Electrolytes in replacement therapy: Oral Rehydration Salt (ORS) MCQs With Answer

Electrolytes in replacement therapy: Oral Rehydration Salt (ORS) is a core topic for B. Pharm students linking pharmaceutics, pharmacology, and therapeutics. ORS uses precise electrolyte and glucose ratios to drive sodium–glucose co-transport (SGLT1), restoring water and electrolyte balance in dehydration from diarrhea, cholera, and gastroenteritis. Understanding WHO ORS composition (sodium, potassium, chloride, citrate, glucose), reduced osmolarity (≈245 mOsm/L), mechanism, indications, contraindications, and dosing plans (Plan A/B/C) is essential for rational therapy. You’ll also revisit stability, preparation (1 L safe water), adjunct zinc, and special cases (severe dehydration, renal failure, SAM/ReSoMal). This knowledge supports evidence-based, cost-effective oral rehydration strategies in public health.

Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary mechanism by which ORS corrects dehydration in diarrheal illness?

• Coupled sodium–glucose absorption via SGLT1 in small intestinal enterocytes
• Passive water diffusion independent of solutes
• Inhibition of CFTR-mediated chloride secretion
• Increased renal water reabsorption via ADH

Correct Answer: Coupled sodium–glucose absorption via SGLT1 in small intestinal enterocytes

Q2. The sodium concentration in WHO reduced-osmolarity ORS is:

• 75 mmol/L
• 90 mmol/L
• 50 mmol/L
• 140 mmol/L

Correct Answer: 75 mmol/L

Q3. The glucose concentration in WHO reduced-osmolarity ORS is:

• 75 mmol/L
• 111 mmol/L
• 13.5 mmol/L
• 20 mmol/L

Correct Answer: 75 mmol/L

Q4. The total osmolarity of WHO reduced-osmolarity ORS is approximately:

• 245 mOsm/L
• 311 mOsm/L
• 150 mOsm/L
• 500 mOsm/L

Correct Answer: 245 mOsm/L

Q5. The base component in modern WHO ORS that corrects metabolic acidosis is:

• Trisodium citrate dihydrate
• Sodium bicarbonate
• Calcium carbonate
• Sodium lactate

Correct Answer: Trisodium citrate dihydrate

Q6. The potassium concentration in WHO reduced-osmolarity ORS is:

• 20 mmol/L
• 10 mmol/L
• 40 mmol/L
• 5 mmol/L

Correct Answer: 20 mmol/L

Q7. The chloride concentration in WHO reduced-osmolarity ORS is:

• 65 mmol/L
• 90 mmol/L
• 35 mmol/L
• 110 mmol/L

Correct Answer: 65 mmol/L

Q8. Which statement best describes the purpose of ORS?

• Prevents dehydration by enhancing sodium and water absorption without necessarily stopping diarrhea
• Directly kills enteric pathogens to cure diarrhea
• Completely halts intestinal chloride secretion
• Acts as a diuretic to remove excess fluid

Correct Answer: Prevents dehydration by enhancing sodium and water absorption without necessarily stopping diarrhea

Q9. For a child with moderate dehydration (Plan B), the recommended ORS volume is:

• 75 mL/kg over 4 hours
• 20 mL/kg bolus over 15 minutes
• 100 mL/kg over 24 hours
• Ad libitum only, no calculation required

Correct Answer: 75 mL/kg over 4 hours

Q10. For children aged 2–10 years with no dehydration (Plan A), the approximate ORS amount after each loose stool is:

• 10–20 mL
• 50 mL
• 100–200 mL
• 500 mL

Correct Answer: 100–200 mL

Q11. Which is the correct instruction for preparing a standard ORS sachet?

• Dissolve the entire sachet in exactly 1 liter of safe water
• Dissolve half the sachet in 500 mL to make a more concentrated solution
• Boil the dry powder before mixing with water
• Add extra sugar to improve efficacy

Correct Answer: Dissolve the entire sachet in exactly 1 liter of safe water

Q12. After preparation, ORS solution should be:

• Used within 24 hours of preparation
• Stored for 1 week in the refrigerator
• Kept at room temperature for 72 hours
• Frozen for future use

Correct Answer: Used within 24 hours of preparation

Q13. In severe dehydration with shock, the initial fluid of choice is:

• Intravenous Ringer’s lactate is first-line
• Begin with ORS alone
• Intramuscular normal saline
• Oral plain water only

Correct Answer: Intravenous Ringer’s lactate is first-line

Q14. Why is ORS effective in cholera?

• Sodium–glucose co-transport remains intact despite cholera toxin
• Cholera toxin blocks SGLT1
• ORS is ineffective in cholera due to secretory diarrhea
• ORS worsens stool output in cholera and is contraindicated

Correct Answer: Sodium–glucose co-transport remains intact despite cholera toxin

Q15. Which is a contraindication to oral rehydration therapy?

• Paralytic ileus
• Mild dehydration
• Ability to drink without vomiting
• Afebrile state

Correct Answer: Paralytic ileus

Q16. In severe acute malnutrition (SAM), the preferred oral rehydration solution is:

• ReSoMal (Rehydration Solution for Malnutrition)
• Hypertonic saline
• Plain water
• 5% dextrose (D5W)

Correct Answer: ReSoMal (Rehydration Solution for Malnutrition)

Q17. The amount of sodium chloride per liter in WHO reduced-osmolarity ORS sachet is:

• 2.6 g
• 5.85 g
• 1.0 g
• 3.5 g

Correct Answer: 2.6 g

Q18. The amount of anhydrous glucose per liter in WHO reduced-osmolarity ORS sachet is:

• 13.5 g
• 20.5 g
• 75 g
• 6.75 g

Correct Answer: 13.5 g

Q19. The amount of potassium chloride per liter in WHO reduced-osmolarity ORS sachet is:

• 1.5 g
• 2.9 g
• 0.75 g
• 3.0 g

Correct Answer: 1.5 g

Q20. The amount of trisodium citrate dihydrate per liter in WHO reduced-osmolarity ORS sachet is:

• 2.9 g
• 1.5 g
• 13.5 g
• 4.0 g

Correct Answer: 2.9 g

Q21. Best initial management for an adult with mild dehydration from gastroenteritis is:

• Oral reduced-osmolarity ORS with continued feeding
• NPO for 24 hours to rest the gut
• Immediate IV fluids for all adults
• Broad-spectrum antibiotics alone

Correct Answer: Oral reduced-osmolarity ORS with continued feeding

Q22. Why is citrate used instead of bicarbonate in modern ORS?

• Metabolizes to bicarbonate to correct acidosis and improves solution stability compared to bicarbonate
• Directly inhibits intestinal chloride channels
• Acts as primary energy source
• Causes bronchodilation

Correct Answer: Metabolizes to bicarbonate to correct acidosis and improves solution stability compared to bicarbonate

Q23. The recommended “home-made” ORS recipe (when sachets are unavailable) is:

• 6 level teaspoons of sugar and 1/2 level teaspoon of salt in 1 liter of clean water
• 3 tablespoons sugar and 1 teaspoon salt in 500 mL water
• 6 level teaspoons sugar only in 1 liter water
• 1/2 teaspoon salt only in 1 liter water

Correct Answer: 6 level teaspoons of sugar and 1/2 level teaspoon of salt in 1 liter of clean water

Q24. Which finding suggests successful rehydration with ORS?

• Improved urine output and moist mucous membranes
• Persistently sunken eyes
• Increasing lethargy
• Worsening thirst

Correct Answer: Improved urine output and moist mucous membranes

Q25. Compared with older (higher osmolarity) ORS, the reduced-osmolarity WHO ORS:

• Lowers stool output and vomiting compared with older ORS
• Increases risk of hypernatremia
• Eliminates need for zinc supplementation
• Causes severe hypoglycemia

Correct Answer: Lowers stool output and vomiting compared with older ORS

Q26. The key apical membrane transporter enabling ORS action is:

• SGLT1 on apical membrane of enterocytes
• GLUT2 on basolateral membrane only
• Na+/K+ ATPase on brush border
• CFTR in crypt cells

Correct Answer: SGLT1 on apical membrane of enterocytes

Q27. Which fluids are NOT recommended as substitutes for ORS?

• Carbonated soft drinks and undiluted fruit juices due to high osmolality
• Clean water if ORS unavailable
• Breast milk for infants
• Rice water or soups (appropriately salted)

Correct Answer: Carbonated soft drinks and undiluted fruit juices due to high osmolality

Q28. Which statement about plain water in acute diarrhea is true?

• Exclusive plain water intake in diarrhea can precipitate hyponatremia
• Plain water is superior to ORS for rehydration
• Plain water prevents electrolyte imbalance better than ORS
• Plain water is contraindicated in any illness

Correct Answer: Exclusive plain water intake in diarrhea can precipitate hyponatremia

Q29. WHO-recommended zinc supplementation with ORS for children is:

• 20 mg elemental zinc daily for 10–14 days for children ≥6 months; 10 mg daily for <6 months
• 50 mg elemental zinc twice daily for 3 days for all ages
• Zinc is not recommended with ORS
• 5 mg zinc weekly for 1 month

Correct Answer: 20 mg elemental zinc daily for 10–14 days for children ≥6 months; 10 mg daily for <6 months

Q30. ORS should be used with caution in which patients?

• Advanced renal failure with oliguria/anuria due to potassium load risk
• Mild hypertension
• Uncomplicated GERD
• Healthy athletes

Correct Answer: Advanced renal failure with oliguria/anuria due to potassium load risk

Q31. Calculate the Plan B ORS volume for an 8-kg child with moderate dehydration:

• 600 mL of ORS over 4 hours
• 300 mL over 4 hours
• 800 mL over 8 hours
• 1000 mL over 1 hour

Correct Answer: 600 mL of ORS over 4 hours

Q32. Which statement regarding ORS use in cholera is correct?

• Reduced-osmolarity ORS is recommended across age groups, including cholera, with clinical monitoring
• Only high-sodium ORS should be used in cholera; reduced ORS is contraindicated
• ORS should not be used in cholera at all
• ORS must be given via nasogastric tube only

Correct Answer: Reduced-osmolarity ORS is recommended across age groups, including cholera, with clinical monitoring

Q33. The main role of potassium in ORS is to:

• Replace stool potassium losses and help prevent hypokalemia
• Inhibit SGLT1
• Cause osmotic diuresis
• Neutralize gastric acid directly

Correct Answer: Replace stool potassium losses and help prevent hypokalemia

Q34. In Plan B, when should reassessment occur and what is the next step?

• Reassess after 4 hours and choose Plan A or C based on signs
• Continue Plan B indefinitely
• Switch to IV fluids regardless of improvement
• Stop all feeding until stools cease

Correct Answer: Reassess after 4 hours and choose Plan A or C based on signs

Q35. ORS remains effective in secretory diarrhea primarily because:

• ORS remains effective because sodium–glucose co-transport is preserved
• ORS fails because secretion overwhelms absorption
• ORS works only in osmotic diarrhea
• ORS is contraindicated in secretory diarrhea

Correct Answer: ORS remains effective because sodium–glucose co-transport is preserved

Q36. Proper storage advice for unopened ORS sachets is:

• Store sachets in a cool, dry place away from moisture and heat
• Keep sachets unsealed to air them out
• Refrigeration is mandatory for sachets
• Expose sachets to sunlight to prevent contamination

Correct Answer: Store sachets in a cool, dry place away from moisture and heat

Q37. Why is accurate reconstitution volume crucial for ORS?

• Using less water than directed can dangerously increase osmolarity and sodium concentration
• Using less water has no clinical impact
• Using more water increases efficacy
• Any volume is acceptable if patient drinks enough

Correct Answer: Using less water than directed can dangerously increase osmolarity and sodium concentration

Q38. Which set of signs indicates the need for urgent referral/IV therapy instead of ORS alone?

• Lethargy/unconsciousness, inability to drink, repeated vomiting, or shock
• Mild thirst with normal activity
• Single loose stool
• Low-grade fever only

Correct Answer: Lethargy/unconsciousness, inability to drink, repeated vomiting, or shock

Q39. Rice-based ORS has shown particular benefit in:

• Particularly reduces stool volume in cholera compared with glucose-based ORS
• Increases risk of hypoglycemia
• Contraindicated in adults
• Has no role in any diarrhea

Correct Answer: Particularly reduces stool volume in cholera compared with glucose-based ORS

Q40. A common error that predisposes to hyponatremia during ORT is:

• Over-dilution of ORS with excess water
• Preparing with exactly 1 liter of water
• Using reduced-osmolarity WHO formula as directed
• Giving with continued feeding

Correct Answer: Over-dilution of ORS with excess water

Q41. The primary site of ORS-mediated absorption is the:

• Small intestinal enterocytes (jejunum and ileum)
• Gastric parietal cells
• Colonic goblet cells
• Renal proximal tubules

Correct Answer: Small intestinal enterocytes (jejunum and ileum)

Q42. Which best explains why citrate is preferred over bicarbonate in ORS sachets?

• Better shelf-stability in dry mix and avoids CO2 loss; still corrects acidosis after metabolism
• Cheaper taste enhancer without metabolic effects
• Promotes diarrhea to clear pathogens
• Increases solution osmolarity excessively

Correct Answer: Better shelf-stability in dry mix and avoids CO2 loss; still corrects acidosis after metabolism

Q43. Regarding antibiotics and ORS in cholera, which is correct?

• Start ORS immediately; antibiotics are adjuncts that shorten illness but do not replace rehydration
• Withhold ORS until antibiotics have started
• Use antibiotics alone without ORS
• ORS only after 24 hours of fasting

Correct Answer: Start ORS immediately; antibiotics are adjuncts that shorten illness but do not replace rehydration

Q44. For adults, a practical per-stool ORS intake guidance is:

• Typically 200–400 mL ORS after each loose stool, adjusted to thirst
• Exactly 50 mL after each stool regardless of size
• No fluids for 6 hours after each stool
• Only IV fluids are appropriate in adults

Correct Answer: Typically 200–400 mL ORS after each loose stool, adjusted to thirst

Q45. A simple safety check for home-prepared ORS is:

• Solution should not taste saltier than tears
• Solution must taste very salty to be effective
• Add honey to ensure isotonicity
• Color must be dark brown

Correct Answer: Solution should not taste saltier than tears

Q46. If a patient vomits while taking ORS, the recommended action is to:

• Wait 5–10 minutes, then resume with small, frequent sips
• Stop ORS permanently
• Switch immediately to IV fluids in all cases
• Double the volume at next attempt

Correct Answer: Wait 5–10 minutes, then resume with small, frequent sips

Q47. Which statement about ORS handling is incorrect?

• Boiling the prepared ORS solution is recommended to improve efficacy
• Prepared ORS should be discarded after 24 hours
• Use safe water for reconstitution
• Do not add extra salt or sugar beyond instructions

Correct Answer: Boiling the prepared ORS solution is recommended to improve efficacy

Q48. For infants under 6 months with diarrhea, the correct advice is:

• Continue breastfeeding and give ORS as needed in small amounts
• Stop breastfeeding until diarrhea resolves
• Give only water instead of ORS
• Use adult sports drinks in place of ORS

Correct Answer: Continue breastfeeding and give ORS as needed in small amounts

Q49. A key advantage of ORS over IV fluids in mild-to-moderate dehydration is:

• Can be administered safely at home without sterile equipment
• Provides faster plasma expansion than IV bolus in shock
• Has higher risk of electrolyte imbalance than IV fluids
• Requires medical supervision for every dose

Correct Answer: Can be administered safely at home without sterile equipment

Q50. Identify the incorrect ion–benefit pairing for ORS:

• Chloride — treats hypoglycemia
• Sodium — restores extracellular fluid volume
• Potassium — helps prevent ileus and muscle weakness from hypokalemia
• Citrate — corrects metabolic acidosis after hepatic metabolism

Correct Answer: Chloride — treats hypoglycemia