MLT GlobeQCHP exam preparation (Qatar) – page 27
749 practice MCQs for the QCHP medical laboratory exam. Level: Intermediate.
Bg (Bennett-Goodspeed) antigens on red cells are:
Bga, Bgb and Bgc correspond to HLA-B7, B17 and A28. Their antibodies are usually weak and rarely clinically significant.
Anti-Lea rarely destroys transfused red cells mainly because:
Lewis antigens are adsorbed from plasma; soluble Lewis substance in transfused plasma neutralizes the antibody, and donor cells shed Lewis antigens in the recipient.
Immune anti-D is mainly of which IgG subclasses?
Anti-D is mostly IgG1 and IgG3, which bind Fc receptors on macrophages well, causing extravascular hemolysis.
In people of European descent, the most common Rh haplotype is:
R1 is found in about 42% of European haplotypes, followed by r (about 37%). R0 is most common in people of African descent.
A D-negative woman delivered a D-positive baby 5 days ago but did not receive RhIG. What is recommended?
RhIG is best given within 72 hours, but if missed it should still be given as soon as possible, since some protection may occur up to 28 days.
Which organism group most often contaminates platelet units and causes septic transfusion reactions?
Platelets are stored at room temperature, so skin flora such as Staphylococcus species (and other Gram-positive cocci) grow well. Yersinia is typically linked to refrigerated red cells.
In a person with normal kidneys, glucose usually appears in the urine when plasma glucose exceeds about:
The renal threshold for glucose is about 160–180 mg/dL (9–10 mmol/L); above this the tubular reabsorption capacity is exceeded and glycosuria occurs.
Why is C-peptide, rather than insulin, used to assess the body's own insulin secretion in a patient treated with insulin injections?
C-peptide is released in equal amounts with insulin from proinsulin, but commercial insulin lacks it, so C-peptide reflects endogenous secretion only. It is cleared by the kidney and rises in renal failure.
A diabetic patient with nephrotic syndrome (albumin 2.0 g/dL, 20 g/L) has a low fructosamine that does not match high home glucose readings. Why?
Fructosamine measures glycated serum proteins, mostly albumin. Low albumin or fast protein turnover gives falsely low results. Fructosamine reflects the previous 2–3 weeks, not 3 months.
In a polarographic glucose oxidase method, the glucose concentration is proportional to:
Glucose oxidase uses oxygen to oxidize glucose; an oxygen electrode measures how fast O2 is consumed. The quinoneimine dye is the end point of the colorimetric (Trinder) version.
A patient with polyuria has no rise in urine osmolality after water deprivation. After desmopressin, urine osmolality still does not rise. This is most consistent with:
In nephrogenic DI the kidney does not respond to ADH, so neither dehydration nor desmopressin concentrates the urine. In central DI, urine osmolality rises clearly after desmopressin.
As blood passes through tissue capillaries, bicarbonate leaves the red cells. To keep electrical neutrality, which ion moves into the red cells?
This is the chloride (Hamburger) shift: bicarbonate formed by carbonic anhydrase moves out of the RBC and chloride moves in, which is why venous RBCs contain more chloride.
Why are ionized calcium samples collected in syringes with "balanced" (calcium-titrated) heparin?
Unbalanced heparin binds free calcium, especially at high heparin concentration, causing falsely low ionized calcium. CO2 loss is prevented by anaerobic handling, not by the type of heparin.
A woman with pre-eclampsia on IV magnesium sulfate develops loss of deep tendon reflexes and slow breathing. Which result is most likely?
Magnesium sulfate therapy can cause magnesium toxicity; loss of reflexes and respiratory depression appear as serum Mg rises well above the therapeutic range. Serum Mg should be checked urgently.
A serum tube for electrolytes is only one-quarter filled and left uncapped before analysis. Which result is most likely falsely low?
CO2 escapes from the sample into the air space of an underfilled or uncapped tube, falsely lowering total CO2 and falsely raising the calculated anion gap. Sodium and chloride are not volatile.
A 2-year-old has an abdominal mass. Which urine tests are most useful to support neuroblastoma?
Most neuroblastomas secrete catecholamines, so urine HVA and VMA (expressed per creatinine) are raised. 5-HIAA is used for carcinoid (serotonin-secreting) tumors.
A patient is collecting a 24-hour urine for 5-HIAA. Which instruction is most important?
These foods are rich in serotonin and can falsely raise urine 5-HIAA. The test screens for serotonin-producing neuroendocrine (carcinoid) tumors.
A patient with small cell lung cancer has sodium 122 mmol/L, serum osmolality 255 mOsm/kg, urine osmolality 480 mOsm/kg and urine sodium 50 mmol/L. He is euvolemic. The most likely diagnosis is:
Low serum osmolality with inappropriately concentrated urine and high urine sodium in a euvolemic patient fits SIADH. Primary polydipsia gives dilute urine (< 100 mOsm/kg).
Which single test is best for screening a patient suspected of acromegaly?
IGF-1 reflects integrated GH secretion and is stable through the day. Random GH is pulsatile and unreliable; failure of GH suppression during an OGTT is used to confirm.
A tall young man with small testes and infertility has high LH and FSH and low testosterone. Which karyotype is most likely?
Klinefelter syndrome (47,XXY) causes primary testicular failure: low testosterone with high gonadotropins. Kallmann syndrome has low LH and FSH.