Most fluid and electrolyte questions that feel unfair are questions about two electrolytes disguised as questions about one. The potassium that will not come up. The tetany in a patient whose calcium is normal. The sodium of 128 that nobody treats. Each has the same shape: one value is coupled to another, and the exam is checking whether you know the coupling. Here are the six that generate most of those questions, with the mechanism and the nursing move for each.
Why will potassium not correct until magnesium is replaced?
Magnesium sits on the ROMK channel in the kidney’s distal tubule and blocks it. When magnesium is low, the block comes off, the channel dumps potassium into the urine, and every dose you infuse leaves almost as fast as it goes in. That is refractory hypokalemia, and the fix is to replace magnesium first — the potassium often comes up on its own afterwards. The same deficiency suppresses parathyroid hormone, so calcium refuses to correct too. On a question, “low potassium and low magnesium” means magnesium is the priority replacement.
Try it · hang potassium, then magnesium
Mg²⁺ and K⁺ simulator
See the whole patient respond.
Click or hover any value to see why it changed.
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Bedside monitor · pattern
Hypomagnesemia
Low magnesium removes a membrane brake and unblocks renal potassium wasting, so potassium and calcium fall with it.
QTc 489 msFlattening T waves and an emerging U wave — the earliest ECG evidence that potassium is low.
Extracellular volume at baseline. The dashed line marks euvolemia.
What the nurse would notice
Hypomagnesemia across body systems
Prolonged QT, torsades de pointes, digoxin sensitivity
Tremor, cramps, tetany, hyperreflexia, Chvostek and Trousseau
Agitation, confusion; seizures when severe
Nausea, anorexia, decreased motility
Teaching model, not a clinical calculator. Patterns show high-yield directional relationships under the stated assumption. Real symptoms, ranges and treatment depend on cause, acuity, comorbidities and local protocols.
Signs & symptoms
Present on this panel. Tap any sign to see which values produce it.
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Now see if it holds up on exam questions
A free practice test puts fluids and electrolytes next to everything else the NCLEX asks, with a rationale on every answer.
Why do calcium and phosphate move in opposite directions?
They bind each other. When phosphate rises — classically in kidney failure — it complexes with calcium and pulls the ionized calcium down, so the symptoms of hyperphosphatemia are really hypocalcemia: tingling, cramps, tetany, a long QT. When their product (calcium times phosphate, both in mg/dL) climbs past about 55, the salts precipitate into vessels and soft tissue. The nursing logic follows: phosphate binders with meals, and caution about pushing calcium while the phosphate is still high.
How does pH move potassium?
Hydrogen ions and potassium trade places across the cell membrane. In acidosis, hydrogen moves into cells and potassium comes out, so the serum potassium rises — roughly half a point for every 0.1 fall in pH in a mineral acidosis, less predictably in DKA. In alkalosis the trade runs the other way and the potassium falls. Two exam consequences: the high potassium in DKA is a shifted potassium in a patient who is actually depleted, and it will crash the moment insulin starts; and bicarbonate, by raising the pH, lowers the potassium — which is why it is one of the shift treatments for hyperkalemia.
The acid–base side of this — why the pH moved in the first place, and how to read the gas — is its own simulator and guide: how to learn acid–base balance.
Why does alkalosis cause tetany when the calcium is normal?
About half of serum calcium rides on albumin, biologically inactive. Alkalosis increases that binding, so the ionized calcium — the only fraction the membrane feels — falls while the total calcium reads normal. The patient gets perioral tingling, a positive Trousseau sign and carpopedal spasm at a calcium of 9.5. Treat the alkalosis and order an ionized calcium; do not chase the total. The reverse also matters: acidosis frees calcium from albumin, so a low total calcium in an acidotic patient may be less dangerous than it looks.
Why is sodium a water problem?
Serum sodium is a concentration, so it reads low when there is too much water and high when there is too little, regardless of how much sodium the body holds. That single idea decides treatment. Hyponatremia with normal or high volume — SIADH, heart failure, thiazides — is treated by restricting water; hyponatremia with volume depletion is treated with isotonic saline. Hypernatremia is nearly always a water deficit and is treated with free water, slowly. And a high glucose dilutes the sodium about 1.6 mEq/L per 100 mg/dL above normal, so the sodium of 128 in a DKA patient with a glucose of 620 corrects to roughly 136 and needs no treatment at all.
Why does low potassium make digoxin dangerous?
Digoxin and potassium compete for the same binding site on the sodium–potassium pump. When potassium is low, digoxin binds more avidly, and toxicity — nausea, visual halos, bradycardia, new arrhythmias — appears at a therapeutic digoxin level. The nursing rule is mechanical: check the potassium before the dose, and treat a potassium under 3.5 in a digoxin patient as a hold-and-call finding.
How to make these permanent
Do not read them again. Break each one on the simulator: set magnesium to 1.0 and hang potassium; drag phosphate to 7 and watch calcium fall; drag the pH and watch potassium move the other way; drag bicarbonate up and watch the ionized calcium drop while the total sits still. Five minutes of that beats an hour of rereading — the evidence for why is in the best way to study for nursing school, and the whole unit’s method is in how to learn fluid and electrolytes.
Practise the two-electrolyte questions
Free account, no card. Questions built on exactly these couplings, with a rationale on every option — including the ones that look right.
Common questions
Why does hypomagnesemia cause hypokalemia?
Magnesium blocks the ROMK potassium channel in the distal nephron. When magnesium is low the block comes off, the kidney wastes potassium in the urine, and potassium replacement is refractory until magnesium is replaced first. Low magnesium also suppresses PTH, so calcium stays low too.
Why do calcium and phosphate have an inverse relationship?
They bind each other. When phosphate rises, typically in kidney failure, it complexes with calcium and lowers the ionized calcium, so the symptoms of hyperphosphatemia are those of hypocalcemia. When the calcium × phosphate product exceeds about 55, the salts precipitate into vessels and soft tissue.
How does pH affect potassium?
Hydrogen and potassium trade places across the cell membrane. Acidosis moves hydrogen into cells and potassium out, raising serum potassium roughly 0.5 mEq/L per 0.1 fall in pH in a mineral acidosis; alkalosis lowers it. Total-body potassium may be unchanged or depleted — as in DKA, where the high potassium crashes once insulin starts.
Why does alkalosis cause tetany when total calcium is normal?
Alkalosis increases calcium binding to albumin, so the ionized (active) calcium falls while the total reads normal. The patient gets perioral tingling, a positive Trousseau sign and carpopedal spasm. Treat the alkalosis and order an ionized calcium.