MLT GlobeChemistry: Electrolytes & osmolality – page 4
77 Chemistry MCQs on Electrolytes & osmolality with answers and explanations.
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.
In the enzymatic method for serum bicarbonate (total CO2), which reaction is measured?
PEP carboxylase fixes HCO3− to form oxaloacetate, which malate dehydrogenase reduces while oxidizing NADH; the fall at 340 nm is proportional to bicarbonate. Hexokinase is used for glucose.
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 patient taking a loop diuretic (furosemide) for heart failure has serum potassium of 3.0 mmol/L. The most likely cause is:
Loop diuretics increase sodium delivery to the distal nephron, promoting renal potassium excretion and hypokalemia. Hemolysis and EDTA contamination cause falsely high potassium.
An elderly patient with poor fluid intake has serum sodium of 156 mmol/L. This is best described as:
A sodium above 145 mmol/L is hypernatremia. In elderly patients with reduced thirst or access to water, loss of water relative to sodium is the usual cause.
Blood is drawn from the same arm as a running 5% dextrose infusion. Which result is most likely to be falsely increased?
Contamination with dextrose infusion fluid greatly increases glucose and dilutes other analytes. Samples should be drawn from the opposite arm or below the infusion site.
Na 138, K 4.0, Cl 104, HCO3 24 mmol/L, albumin 20 g/L (2.0 g/dL). The anion gap without K is 10 mmol/L. What is the albumin-corrected anion gap?
Corrected AG = AG + 2.5 × (40 − albumin g/L)/10 = 10 + 2.5 × 2 = 15 mmol/L. Low albumin lowers the measured gap and can hide a high-anion-gap acidosis.
An unexpectedly low or negative anion gap may be caused by:
Cationic IgG paraproteins add unmeasured cations and lower the gap; hypoalbuminemia and bromide interference also do so. The other options raise the anion gap.
Why may a vapor pressure osmometer miss a raised osmolal gap in methanol poisoning?
Volatile compounds like methanol and ethanol escape into the vapor phase and are not counted by vapor pressure osmometry. Freezing point depression osmometry detects them.
Total calcium is 1.90 mmol/L (7.6 mg/dL) and albumin 25 g/L (2.5 g/dL). Using the common correction formula, the adjusted calcium is about:
Adjusted Ca (mg/dL) = measured + 0.8 × (4.0 − albumin g/dL) = 7.6 + 0.8 × 1.5 = 8.8 mg/dL (2.20 mmol/L). Ionised calcium measurement is preferred when accuracy matters.
In the ammonium molybdate method for inorganic phosphate, the unreduced phosphomolybdate complex is measured at:
Many automated methods read the unreduced ammonium phosphomolybdate complex in the UV at 340 nm. Reduction to molybdenum blue (read near 600–700 nm) is an older alternative.
Chloride measured with an ISE may be falsely increased in a patient taking which substance?
Chloride electrodes respond to other halides; bromide and iodide give positive interference and a falsely low anion gap. Lithium does not affect chloride ISEs.
Which finding suggests that hyponatremia is due to hypothyroidism or adrenal insufficiency rather than a laboratory artefact?
True hypo-osmolar hyponatremia shows low measured osmolality. Pseudohyponatremia from lipid or protein excess has normal measured osmolality because osmolality depends on plasma water solutes.
A patient has normal anion gap metabolic acidosis. Urine Na + K − Cl (urine anion gap) is clearly negative. This suggests:
A negative urine anion gap indicates high urinary ammonium (as NH4Cl), the normal kidney response to diarrhea. In distal RTA the kidney cannot excrete ammonium, so the urine anion gap is positive.
In the o-cresolphthalein complexone method for calcium, 8-hydroxyquinoline is added to the reagent to:
o-Cresolphthalein complexone also binds magnesium; 8-hydroxyquinoline chelates Mg so that only calcium forms the red complex. The alkaline pH is provided by a separate buffer.
A freezing point osmometer reads a sample freezing point of −0.558 °C. Given 1 Osm/kg water lowers the freezing point by 1.86 °C, the osmolality is:
Osmolality = 0.558 ÷ 1.86 = 0.300 Osm/kg = 300 mOsm/kg. Reading the freezing point value directly as 558 ignores the 1.86 °C constant.
A patient with chronic lymphocytic leukemia (WBC 250 × 10^9/L) has K 2.9 mmol/L on a sample left at room temperature for 6 hours; a promptly separated sample shows K 4.1 mmol/L. The low result is due to:
Large numbers of active leukocytes continue to take up potassium in vitro, causing pseudohypokalemia, especially at warm temperatures. Hemolysis would raise, not lower, potassium.