MLT GlobeMOH exam preparation (UAE) – page 29
700 practice MCQs for the MOH medical laboratory exam. Level: Basic to intermediate.
The plasma half-life of albumin is approximately:
Albumin has a half-life of about 15–20 days, so its level changes slowly and is a poor marker of acute changes in nutrition or liver synthesis. 120 days is the red cell lifespan.
Which method is commonly used on automated analyzers for the low protein concentrations in urine and CSF?
Pyrogallol red–molybdate (or turbidimetric methods such as benzethonium chloride) can detect milligram-per-deciliter protein levels. Biuret and refractometry are not sensitive enough.
A phenytoin dose is raised by 10%, but the serum level rises by 60% and toxicity develops. This is because phenytoin:
Phenytoin metabolism saturates near therapeutic levels (Michaelis–Menten kinetics), so small dose increases cause disproportionately large rises in level. First-order drugs change level in proportion to dose.
In the Trinder method for salicylate, what forms the colored product measured?
Ferric ions (Fe3+) in acid react with salicylate to form a violet complex read near 540 nm. Picric acid is used for creatinine (Jaffe), not salicylate.
A man drank homemade spirits and now has blurred vision, high anion gap metabolic acidosis and a raised osmolal gap. Which toxic metabolite causes the eye damage?
Methanol is converted by alcohol dehydrogenase to formaldehyde and then formic acid, which damages the optic nerve and causes acidosis. Oxalic acid comes from ethylene glycol.
A workplace urine specimen has creatinine 15 mg/dL (1.3 mmol/L) and specific gravity 1.002. Under common US federal validity criteria, it is classified as:
Creatinine ≥2 but <20 mg/dL with specific gravity >1.0010 but <1.0030 meets the dilute criteria. Substituted requires creatinine below 2 mg/dL with abnormal specific gravity.
A standard microhematocrit is centrifuged at about 10,000–15,000 g. For how long is the tube usually spun?
CLSI guidance uses a high-speed microhematocrit centrifuge for about 5 minutes to reach maximum red cell packing. Much shorter spins leave extra plasma in the column and give falsely high results.
A spun microhematocrit is usually slightly higher than the hematocrit calculated by an analyzer mainly because:
Even after full centrifugation, some plasma remains trapped in the red cell column, so the spun value runs about 1–3% higher. The analyzer calculates Hct from RBC count × MCV and has no trapped plasma.
For a manual RBC count, blood is diluted 1:200. A total of 450 red cells are counted in 5 small squares (each 0.04 mm²) of a hemacytometer 0.1 mm deep. What is the RBC count?
Volume counted = 5 × 0.04 mm² × 0.1 mm = 0.02 µL. Count = 450 × 200 ÷ 0.02 = 4,500,000/µL = 4.5 × 10^12/L. Forgetting the depth or area gives the wrong multiples.
A child with sickle cell anemia develops osteomyelitis. Besides Staphylococcus aureus, which organism is especially associated with this in sickle cell disease?
Bone infarcts and poor splenic function make Salmonella a characteristic cause of osteomyelitis in sickle cell disease. N. meningitidis causes sepsis and meningitis, not typical osteomyelitis.
HbF can be measured by the alkali denaturation (Betke) method because HbF:
Adult hemoglobins are denatured and precipitated by alkali, while HbF stays in solution and is measured in the filtrate. Precipitation in reducing phosphate buffer is the principle of the sickle solubility test.
A patient on very high-dose intravenous penicillin develops IgG-mediated extravascular hemolysis. The eluate reacts only with penicillin-treated red cells. The mechanism is:
Penicillin binds firmly to the red cell membrane, and IgG anti-penicillin attaches to the coated cells, so the eluate reacts only with drug-treated cells. A true autoantibody, as with methyldopa, reacts with untreated cells.
According to CLSI guidance, uncentrifuged or centrifuged citrated whole blood for PT/INR, kept capped at room temperature, should be tested within:
PT samples are stable for up to 24 hours at room temperature. Samples for aPTT (non-heparinized) should be tested within 4 hours because factor VIII is labile.
The recommended first-line screening test for inherited antithrombin deficiency is:
A heparin-cofactor activity assay detects both type I (low amount) and type II (abnormal function) deficiency. Antigen alone misses type II deficiency.
A patient given a GP IIb/IIIa inhibitor such as eptifibatide during coronary stenting would show which aggregation pattern?
Blocking GP IIb/IIIa prevents fibrinogen bridging, giving a Glanzmann-like pattern; ristocetin agglutination via GP Ib–VWF is preserved. Aspirin affects mainly the arachidonic acid response.
Flow cytometry of platelets from a bleeding patient shows absent CD41 and CD61. The diagnosis is:
CD41 and CD61 are GP IIb and GP IIIa, which are missing in Glanzmann thrombasthenia. Bernard-Soulier syndrome lacks CD42a/CD42b (GP IX/GP Ib).
A patient has repeated poor 1-hour platelet increments after fresh ABO-compatible platelets, with no fever, sepsis or splenomegaly. The most common immune cause is antibodies to:
Immune platelet refractoriness is most often caused by HLA class I antibodies from earlier transfusion or pregnancy. HLA-matched or crossmatched platelets are used. HPA antibodies are less common.
In a patient with suspected HIT, the '4Ts' score is used to:
The 4Ts (thrombocytopenia, timing, thrombosis, other causes) estimate clinical probability. A low score has a high negative predictive value, and lab testing is directed at intermediate/high scores.
Zinc protoporphyrin (ZPP) in red cells rises when zinc is inserted into protoporphyrin instead of iron. ZPP is typically increased in:
ZPP rises when iron is unavailable (iron deficiency) or ferrochelatase is blocked (lead). It is usually normal in thalassemia trait, which helps separate it from iron deficiency.
Lead causes anemia mainly by inhibiting which two enzymes of heme synthesis?
Lead inhibits ALA dehydratase (raising urinary ALA) and ferrochelatase (raising ZPP). Heme oxygenase and biliverdin reductase act in heme breakdown, not synthesis.