Every practice question answered, with the reasoning shown.
This is your practice set with every answer written in — correct option first, then why each other option is wrong. Tap "Hide answers" to quiz yourself, and tap "Show the answer" on any question to check your reasoning.
Ask yourself these five questions in this order on every electrolyte item. The answer to question 5 is almost always the correct option.
🤖 Ask Claire: "Give me an electrolyte value and one sentence of assessment data. Make me say high or low, which system, and what my first action is — then tell me if I picked the right priority."
You cannot label a value high or low if you do not know the range. Every question below starts here.
| Lab | Normal range (with units) | The number that should scare you |
|---|---|---|
| Sodium (Na⁺) | 135–145 mEq/L | <120 or >160 — seizure and coma range either way |
| Potassium (K⁺) | 3.5–5.0 mEq/L | <2.5 or >6.5 — and any level with ECG changes |
| Calcium, total | 9.0–10.5 mg/dL | <7 (tetany, laryngospasm) or >13 (dysrhythmias, coma) |
| Calcium, ionized | 4.5–5.6 mg/dL — the active form; this is the one that causes symptoms | Falls when albumin is low or when the patient is alkalotic |
| Magnesium (Mg²⁺) | 1.3–2.1 mEq/L (about 1.8–2.6 mg/dL) | >4 — reflexes disappear, then respirations do |
| Phosphorus | 3.0–4.5 mg/dL | Moves opposite calcium — high phos usually means low calcium |
| Chloride (Cl⁻) | 98–106 mEq/L | Tracks sodium and acid–base; low Cl⁻ with vomiting = metabolic alkalosis |
| Serum osmolality | 275–295 mOsm/kg | High = cells are drying out · Low = cells are swelling |
Memory hook: 135–145 sodium, 3.5–5.0 potassium, 9–10.5 calcium, 1.3–2.1 magnesium. Say them as one string until they are automatic.
The whole test is this table. Almost every stem gives you signs and asks you to name the imbalance, or gives you the imbalance and asks what you will see.
| Hyponatremia <135 | Hypernatremia >145 | |
|---|---|---|
| What water does | Water moves INTO the cell → cells swell → brain swells | Water pulled OUT of the cell → cells shrink → brain shrivels |
| Signs | Headache, confusion, lethargy, muscle cramps, nausea, seizures, coma | Intense thirst, dry sticky mucous membranes, flushed skin, restlessness → agitation, fever, weakness, seizures if severe |
| Usual cause | Too much water — SIADH, excess D5W or hypotonic fluid, heart or liver failure, water intoxication, diuretics | Too little water — dehydration, fever, tube feeds without free water, diabetes insipidus, no access to fluids |
| Priority | Neuro checks + seizure precautions. Fluid restriction if dilutional; 3% saline only if severe and symptomatic | Give free water — PO water or hypotonic IV fluid slowly; correcting too fast causes cerebral edema |
| Hypokalemia <3.5 | Hyperkalemia >5.0 | |
|---|---|---|
| Muscle | Weak, flabby, hypoactive — weakness, leg cramps, decreased reflexes, paralytic ileus (silent belly) | Early twitching and cramping, then weakness and flaccid paralysis; hyperactive bowel → diarrhea |
| ECG | Flattened / inverted T waves, ST depression, prominent U waves, PVCs | Tall peaked T waves → flat/absent P → widened QRS → sine wave → VF or asystole |
| Usual cause | Vomiting, diarrhea, NG suction, loop and thiazide diuretics, corticosteroids, alkalosis, insulin | Kidney failure, K⁺-sparing diuretics, ACE inhibitors/ARBs, acidosis, crush injury/burns, blood transfusions, salt substitutes |
| Priority | Replace K⁺ — never IV push, never as a bolus; always diluted, on a pump, max about 10 mEq/hr on a general unit. Check urine output first and check magnesium — low Mg blocks K⁺ correction | ECG / telemetry first. Then calcium gluconate to protect the heart → insulin + D50 or albuterol to shift → binder or dialysis to remove |
| Hypocalcemia <9.0 | Hypercalcemia >10.5 | |
|---|---|---|
| Nerve/muscle | Everything is turned UP. Tingling around the mouth and fingertips, muscle twitching, cramps, tetany, + Chvostek and + Trousseau, seizures, laryngospasm | Everything is turned DOWN. Lethargy, confusion, profound muscle weakness, decreased deep tendon reflexes, constipation, bone pain, kidney stones |
| ECG | Prolonged ST and prolonged QT → risk of torsades | Shortened QT, bradycardia, heart block |
| Usual cause | Thyroid/parathyroid surgery, low vitamin D, low albumin, pancreatitis, low magnesium, massive transfusion (citrate), alkalosis | Hyperparathyroidism, malignancy with bone metastases, prolonged immobility, thiazides, excess calcium or vitamin D intake |
| Priority | Seizure precautions and airway at the bedside — laryngospasm is the killer. IV calcium gluconate for severe/symptomatic | Isotonic IV fluid to dilute and flush, mobilize the patient, fall precautions, telemetry, strain urine for stones |
| Hypomagnesemia <1.3 | Hypermagnesemia >2.1 | |
|---|---|---|
| Picture | Looks exactly like low calcium — hyperactive reflexes, tremor, twitching, + Chvostek and + Trousseau, tetany, seizures, torsades | Looks sedated — flushing, warmth, hypotension, drowsiness, diminished then absent deep tendon reflexes, shallow respirations, bradycardia, cardiac arrest |
| Usual cause | Alcohol use disorder, malnutrition, diarrhea, NG suction, loop diuretics, PPIs | Renal failure (cannot excrete it), magnesium-containing antacids and laxatives, IV magnesium for preeclampsia |
| Priority | Seizure precautions; replace magnesium and recheck potassium and calcium — they will not correct until magnesium does | Check deep tendon reflexes and respiratory rate every hour — reflexes disappear before breathing does, so they are your early warning. Stop all Mg sources. Antidote = IV calcium gluconate |
The one-line version. If a stem gives you one of these words, it is telling you the answer.
| Electrolyte | LOW | HIGH |
|---|---|---|
| Sodium | Swollen brain — confused, headache, seizing | Dry brain — thirsty, sticky mouth, restless |
| Potassium | Weak and sluggish — flat T waves, U waves, silent bowel | Deadly heart — peaked T waves, wide QRS, arrest |
| Calcium | Twitchy — tingling lips, tetany, Chvostek, Trousseau | Sedated — weak, constipated, stones, bones, groans |
| Magnesium | Hyperactive — tremors, brisk reflexes, seizures (calcium's twin) | Depressed — no reflexes, shallow breathing, low BP |
| If the stem says… | Think… |
|---|---|
| Peaked T waves | Hyperkalemia — get the ECG and call, this is the emergency |
| Flat T waves + U wave | Hypokalemia |
| Numbness/tingling around the mouth | Hypocalcemia (or hypomagnesemia — they mimic each other) |
| Chvostek (cheek tap → face twitch) or Trousseau (BP cuff → hand spasm) | Hypocalcemia — neuromuscular irritability |
| Absent deep tendon reflexes | Hypermagnesemia — check the respiratory rate next |
| New confusion + low sodium | Cerebral edema — neuro checks, seizure precautions, this patient is seen first |
| Furosemide, thiazide, vomiting, diarrhea, NG suction | Hypokalemia (plus low magnesium and low chloride) |
| Kidney failure / dialysis patient | Hyperkalemia, hypermagnesemia, hyperphosphatemia, hypocalcemia |
| Spironolactone, ACE inhibitor, ARB, salt substitute | Hyperkalemia — teach to avoid high-potassium foods |
| Thyroidectomy / parathyroid surgery | Hypocalcemia — tingling and tetany, keep an airway at the bedside |
| Cancer with bone metastasis, immobility | Hypercalcemia — weakness, constipation, stones |
| Alcohol use disorder, malnutrition | Hypomagnesemia — and the potassium will not correct until you fix it |
When two patients both have an abnormal electrolyte, this is how you choose. Assess the one whose organ can stop in the next few minutes.
Never choose the mildly abnormal lab with a comfort symptom over a near-normal lab with a lethal rhythm change. The ECG change outranks the number every single time.
Answer: 2 — confusion. Sodium 124 mEq/L is well below 135. Water follows the low sodium into the brain cells, they swell, and intracranial pressure rises — so confusion is the first sign that the brain is in trouble and the last warning before seizures and coma.
Why the others are wrong:
1. Dry skin points to fluid volume deficit or high sodium, not low, and dry skin never killed anyone.
3. Constipation belongs to hypercalcemia and hypokalemia, and it is a comfort problem, not an emergency.
4. Bradycardia is the cardiac slowing you see with hyperkalemia, hypercalcemia, and hypermagnesemia — low sodium acts on the brain, not the conduction system.
Answer: FALSE. Hypernatremia is usually caused by too little water, not too much salt. The sodium looks high because the water it was dissolved in is gone. Think fever, dehydration, diarrhea, diabetes insipidus, tube feedings without free-water flushes, and older adults who cannot get themselves a drink.
Why "true" is wrong: too much water does the opposite — it dilutes the sodium and causes hyponatremia (SIADH, water intoxication, too much D5W). High sodium = dry. Low sodium = waterlogged.
Answer: 3 — ECG rhythm. Potassium controls how cardiac cells repolarize, so a high level can flip the heart into ventricular fibrillation or asystole with almost no warning. Put the patient on telemetry and get a 12-lead: peaked T waves → flattened P waves → widened QRS is the countdown.
Why the others are wrong:
1. Skin turgor assesses hydration — useful data, zero urgency.
2. Lung sounds screen for fluid overload; potassium does not cause crackles.
4. Hyperkalemia does cause hyperactive bowel sounds and diarrhea, and that is a real finding — but a noisy belly is not what kills this patient.
Answer: 1 — diarrhea and muscle weakness. GI losses are the most common cause of hypokalemia — stool is potassium-rich. Low potassium makes muscle cells harder to depolarize, so the patient is weak and flabby with sluggish reflexes and a quiet abdomen. Cause plus symptom in one option is the giveaway.
Why the others are wrong:
2. Peaked T waves are the signature of hyperkalemia — the exact opposite. Hypokalemia flattens the T wave and adds a U wave.
3. Bradycardia with absent deep tendon reflexes is hypermagnesemia — the sedated picture.
4. Hypertension with edema describes fluid volume excess. It says nothing about potassium, and the swelling and high pressure point you toward sodium and water, not K⁺.
Answer: TRUE. Tall, narrow, peaked (tented) T waves are the earliest ECG change in hyperkalemia, because excess potassium speeds up repolarization. As the level climbs the P wave flattens and disappears, the QRS widens into a sine wave, and the rhythm degenerates into VF or asystole.
Why "false" is wrong: the only other T-wave pattern you need is the opposite one — flat or inverted T waves with a U wave means hypokalemia. Peaked = high, flat = low.
Answer: 2 — dry mucous membranes and thirst. Sodium 154 mEq/L pulls water out of the cells into the bloodstream, so the cells are dehydrated. Thirst is the body's first defense, and dry sticky mucous membranes are the most reliable bedside sign of a true water deficit.
Why the others are wrong:
1. Cerebral edema is a hyponatremia problem — cells swell when sodium is low. High sodium shrinks brain cells. (Hypernatremia can still seize, but the mechanism is shrinkage or over-rapid correction, not edema.)
3. Tetany is hypocalcemia (or hypomagnesemia).
4. Bounding pulses mean fluid volume excess; this patient is volume depleted, so expect weak, thready pulses and tachycardia.
Answer: 3 — hypocalcemia. Calcium is what keeps nerve membranes calm. When it drops, the threshold for firing drops with it, so nerves fire spontaneously → perioral tingling, twitching, carpopedal spasm, tetany, laryngospasm, and seizures. Chvostek's and Trousseau's signs come from the same mechanism.
Why the others are wrong:
1. Hypercalcemia does the reverse — it sedates. Expect lethargy, weakness, and decreased reflexes.
2. Hypermagnesemia is also a depressant: absent reflexes, shallow breathing, hypotension. (It is hypomagnesemia that seizes.)
4. Hypernatremia causes thirst, restlessness, and agitation; severe cases can seize, but tetany is not its picture.
Answer: TRUE. Low calcium = twitchy. Calcium stabilizes the nerve cell membrane, so less calcium means the nerve fires more easily. That is why you get tingling, cramps, hyperactive deep tendon reflexes, tetany, positive Chvostek and Trousseau signs, and a prolonged QT interval.
Why "false" is wrong: decreased excitability — sedation, weakness, constipation, sluggish reflexes — is what high calcium (and high magnesium) look like.
Answer: 2 — hypokalemia. Furosemide is a loop diuretic: it blocks reabsorption in the loop of Henle, so sodium, potassium, chloride, calcium, and magnesium all leave in the urine together. Potassium loss is the one that shows up on the monitor, so check K⁺ before you give it and teach potassium-rich foods or give the ordered supplement.
Why the others are wrong:
1. Hyperkalemia comes from potassium-sparing diuretics (spironolactone), ACE inhibitors/ARBs, and kidney failure — not loops.
3. Loops waste calcium; it is thiazides that hold on to calcium and can raise it.
4. Loops waste magnesium too, so the risk is hypomagnesemia — and that low magnesium is often why the potassium will not come back up.
Answer: 2 — potassium 6.4 with a widened QRS. Hyperkalemia plus an ECG change is a pre-arrest rhythm. A widening QRS means the conduction system is already failing and the next step is a sine wave, then VF. This patient needs IV calcium gluconate to stabilize the myocardium immediately.
Why the others are wrong:
1. Sodium 130 is mildly low with a mild symptom — watch the neuro status, but nothing is imminent.
3. Calcium 8.0 is only slightly low and constipation is a comfort issue; there is no tetany or airway symptom in the stem.
4. Magnesium 1.7 mEq/L is normal. Nausea has many causes and none of them beat a widening QRS.
Answer: TRUE. Hyperkalemia is the electrolyte most likely to kill your patient today. Excess extracellular potassium keeps cardiac cells partially depolarized so they cannot reset, which produces peaked T waves, a widening QRS, then ventricular fibrillation or asystole. Continuous cardiac monitoring is non-negotiable.
Why "false" is wrong: nothing about hyperkalemia is benign at high levels — and the danger does not track neatly with the number. A patient at 6.0 with a wide QRS is sicker than a patient at 6.8 with a normal ECG.
Answer: 2 — hypocalcemia. Perioral numbness and tingling is the classic first symptom of low calcium, along with paresthesia of the fingers and toes. Irritable nerve membranes fire on their own. Follow it up by checking Chvostek's and Trousseau's signs and keeping the airway equipment nearby — laryngospasm is the feared next step.
Why the others are wrong:
1. Hypernatremia gives thirst, dry sticky mucous membranes, and restlessness — not focal tingling.
3. Hypermagnesemia depresses — drowsiness and disappearing reflexes, the opposite of tingling.
4. Hyperkalemia can cause paresthesia, but its hallmark is the ECG change and muscle weakness, and it does not favor the perioral area.
Answer: 2 — hyperkalemia. The kidney is the only meaningful route for getting potassium out of the body. When the GFR falls, potassium accumulates, which is why CKD patients get potassium-restricted diets, avoid salt substitutes, and are watched closely on ACE inhibitors and spironolactone.
Why the others are wrong:
1. Hypokalemia is a losing problem — diuretics, vomiting, diarrhea — not a retaining one.
3. Dilutional hyponatremia genuinely does occur in CKD from water retention, but it is not the imbalance most likely to kill and is not the classic association.
4. Hypocalcemia is also real in CKD (the kidney cannot activate vitamin D and phosphorus rises), but it develops over months. Potassium is the answer because it is both the most characteristic and the most lethal.
Answer: TRUE. They are clinical twins — both cause hyperactive reflexes, tremors, twitching, positive Chvostek and Trousseau signs, tetany, and seizures. Low magnesium also suppresses parathyroid hormone, which drops the calcium as well, so the two frequently occur together.
The exam point: if the calcium will not come up despite replacement, check the magnesium. Correcting magnesium is what lets calcium — and potassium — be corrected at all.
Answer: 2 — hypokalemia. All three findings line up: flattened T waves (often with a U wave), an irregular pulse from PVCs, and generalized weakness. Low potassium makes both cardiac and skeletal muscle sluggish to depolarize. Also expect decreased reflexes, leg cramps, and a quiet abdomen from paralytic ileus.
Why the others are wrong:
1. Hyperkalemia gives peaked T waves, not flat ones.
3. Hypercalcemia causes weakness too, but the ECG shows a shortened QT and the story usually includes constipation, stones, and bone pain.
4. Hypernatremia is a neuro and hydration picture — thirst and dry membranes — with no signature T-wave change.
Answer: 2 — hypokalemia. Vomiting loses potassium two ways: directly in the gastric fluid, and indirectly through the metabolic alkalosis it creates. Losing acid raises the pH, and potassium shifts into the cells in exchange for hydrogen ions — so the serum level falls even further. The same is true of continuous NG suction.
Why the others are wrong:
1. Hyperkalemia goes with acidosis and kidney failure, not with losing acid out the top.
3. Hypermagnesemia requires retaining magnesium (renal failure, Mg antacids); vomiting loses magnesium.
4. Sodium is lost in vomit as well, so hypernatremia is not the expected direction — and volume loss, not sodium excess, is what you see.
Answer: FALSE. High calcium depresses neuromuscular activity — lethargy, confusion, profound muscle weakness, diminished deep tendon reflexes, constipation, and a shortened QT. Extra calcium raises the threshold nerves must reach in order to fire.
Why "true" is wrong: hyperactive reflexes and tetany are the hypocalcemia picture. Remember it as calcium low = twitchy · calcium high = sleepy, and add "stones, bones, groans, and psychiatric overtones" for the high side.
Answer: 1 — hypermagnesemia. Magnesium is the body's natural sedative. Too much of it blocks acetylcholine release at the neuromuscular junction, so you get flushing, hypotension, drowsiness, reflexes that fade and then disappear, and respiratory depression. Assess deep tendon reflexes and respiratory rate before every dose of IV magnesium — losing the patellar reflex is your warning to stop. Antidote: IV calcium gluconate.
Why the others are wrong:
2. Hypocalcemia gives the opposite: brisk reflexes, twitching, tetany.
3. Hypernatremia presents with thirst, dry membranes, and restlessness.
4. Hypokalemia does cause weakness and can weaken the respiratory muscles, but reflexes are decreased rather than absent, and the giveaway ECG finding (flat T, U wave) is missing here while the whole picture screams sedation.
Answer: 2 — place on telemetry. Continuous cardiac monitoring is the priority because the fatal complication is a dysrhythmia that can appear before any other symptom does. The monitor is what tells you when to move from "watch and treat" to "push calcium gluconate now."
Why the others are wrong:
1. Bananas are high in potassium — this would make the emergency worse. Never add potassium to a hyperkalemic patient.
3. Fluid restriction does nothing for potassium; if anything, patients who still make urine may get fluids plus a loop diuretic to help excrete it.
4. Ambulation is unrelated, and a weak patient with an unstable rhythm should not be walking the hall.
Answer: TRUE. When serum sodium falls, the blood becomes hypotonic relative to the cell, so water moves down its gradient into the cell. Brain cells swell inside a rigid skull → rising ICP → headache, confusion, lethargy, seizures, coma.
Why "false" is wrong: the shrinking side belongs to hypernatremia, where the blood is hypertonic and pulls water out of brain cells. Low sodium = swollen brain. High sodium = dry brain.
Answer: 3 — 3% saline. This is symptomatic hyponatremia: the confusion means the brain is already swelling. Hypertonic saline raises serum osmolality so water is pulled back out of the brain cells. Give it on an infusion pump, recheck sodium frequently, and never correct sodium faster than about 8–12 mEq/L in 24 hours — too-fast correction causes osmotic demyelination.
Why the others are wrong:
1. 0.45% NS is hypotonic — it drives more water into the cells and worsens the cerebral edema.
2. D5W is isotonic in the bag but acts hypotonic once the dextrose is metabolized, leaving free water behind. Same problem.
4. Sterile water is never given IV — it causes massive hemolysis of red blood cells.
Answer: 2 — hypocalcemia. Chvostek's sign = tap over the facial nerve just in front of the ear and the facial muscles twitch. Trousseau's sign = inflate a BP cuff above systolic for a few minutes and the hand goes into carpal spasm. Both prove the nerve is hyper-irritable from low calcium. (Low magnesium produces the same two signs — check both labs.)
Why the others are wrong:
1. Hyperkalemia shows itself on the ECG and as weakness, not facial twitching.
3. Hypernatremia produces thirst, dry membranes, and restlessness.
4. Hypermagnesemia makes reflexes vanish — a positive Chvostek would be the opposite of what you expect.
Answer: TRUE. Magnesium is excreted by the kidney, so failing kidneys let it build up. The classic exam scenario is a renal patient given a magnesium-containing antacid or laxative — teach patients with CKD to check labels for magnesium (milk of magnesia, Maalox, Mag-Ox) before taking any OTC product.
Why "false" is wrong: the same logic that makes renal patients hyperkalemic makes them hypermagnesemic — anything the kidney normally throws away accumulates. Watch reflexes and respirations.
Answer: 3 — banana. Spironolactone is a potassium-sparing diuretic, so it blocks aldosterone and the body holds onto potassium. Adding high-potassium foods on top of that risks hyperkalemia. Teach the patient to limit bananas, oranges and orange juice, potatoes, tomatoes, avocado, spinach, cantaloupe, dried fruit — and especially salt substitutes, which are potassium chloride.
Why the others are wrong:
1. Chicken contains some potassium but is a standard protein choice and is not a teaching red flag.
2. Rice is low in potassium — it is actually on the "safe" list.
4. Bread is low in potassium as well. The question wants the one food that pushes potassium higher when the drug already is.
Answer: 2 — cerebral edema. Sodium 120 mEq/L is severe hyponatremia, and the headache plus lethargy tell you the brain is already involved. Water has shifted into brain cells and ICP is rising. Priority nursing actions: frequent neuro checks, seizure precautions, protect the airway, restrict free water, and anticipate hypertonic saline on a pump.
Why the others are wrong:
1. Constipation is not a hyponatremia finding, and it is never the priority over a neuro change.
3. Dehydration may or may not be present — most hyponatremia is a water excess problem — and it does not explain the headache and lethargy.
4. Hyperactivity is the opposite of what you see; low sodium depresses the level of consciousness.
🤖 Ask Claire: "Give me ten more electrolyte questions where two options are both abnormal labs, and make me justify which patient I would see first before you tell me the answer."
If you review nothing else the morning of the exam, review these eight.