MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 21
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
A non-diabetic nurse has glucose 38 mg/dL (2.1 mmol/L), very high insulin and very low C-peptide. The most likely cause is:
Pharmaceutical insulin contains no C-peptide and suppresses the patient's own secretion, so insulin is high and C-peptide low. Insulinoma and sulfonylureas raise both insulin and C-peptide.
Which condition can cause pseudohyponatremia when sodium is measured by indirect ISE?
High paraprotein reduces the plasma water fraction and causes a falsely low sodium by indirect ISE. Hyperglycemia causes true (translocational) hyponatremia, not a measurement artefact.
A patient with SIADH is expected to have:
In SIADH, ADH excess causes water retention, hyponatremia and low plasma osmolality, while urine osmolality stays inappropriately high (often >100 mOsm/kg) with ongoing urine sodium excretion.
Serum sodium is 125 mmol/L with serum osmolality 300 mOsm/kg. This hyperosmolar hyponatremia is most likely due to:
Glucose draws water out of cells, diluting sodium while raising osmolality. SIADH, hypothyroidism and water intoxication give hypo-osmolar hyponatremia.
A patient has low TSH and low free T4 with symptoms of hypothyroidism. The most likely cause is:
In central hypothyroidism the pituitary fails to produce TSH, so both TSH and free T4 are low or inappropriately normal TSH is seen. Hashimoto thyroiditis gives high TSH.
Late-night salivary cortisol is used as a screening test for Cushing syndrome because:
Normal cortisol is lowest around midnight; loss of this circadian nadir is sensitive for Cushing syndrome. Salivary cortisol reflects free, not bound, cortisol.
A patient taking high-dose biotin supplements has a sandwich (two-site) immunoassay using streptavidin–biotin capture. The result is expected to be:
Excess biotin blocks streptavidin binding, so less labelled complex is captured, giving falsely low sandwich assay results (e.g. TSH). In competitive assays (e.g. free T4) results are falsely high.
Predominance of LD-5 in serum is most consistent with damage to:
LD-5 (M4) is the main isoenzyme in liver and skeletal muscle. Heart and red cells are rich in LD-1 and LD-2.
For a troponin assay to be called 'high-sensitivity', it must have which analytical performance at the 99th percentile upper reference limit?
High-sensitivity assays have a CV ≤10% at the 99th percentile and measure troponin in at least half of healthy people. Detection in healthy people is expected, not zero.
The Friedewald equation should not be used to calculate LDL cholesterol when triglycerides exceed:
Above 400 mg/dL the TG/VLDL-cholesterol ratio is no longer about 5:1, so the estimate is unreliable; direct LDL or another equation is used. 150 mg/dL is only the desirable TG limit.
A routine non-fasting lipid profile shows triglycerides of 520 mg/dL (5.9 mmol/L). What is the best next step?
Non-fasting samples are acceptable for routine screening, but when non-fasting triglycerides exceed 400 mg/dL a fasting profile is recommended. Friedewald is not valid at this level.
In HDL cholesterol precipitation methods, phosphotungstate with magnesium is added to:
Polyanions with divalent cations precipitate VLDL, LDL and Lp(a); cholesterol left in the supernatant is HDL cholesterol. HDL itself is not precipitated.
A young adult has mild conjugated hyperbilirubinemia with normal liver enzymes. Liver biopsy shows dark black-brown pigment in hepatocytes. The most likely diagnosis is:
Dubin–Johnson syndrome is a defect of the canalicular transporter MRP2, causing conjugated hyperbilirubinemia and a dark liver pigment. Rotor syndrome looks similar but without pigment.
In a patient with severe inflammation and low albumin, bromocresol green (BCG) gives a higher albumin result than bromocresol purple (BCP). The main reason is that BCG:
BCG binds some non-albumin proteins (acute-phase alpha and beta globulins), so it overestimates albumin, especially when albumin is low; short reading times reduce this. BCP is more specific for albumin.
When urine containing Bence Jones protein is heated, the protein typically:
Free light chains show this unusual heat behaviour: they come out of solution at 40–60 °C and go back into solution near boiling. Precipitation in the cold describes cryoglobulins, not Bence Jones protein.
A patient with cadmium exposure has increased urine beta-2 microglobulin with little albumin. This pattern indicates:
Low-molecular-weight proteins like beta-2 microglobulin are normally filtered and reabsorbed by proximal tubules; tubular damage lets them escape into urine. Glomerular damage mainly causes albuminuria.
A creatinine clearance is reported as 25 mL/min in a healthy young athlete with normal serum creatinine. The most likely cause is:
An under-collected urine lowers the urine creatinine total and falsely reduces clearance. Checking 24-hour creatinine excretion against expected output (based on body weight) detects this. High muscle mass would not lower clearance.
Which variables are required for the 2021 CKD-EPI creatinine equation?
The 2021 CKD-EPI creatinine equation uses serum creatinine, age and sex; the race coefficient was removed. Weight is used by Cockcroft-Gault, not CKD-EPI.
Cystatin C is useful for estimating GFR mainly because it:
Cystatin C is made by all nucleated cells at a fairly constant rate and is freely filtered, so it is less affected by muscle mass than creatinine. However, it is affected by thyroid status and corticosteroids.
According to KDIGO, a urine albumin-to-creatinine ratio (ACR) of 45 mg/g (4.5 mg/mmol) is classified as:
KDIGO categories: A1 <30 mg/g (<3 mg/mmol), A2 30–300 mg/g (3–30 mg/mmol), A3 >300 mg/g (>30 mg/mmol). 45 mg/g falls in A2, formerly called microalbuminuria.