Electrolyte and ECG safety · Renal & GI
Fluid & Electrolyte Balance MCQ
Body fluid compartments, sodium and potassium imbalances with their classic signs, calcium regulation by PTH and vitamin D, and acid-base buffering. 25 MCQs and 9 INBDE patient cases.
25 practice MCQsQuick-reference tableMnemonics + clinical pearlsFull distractor explanations
High-yield review
Concept summary and clinical relevance.
Quick-reference structure first, then detailed coverage. Mnemonics in amber, clinical pearls in blue.
Electrolyte questions on the INBDE focus on three patterns: where each ion lives (ICF vs ECF), what symptoms appear when the ion is too high or too low, and how diuretics and hormones change the balance. Calcium gets extra attention for dental relevance, it's the foundation of enamel and dentin and a critical clotting cofactor.
Body fluid compartments
| Compartment | % of TBW | Major cation | Notes |
|---|---|---|---|
| Total body water (TBW) | ~60% body weight | - | Lower in elderly, obese, women |
| Intracellular fluid (ICF) | ~2/3 of TBW | K⁺ | Largest compartment |
| Extracellular fluid (ECF) | ~1/3 of TBW | Na⁺ | Plasma + interstitial |
| Plasma | 1/4 of ECF | Na⁺ | Inside vessels |
| Interstitial fluid | 3/4 of ECF | Na⁺ | Between cells |
Electrolyte imbalances: classic findings
| Imbalance | Classic findings | Common causes |
|---|---|---|
| Hyponatremia | Seizures, confusion, weakness | SIADH, thiazides |
| Hypernatremia | Thirst, dry mucosa, irritability | Dehydration, diabetes insipidus |
| Hypokalemia | Muscle weakness, arrhythmia, flat T waves, U waves | Loop/thiazide diuretics, vomiting |
| Hyperkalemia | Peaked T waves, fatal arrhythmia risk | K⁺-sparing diuretics, ACE-I, CKD |
| Hypocalcemia | Tetany, Chvostek/Trousseau signs, perioral tingling | CKD (low active vit D), hypoparathyroidism |
| Hypercalcemia | “Stones, bones, groans, psychiatric overtones” | Hyperparathyroidism, malignancy |
Hormonal regulation
| Hormone | Action |
|---|---|
| ADH (vasopressin) | ↑ water reabsorption (collecting duct aquaporins) |
| Aldosterone | ↑ Na⁺ reabsorption, ↑ K⁺ secretion |
| ANP | Opposes RAAS → ↑ Na⁺ and water excretion |
| PTH | ↑ plasma Ca²⁺, ↓ plasma phosphate (renal effects) |
| Vitamin D (calcitriol) | ↑ Ca²⁺ AND phosphate absorption from gut |
| Calcitonin | ↓ plasma Ca²⁺ (minor role in adults) |
Clinical pearl, Why this matters in dentistry
Calcium is the structural ion of enamel, dentin, and bone, and it's a clotting cofactor. Hypocalcemia patients can have impaired hemostasis after extraction. Potassium imbalances are dangerous before sedation or epinephrine use because they raise arrhythmia risk. Always check the medication list: thiazides and loop diuretics drop K⁺; spironolactone and ACE inhibitors raise it.
Clinical pearl, CKD electrolyte triad to remember
Chronic kidney disease patients commonly present with hyperkalemia (failed K⁺ excretion → arrhythmia risk), hypocalcemia (failed activation of vitamin D → reduced gut Ca²⁺ absorption → secondary hyperparathyroidism), and metabolic acidosis (failed H⁺ excretion). All three affect medication choices and emergency planning.
Clinical pearl, Hypercalcemia mnemonic
“Stones (kidney stones), Bones (bone pain, osteopenia), Groans (abdominal pain, constipation), Psychiatric overtones (confusion, depression).” The classic four-part presentation that shows up in malignancy and primary hyperparathyroidism.
Mnemonic, Cation by compartment
Na⁺ = NAvigator of volume (ECF). K⁺ = Kontrols excitability (ICF, cardiac arrhythmias). Ca²⁺ = Clotting + Contraction + Calcification.
Mnemonic, Hypocalcemia signs
Chvostek (facial nerve tap → twitch) and Trousseau (BP cuff → carpal spasm). Both reflect neuromuscular hyperexcitability from low ionized calcium.
Sodium
- Major ECF cation; principal determinant of ECF volume.
- Hyponatremia → seizures, confusion, weakness; classic causes include SIADH and thiazides.
- Hypernatremia → thirst, dry mucosa, irritability; usually from water loss (dehydration, diabetes insipidus).
Potassium
- Major ICF cation; controls cardiac and skeletal muscle excitability.
- Hypokalemia → muscle weakness, arrhythmias, flat T waves, U waves. Caused by thiazides, loop diuretics, vomiting/diarrhea.
- Hyperkalemia → peaked T waves; risk of fatal arrhythmia. Caused by K⁺-sparing diuretics, ACE inhibitors, CKD.
Calcium
- 99% in bone; ~1% in plasma. Regulated by PTH, vitamin D, and (minor in adults) calcitonin.
- PTH ↑ Ca²⁺ and ↓ phosphate (acts on bone, kidney, and indirectly via vitamin D activation).
- Vitamin D (calcitriol): ↑ Ca²⁺ AND phosphate absorption from the gut.
- Hypocalcemia: tetany, Chvostek/Trousseau signs, perioral and finger tingling.
- Hypercalcemia: stones, bones, groans, psychiatric overtones.
- Dental relevance: enamel and dentin mineralization; clotting cofactor (factor IV).
Acid-base & buffers
- Bicarbonate (HCO₃⁻) is the major plasma buffer; works with CO₂ (lungs) and renal HCO₃⁻ reabsorption (kidneys).
- Chloride is the major ECF anion, often shifting in parallel with sodium.
Core Recall Check
25 board-style MCQs.
Active recall is the highest-yield study method. Pick an answer, check it, and read why every distractor is wrong.
0 of 25 answered · 0 correct
Keys: A-E or 1-5, Enter to check
- Question 1EasyWhich fluid compartment contains the largest proportion of total body water?
- Question 2EasyWhich ion is the major extracellular cation?
- Question 3EasyWhich ion is the major intracellular cation?
- Question 4ModerateWhich clinical feature is most associated with hyponatremia?
- Question 5ModerateWhich ECG finding is typical of hyperkalemia?
- Question 6ModerateWhich symptom is most associated with hypokalemia?
- Question 7EasyWhich ion is most important for enamel and dentin mineralization?
- Question 8ModerateWhich hormone raises plasma calcium while lowering plasma phosphate?
- Question 9ModerateWhich clinical feature is most typical of hypocalcemia?
- Question 10ModerateWhich phrase best summarizes the clinical features of hypercalcemia?
- Question 11EasyWhich buffer system is most important for maintaining blood pH?
- Question 12ModerateWithin the extracellular fluid, the plasma volume is best described as:
- Question 13EasyPlasma osmolality is determined mainly by:
- Question 14ModerateHypernatremia most often results from:
- Question 15ModerateWhich is a common cause of hyperkalemia?
- Question 16EasyWhich is a common cause of hypokalemia?
- Question 17EasyWhich hormone lowers plasma calcium?
- Question 18ModerateActive vitamin D (calcitriol) raises plasma calcium mainly by:
- Question 19HardPersistent hypokalemia that will not correct despite potassium replacement should prompt a check of:
- Question 20ModerateChvostek and Trousseau signs indicate:
- Question 21ModerateThe anion gap is used to:
- Question 22ModerateWhich condition classically causes a high anion gap metabolic acidosis?
- Question 23EasyProlonged vomiting tends to cause:
- Question 24ModerateDeep, rapid Kussmaul respirations are the body's attempt to compensate for:
- Question 25ModerateCompared with adults, infants are more vulnerable to dehydration partly because they have:
Clinical Reasoning Cases
INBDE patient cases.
9 ADA INBDE-format patient cases on fluid & electrolytes. Each case is a shared patient box plus linked questions with full distractor explanations.
INBDE Patient Cases
Fluid & Electrolytes INBDE Patient Cases →
9 patient cases · 45 linked questions
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Author
Dr. Isaac Sun, DDS
Founder, KYT Dental Services. These MCQs are reviewed by a practicing clinician and offered as an educational reference for dental students.
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