MLT GlobeDHA exam preparation (Dubai) – page 29
700 practice MCQs for the DHA medical laboratory exam. Level: Basic to intermediate.
In the fully enzymatic creatinine method, which enzyme performs the first step, converting creatinine to creatine?
Creatininase converts creatinine to creatine; creatinase then forms sarcosine, and sarcosine oxidase produces hydrogen peroxide for a Trinder-type color. Creatine kinase is not part of this assay.
An outpatient has creatinine drawn 2 hours after a large meal of cooked meat. What effect is expected?
Cooking converts muscle creatine into creatinine, which is absorbed and can temporarily raise serum creatinine. Fasting or avoiding cooked meat before testing reduces this effect.
A patient with HbC disease has many target cells. The WBC count is unexpectedly high and does not match the smear estimate. The most likely cause is:
Target cells in HbC, HbS, thalassemia, liver disease and neonates may resist lysis, and the unlysed stroma is counted as WBCs. Many analyzers flag this; extended lysis or a manual count solves it.
A manual 100-cell differential is poor at estimating cell types present in low numbers, such as basophils. The main reason is:
With so few cells counted, random sampling error is large for rare cells; counting more cells or using an analyzer (thousands of cells) improves precision. Basophils do stain with Wright stain.
Blood films made from heparinized blood are unsuitable for routine morphology mainly because heparin:
Heparin causes a blue-tinged background on Romanowsky stains and promotes platelet and leukocyte clumping. EDTA is preferred because it preserves cell morphology best.
A quantitative G6PD assay measures enzyme activity by monitoring:
G6PD converts glucose-6-phosphate and reduces NADP+ to NADPH, which absorbs at 340 nm; the rate of absorbance rise equals activity. p-Nitroaniline at 405 nm is used in chromogenic coagulation assays.
The autoantibody in warm autoimmune hemolytic anemia is usually IgG and most often shows specificity related to:
Warm autoantibodies usually react with nearly all cells and often target Rh-related proteins. Anti-I and anti-P are cold-reacting autoantibodies seen in cold agglutinin disease and PCH.
When thrombin acts on fibrinogen, the first products released are:
Thrombin cleaves fibrinopeptides A and B from the alpha and beta chains, forming fibrin monomers that polymerize. Fragments D and E and D-dimers are plasmin degradation products.
A PT reagent with an ISI of 1.0 compared with one of 2.0 is:
A lower ISI means the thromboplastin behaves close to the WHO reference and is more responsive to factor deficiency, giving more precise INRs. Reagents with ISI near 1.0 are preferred.
About what fraction of the total platelet mass is normally pooled in the spleen?
About one-third of platelets are held in the spleen. In massive splenomegaly more are pooled, causing thrombocytopenia, while splenectomy raises the platelet count.
A patient with chronic kidney failure has bleeding after a minor procedure. Platelet count, PT and aPTT are normal. The most likely cause is:
Uremic toxins impair platelet adhesion and aggregation despite a normal count; dialysis, DDAVP and correcting anemia help. Hemophilia and vitamin K deficiency prolong clotting tests.
Hypereosinophilia, used in the work-up of eosinophilic disorders, is defined as a blood eosinophil count of at least:
Hypereosinophilia is ≥1.5 × 10^9/L on two tests separated in time (or tissue hypereosinophilia). Eosinophilia starts above about 0.5 × 10^9/L.
A patient's WBC rises from 6 to 11 × 10^9/L within minutes after hard exercise, with no left shift. The best explanation is:
About half of blood neutrophils adhere to vessel walls (marginal pool). Exercise, stress or epinephrine release them into the circulating pool within minutes, without immature cells.
A patient has primary adrenal insufficiency with hypotension, hyponatremia and hyperkalemia. Which leukocyte change fits best?
Cortisol normally lowers eosinophil counts; in adrenal insufficiency the lack of cortisol leads to eosinophilia and relative lymphocytosis. Toxic neutrophilia suggests infection.
An unvaccinated infant with paroxysmal cough has WBC 45 × 10^9/L, mostly small mature lymphocytes, some with cleaved nuclei. The most likely cause is:
Pertussis toxin causes a marked absolute lymphocytosis of small mature (often cleaved) lymphocytes. ALL shows blasts, and infectious mononucleosis shows large reactive lymphocytes.
In an ion-selective electrode analyser, the potassium electrode membrane usually contains:
Valinomycin is an ionophore that binds K⁺ selectively and is used in potassium ISE membranes. Sodium is measured with a special sodium-selective glass membrane.
A skin puncture collection needs an EDTA tube, a lithium heparin tube and a serum tube. According to CLSI, which should be collected first?
In skin puncture (after any blood gas sample), EDTA is collected first to limit platelet clumping and clotting, then other additive tubes, and serum last. This differs from the venous order of draw.
During method evaluation, a sample is split and a potential interfering substance is added to one portion. This interference experiment mainly estimates:
The interference experiment measures the difference caused by an added substance, which is usually a constant systematic error. Proportional error is estimated by the recovery experiment.
Under CLIA regulations, calibration verification must be performed at least every:
CLIA requires calibration verification at least every 6 months, and also after major maintenance, reagent lot changes (if needed) or QC problems. Weekly or monthly checks are not the regulatory minimum.
Young colonies of an anaerobic gram-negative rod from a dental abscess show brick-red fluorescence under long-wave UV light before black pigment appears. This suggests:
Pigmented Prevotella and Porphyromonas species fluoresce brick-red under UV light before they develop black-brown pigment. B. fragilis and Fusobacterium do not fluoresce red.