MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 27
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
A pregnant woman has anti-K. Her partner is typed as K-negative, and paternity is certain. What is the risk of HDFN due to anti-K?
If the father is K-negative, the fetus cannot inherit K and is not at risk. Testing the partner's phenotype is a simple first step in assessing risk.
A newborn has severe thrombocytopenia with intracranial bleeding in the first pregnancy. The mother's platelet count is normal. The most likely antibody is:
Fetal and neonatal alloimmune thrombocytopenia is most often due to maternal anti-HPA-1a. Unlike Rh HDFN, it often affects the first pregnancy. A normal maternal count argues against autoimmune ITP.
A patient with the Ko (Kell-null) phenotype has an antibody reacting with all panel cells. The antibody is most likely:
Ko cells lack all Kell antigens, and immunized people make anti-Ku (anti-KEL5), which reacts with all cells except Ko. Kx is present on Ko cells.
The D type of a newborn of a D-negative mother is tested with a method that detects weak D. The purpose is to:
A weak D baby can immunize a D-negative mother, so weak D is tested to decide on postpartum RhIG.
A group O mother received antenatal RhIG at 28 weeks. Her group O, D-positive baby has a weakly positive DAT with anti-IgG and no hemolysis. The most likely cause is:
IgG anti-D from antenatal RhIG can cross the placenta and weakly coat D-positive fetal cells. ABO HDFN is excluded because mother and baby are both group O.
A patient with metabolic alkalosis has urine chloride of 8 mmol/L. Which cause is most likely?
Low urine chloride (below about 20 mmol/L) shows chloride and volume depletion, as in vomiting; this alkalosis corrects with saline. Mineralocorticoid excess and Bartter syndrome give high urine chloride.
A healthy woman in the third trimester of pregnancy has a blood gas. Which finding is normal for this state?
Progesterone stimulates breathing, lowering pCO2 to about 30 mmHg; the kidneys compensate by lowering HCO3−, so pH is near normal or slightly high. Respiratory acidosis is not normal in pregnancy.
Standard bicarbonate is defined as the plasma bicarbonate concentration:
Standard bicarbonate removes the respiratory influence by equilibrating blood to pCO2 40 mmHg at 37 °C, so a change reflects the metabolic component. Actual bicarbonate is at the patient's own pCO2.
Calculate HOMA-IR for fasting glucose 5.0 mmol/L and fasting insulin 18 µU/mL. [HOMA-IR = glucose (mmol/L) × insulin (µU/mL) ÷ 22.5]
HOMA-IR = 5.0 × 18 ÷ 22.5 = 90 ÷ 22.5 = 4.0. Higher values indicate greater insulin resistance; cut-offs vary between populations.
A 45-year-old with resistant hypertension has K 3.0 mmol/L and bicarbonate 32 mmol/L. Which is the recommended screening test for the suspected cause?
Hypertension with hypokalemia and metabolic alkalosis suggests primary aldosteronism; a high aldosterone with suppressed renin gives a raised aldosterone-to-renin ratio. Metanephrines screen for pheochromocytoma.
A patient with diabetic ketoacidosis has K 5.2 mmol/L on admission. After IV insulin, K falls to 3.1 mmol/L. The main reason is:
Insulin stimulates Na/K-ATPase, moving potassium into cells; correcting acidosis adds to this shift. Total body K is usually low in DKA, so plasma K falls quickly once insulin is given.
Two days after total thyroidectomy, a patient has tingling around the mouth. Which laboratory pattern is expected?
Accidental removal or injury of the parathyroid glands causes hypoparathyroidism: low PTH, low calcium and high phosphate because PTH normally promotes phosphate excretion. High PTH with low calcium suggests secondary hyperparathyroidism.
Long-term use of which drug class is well known to cause hypomagnesemia?
Proton pump inhibitors can reduce intestinal magnesium absorption and cause severe hypomagnesemia after months of use. Potassium-sparing diuretics tend to conserve magnesium.
A severely malnourished patient is started on high-calorie feeding. Two days later there is muscle weakness. Which electrolyte fall is most characteristic of this refeeding syndrome?
Insulin released with carbohydrate feeding drives phosphate (with K and Mg) into cells for ATP synthesis, causing marked hypophosphatemia. It is the hallmark lab finding of refeeding syndrome.
A patient with a psychiatric illness has serum Na 124 mmol/L and urine osmolality 70 mOsm/kg. The most likely cause is:
Very dilute urine (below about 100 mOsm/kg) shows ADH is properly suppressed, so hyponatremia results from excess water intake. In SIADH urine is inappropriately concentrated.
In coulometric-amperometric titration of chloride, the endpoint is detected when:
Silver ions are generated at a constant rate and precipitate chloride as AgCl; when all chloride is used, free Ag+ appears and the indicator current rises, stopping the timer. Time is proportional to chloride.
Why are kinetic (multipoint rate) assays preferred over single fixed-time assays for enzyme activity?
Multiple readings allow the analyzer to confirm a constant (zero-order) rate and flag lag phase or substrate depletion. A fixed-time assay assumes linearity without checking it.
What is the ESC-recommended NT-proBNP rule-out cut-off for acute heart failure in a breathless patient in the emergency department?
An NT-proBNP below 300 pg/mL makes acute heart failure unlikely. Higher, age-related values (450, 900, 1800 pg/mL) are used to rule in. The non-acute rule-out value is lower, 125 pg/mL.
Serum tartrate-resistant acid phosphatase isoform 5b (TRAP 5b) is a marker of:
TRAP 5b is secreted by osteoclasts and reflects bone resorption. Prostatic acid phosphatase is inhibited by tartrate. Bone ALP is a marker of osteoblast (formation) activity.
Serum triglycerides are 3000 mg/dL (34 mmol/L). Sodium is 124 mmol/L by indirect ISE and 139 mmol/L by direct ISE on a blood gas analyzer. The best explanation is:
Indirect ISE dilutes the sample and assumes normal plasma water; large lipid volumes reduce the water fraction, causing pseudohyponatremia. Direct ISE measures activity in plasma water and is not affected.