MLT GlobeChemistry: Electrolytes & osmolality
77 Chemistry MCQs on Electrolytes & osmolality with answers and explanations.
Compared with normal plasma, which intravenous solution is hypertonic?
3% saline has roughly three times the osmolality of plasma. 0.9% saline is isotonic, while 0.45% saline and water are hypotonic and would cause red cells to swell.
In osmosis across a semipermeable membrane, what moves and in which direction?
Osmosis is net water movement toward higher solute concentration, since solute cannot cross. Solute movement down its gradient is ordinary diffusion.
Red cells placed in a solution swell and lyse. Relative to the cells, the solution is:
Water enters cells when the outside solution has lower effective osmolality, causing haemolysis. Isotonic solutions cause no change; hypertonic ones, including concentrated saline, shrink (crenate) cells.
Which of these is a trace element (daily requirement well below 100 mg)?
Copper is needed in milligram amounts, for example in ceruloplasmin and cytochrome oxidase. Calcium, phosphorus and magnesium are macrominerals needed in hundreds of milligrams daily.
The Na+/K+-ATPase is best classified as a(n):
It uses ATP directly to pump 3 Na+ out and 2 K+ in, i.e. two ions in opposite directions. Symporters move solutes the same way; channels allow passive flow.
Na 140 mmol/L, Cl 100 mmol/L and HCO3 10 mmol/L. What is the anion gap (without K)?
Anion gap = Na - (Cl + HCO3) = 140 - (100 + 10) = 30 mmol/L. This is high (usual range about 8-16) and suggests a high anion gap acidosis such as DKA, lactic acidosis, renal failure or poisoning.
A potassium of 6.9 mmol/L is reported on a visibly hemolysed sample from a well patient with a normal ECG. The most likely cause is:
Red cells contain far more potassium than plasma, so hemolysis falsely raises the result (pseudohyperkalemia). The sample should be recollected before acting on the value.
Which form of serum calcium is physiologically active?
About 45-50% of serum calcium is ionized and biologically active; roughly 40% is bound to albumin and the rest to anions. Ionized calcium is not affected by albumin levels, unlike total calcium.
The sodium ion-selective electrode most commonly uses a membrane made of:
Sodium ISEs use aluminium silicate glass membranes (or ionophores in some systems). Valinomycin is the ionophore used for potassium electrodes.
In a potassium ion-selective electrode, valinomycin acts as:
Valinomycin is a neutral carrier that selectively binds potassium in the membrane, producing a potential related to K+ activity. The reference electrode provides a constant potential for comparison.
A blood sample for electrolytes was stored refrigerated overnight before separation. Which result is expected?
Cold inhibits Na+/K+-ATPase, so potassium leaks out of cells into plasma. Glucose falls with storage, and phosphate tends to rise with cell leakage.
Repeated fist clenching during venepuncture with a tight tourniquet most likely causes:
Forearm muscle activity releases potassium locally, raising results by up to 1–2 mmol/L. This is a common preanalytical cause of pseudohyperkalemia.
Which condition causes a high anion gap metabolic acidosis?
Ketoacids (beta-hydroxybutyrate, acetoacetate) are unmeasured anions that raise the anion gap. Diarrhoea, RTA and carbonic anhydrase inhibitors cause normal-gap (hyperchloremic) acidosis.
Which method measures osmolality most commonly in clinical laboratories?
Freezing point depression osmometers are most widely used and detect volatile solutes such as ethanol and methanol. Vapor pressure osmometers do not detect volatile alcohols.
In central diabetes insipidus, which laboratory pattern is typical?
Lack of ADH prevents water reabsorption, producing large volumes of dilute urine, with rising plasma sodium and osmolality if water intake is limited.
Which specimen problem gives a falsely high serum phosphate?
Red cells contain more phosphate than plasma, so hemolysis and delayed separation raise results. Marked thrombocytosis can also raise serum phosphate; fasting, heparin plasma and prompt separation do not cause false elevation.
The main form in which total CO2 exists in serum, and is measured as 'bicarbonate' on chemistry analysers, is:
About 90–95% of total CO2 in plasma is bicarbonate ion; dissolved CO2 and carbonic acid make up a small part. Tubes left uncapped lose CO2, giving falsely low results.
Which ion is the main cation inside body cells?
About 98% of body potassium is intracellular, where its concentration is near 150 mmol/L. Sodium is the main extracellular cation.
Aldosterone acts on the renal collecting duct mainly to:
Aldosterone increases sodium channels and Na/K-ATPase activity in principal cells, so sodium is retained and potassium (and H+) is lost. Free-water reabsorption is the action of ADH.
Which dye is commonly used in automated photometric methods for total serum calcium?
Arsenazo III (and o-cresolphthalein complexone) forms colored complexes with calcium. Bromocresol green is used for albumin and ferrozine for iron.