens, is most likely?
AIRE lets medullary thymic cells express tissue-specific antigens so autoreactive thymocytes are deleted; its loss causes APECED (APS-1). FOXP3 defects cause IPEX through absent regulatory T cells.
Which finding in a newborn best supports congenital syphilis?
Maternal IgG crosses the placenta, so a reactive infant IgG treponemal test or an equal RPR titre may be only passive antibody. An infant titre at least fourfold higher than the mother's suggests infant production.
In an indirect IgM assay for rubella, a false-positive IgM result can occur when the specimen contains:
IgM rheumatoid factor binds specific IgG attached to antigen and is detected by anti-IgM conjugate. Removing IgG before testing, or using IgM-capture formats, prevents this.
A spore-forming gram-positive rod grows on chocolate agar in CO2 as well as anaerobically, but forms spores only under anaerobic conditions. It is catalase negative. The most likely organism is:
Clostridium tertium is aerotolerant but forms spores only anaerobically and is catalase negative. Bacillus species are usually catalase positive and form spores aerobically; C. perfringens does not grow in air.