Cathartics: Sodium orthophosphate MCQs With Answer

Cathartics: Sodium orthophosphate MCQs With Answer — This concise introduction covers the pharmacology, clinical use, formulation, dosing, adverse effects and safety considerations of sodium orthophosphate as an osmotic cathartic. Ideal for B.Pharm students preparing for exams, these MCQs focus on mechanism of action, electrolyte disturbances (hyperphosphatemia, hypocalcemia), renal risks including acute phosphate nephropathy, comparative bowel-prep agents (PEG vs sodium phosphate), dosing regimens, contraindications, monitoring and patient counselling. Emphasis is placed on practical pharmacy knowledge: preparation types, storage, QC parameters and vulnerable populations (elderly, renal impairment). Review this core information to master cathartic pharmacotherapy and safe bowel cleansing protocols. ‘Now let’s test your knowledge with 50 MCQs on this topic.’

Q1. Which of the following best describes a cathartic?

• A drug that decreases gastric acid secretion
• A laxative that accelerates defecation by increasing stool bulk or water
• An antiemetic agent
• A drug that reduces intestinal motility

Correct Answer: A laxative that accelerates defecation by increasing stool bulk or water

Q2. Sodium orthophosphate is classified primarily as which type of laxative?

• Bulk-forming laxative
• Osmotic (saline) cathartic
• Stimulant laxative
• Emollient laxative

Correct Answer: Osmotic (saline) cathartic

Q3. What is the main mechanism of action of sodium orthophosphate in the intestine?

• Stimulates serotonin receptors to increase motility
• Deposits surfactant to soften stool
• Creates an osmotic gradient by phosphate ions, drawing water into the lumen
• Inhibits sodium absorption causing secretory diarrhea

Correct Answer: Creates an osmotic gradient by phosphate ions, drawing water into the lumen

Q4. Which clinical use is most common for oral sodium orthophosphate?

• Long-term treatment of chronic constipation
• Bowel cleansing prior to colonoscopy or gastrointestinal procedures
• Mild symptomatic relief of heartburn
• Management of inflammatory bowel disease flare-ups

Correct Answer: Bowel cleansing prior to colonoscopy or gastrointestinal procedures

Q5. Which formulation(s) of sodium orthophosphate are commonly available?

• Oral solution only
• Rectal enema only
• Both oral solution and rectal enema
• Injectable solution for IV use

Correct Answer: Both oral solution and rectal enema

Q6. Typical onset of action for oral sodium orthophosphate bowel-prep is approximately:

• Within 5–20 minutes
• 0.5–6 hours
• 24–48 hours
• 2–3 days

Correct Answer: 0.5–6 hours

Q7. Which patient condition is a major contraindication for sodium orthophosphate use?

• Mild seasonal allergies
• Severe renal impairment or kidney failure
• Well-controlled hypertension
• Uncomplicated upper respiratory infection

Correct Answer: Severe renal impairment or kidney failure

Q8. The most important electrolyte disturbance associated with sodium orthophosphate is:

• Hypernatremia only
• Hyperphosphatemia with secondary hypocalcemia
• Hyperkalemia only
• Hyponatremia with metabolic alkalosis

Correct Answer: Hyperphosphatemia with secondary hypocalcemia

Q9. Which of the following increases the risk of acute phosphate nephropathy after sodium phosphate use?

• Young age and hyperhydration
• Dehydration, advanced age and pre-existing renal impairment
• Concurrent use of topical antibiotics
• Consumption of a high-fiber diet

Correct Answer: Dehydration, advanced age and pre-existing renal impairment

Q10. A commonly recommended adult dosing regimen for oral sodium phosphate bowel preparation is:

• Single dose of 5 mL taken once
• Two 45 mL doses given the evening before and the morning of the procedure
• One 500 mL infusion orally
• 10 tablets taken with meals for 7 days

Correct Answer: Two 45 mL doses given the evening before and the morning of the procedure

Q11. One significant safety concern with sodium orthophosphate is its high sodium content. What clinical problem can this cause?

• Hypotension in all patients
• Fluid overload, hypertension and exacerbation of heart failure
• Marked weight loss over weeks
• Bronchospasm and respiratory failure

Correct Answer: Fluid overload, hypertension and exacerbation of heart failure

Q12. Which ionic species primarily produces the cathartic osmotic effect of sodium orthophosphate?

• Sodium ion
• Phosphate ion
• Chloride ion
• Potassium ion

Correct Answer: Phosphate ion

Q13. Prior to prescribing sodium orthophosphate for bowel prep, which laboratory tests should a pharmacist recommend?

• Serum electrolytes and renal function tests (creatinine, BUN)
• Chest X-ray and liver function tests only
• Complete blood count only
• Fasting blood glucose only

Correct Answer: Serum electrolytes and renal function tests (creatinine, BUN)

Q14. The pathogenesis of acute phosphate nephropathy after sodium phosphate use is best described as:

• Immune-mediated glomerulonephritis
• Obstruction of glomeruli by bacterial biofilm
• Calcium-phosphate crystal deposition in renal tubules
• Primary tubular necrosis from drug metabolites

Correct Answer: Calcium-phosphate crystal deposition in renal tubules

Q15. Compared to polyethylene glycol (PEG) solutions, sodium orthophosphate:

• Is isoosmotic and safer in renal disease
• Is hyperosmotic and has higher risk for electrolyte disturbances
• Has identical safety and electrolyte profile
• Is primarily a stimulant laxative like bisacodyl

Correct Answer: Is hyperosmotic and has higher risk for electrolyte disturbances

Q16. Sodium orthophosphate use in young children is generally:

• Recommended as first-line for constipation
• Not routinely recommended due to risk of severe electrolyte shifts
• Preferable to PEG in infants
• Safe without monitoring

Correct Answer: Not routinely recommended due to risk of severe electrolyte shifts

Q17. Which cardiovascular condition is particularly concerning when considering sodium orthophosphate for bowel preparation?

• Stable angina without heart failure
• Congestive heart failure due to sodium and fluid load
• Mild isolated systolic hypertension controlled by diet
• History of atrial septal defect repair in childhood

Correct Answer: Congestive heart failure due to sodium and fluid load

Q18. Proper storage advice for unopened sodium orthophosphate oral solution typically includes:

• Store frozen to preserve potency
• Store at room temperature away from direct sunlight
• Store in refrigerator at 2–8°C
• Store in a metal container outdoors

Correct Answer: Store at room temperature away from direct sunlight

Q19. Sodium orthophosphate acts as a saline cathartic primarily affecting which segment(s) of the gut?

• Stomach only
• Small intestine and colon by increasing luminal fluid and motility
• Esophagus exclusively
• Pancreas and biliary tree

Correct Answer: Small intestine and colon by increasing luminal fluid and motility

Q20. Common chemical components in many oral sodium phosphate bowel-prep formulations include:

• Sodium dihydrogen phosphate and disodium hydrogen phosphate
• Calcium carbonate and magnesium stearate
• Potassium chloride and sodium bicarbonate
• Activated charcoal and kaolin

Correct Answer: Sodium dihydrogen phosphate and disodium hydrogen phosphate

Q21. Rectal administration of sodium phosphate enema can cause which local adverse effect?

• Mucosal irritation or proctitis
• Permanent bowel dilation
• Sclerosing cholangitis
• Ototoxicity

Correct Answer: Mucosal irritation or proctitis

Q22. The onset of action for a sodium phosphate rectal enema is typically:

• Approximately 5–20 minutes
• 12–24 hours
• 3–5 days
• Immediately and permanently

Correct Answer: Approximately 5–20 minutes

Q23. Why are patients instructed to follow a clear liquid diet and fast before sodium phosphate bowel preparation?

• To reduce the risk of aspiration during sedation and to improve colon cleansing effectiveness
• To increase phosphate absorption into the bloodstream
• To prevent hypertension
• There is no reason; it is a historical myth

Correct Answer: To reduce the risk of aspiration during sedation and to improve colon cleansing effectiveness

Q24. Which class of medications is commonly recommended to be withheld before using sodium phosphate for bowel prep to reduce adverse events?

• Topical corticosteroids
• Loop and thiazide diuretics (due to dehydration and electrolyte risk)
• Oral contraceptives
• Inhaled beta-agonists

Correct Answer: Loop and thiazide diuretics (due to dehydration and electrolyte risk)

Q25. The principal physiological cause of dehydration after sodium orthophosphate ingestion is:

• Excessive urinary retention of water
• Osmotic water retention in the intestinal lumen leading to fluid loss
• Increased sweat production through sympathetic stimulation
• Suppressed thirst center in the hypothalamus

Correct Answer: Osmotic water retention in the intestinal lumen leading to fluid loss

Q26. Which electrolyte change is most likely to accompany acute hyperphosphatemia caused by sodium orthophosphate?

• Hypercalcemia
• Hypocalcemia
• Hypermagnesemia
• Hyperchloremia without calcium changes

Correct Answer: Hypocalcemia

Q27. Among the following, who is at highest risk for severe electrolyte abnormalities with sodium orthophosphate?

• Young healthy adults with no comorbidities
• Patients with chronic kidney disease
• Patients taking vitamin C supplements only
• Individuals on a low-sodium diet

Correct Answer: Patients with chronic kidney disease

Q28. After administration of sodium orthophosphate, typical laboratory trends include:

• Decreased phosphate and increased calcium
• Increased serum phosphate and decreased serum calcium
• No change in electrolytes
• Isolated increase in magnesium only

Correct Answer: Increased serum phosphate and decreased serum calcium

Q29. What essential counseling point should a pharmacist give patients taking sodium orthophosphate for bowel prep?

• Limit fluids to reduce vomiting
• Maintain adequate hydration before and during preparation
• Take extra calcium supplements during prep
• Avoid all activity and remain bedridden

Correct Answer: Maintain adequate hydration before and during preparation

Q30. Management of severe symptomatic hypocalcemia following sodium orthophosphate exposure should include:

• Immediate oral phosphate supplementation
• Intravenous calcium (e.g., calcium gluconate) and supportive care, possible dialysis
• Administration of high-dose magnesium orally
• No treatment; it resolves spontaneously

Correct Answer: Intravenous calcium (e.g., calcium gluconate) and supportive care, possible dialysis

Q31. For patients with significant renal impairment requiring bowel cleansing, the preferred agent is usually:

• Oral sodium phosphate
• Polyethylene glycol (PEG) isotonic lavage solution
• High-dose stimulant laxatives only
• Lactulose syrup overnight

Correct Answer: Polyethylene glycol (PEG) isotonic lavage solution

Q32. In quality control of sodium orthophosphate oral solutions, which tests are commonly performed?

• Assay of phosphate content, pH measurement and microbial limits
• Blood compatibility test and PCR for viruses
• Skin sensitization test on humans
• Explosive residue analysis

Correct Answer: Assay of phosphate content, pH measurement and microbial limits

Q33. To avoid interference with absorption, patients are often advised to time other oral medications relative to sodium phosphate dosing by waiting:

• At least 1–2 hours before or after sodium phosphate administration
• At least 10 minutes before
• No timing adjustment is ever needed
• Take all meds simultaneously with the sodium phosphate dose

Correct Answer: At least 1–2 hours before or after sodium phosphate administration

Q34. Regarding use in pregnancy, sodium orthophosphate for bowel cleansing is best described as:

• Recommended as routine first-line prep in all trimesters
• Use with caution and only if benefits outweigh risks; alternatives often preferred
• Absolutely contraindicated in any pregnancy
• Safe and promotes fetal bone development

Correct Answer: Use with caution and only if benefits outweigh risks; alternatives often preferred

Q35. A typical commercial sodium phosphate enema volume (e.g., Fleet) is approximately:

• 5 mL
• 118 mL (approximately 4 ounces)
• 1,000 mL
• 10 L

Correct Answer: 118 mL (approximately 4 ounces)

Q36. Many oral sodium phosphate preparations are supplied as single-use doses. What is the pharmacist’s advice regarding unused portion?

• Save it for later use at room temperature
• Discard any unused portion; do not store for later dosing
• Refrigerate and reuse within 48 hours
• Freeze and thaw before the next use

Correct Answer: Discard any unused portion; do not store for later dosing

Q37. Which statement about systemic absorption of phosphate after sodium orthophosphate administration is correct?

• No phosphate is absorbed; all is excreted in stool
• Phosphate can be absorbed leading to elevated serum phosphate levels
• Phosphate is converted to carbonate and exhaled
• Phosphate becomes bound to dietary fiber and is retained in colon

Correct Answer: Phosphate can be absorbed leading to elevated serum phosphate levels

Q38. Is sodium orthophosphate recommended for whole-bowel irrigation in acute poisoning?

• No; polyethylene glycol-electrolyte lavage solutions are preferred for whole-bowel irrigation
• Yes; sodium orthophosphate is the first-line for all poisonings
• Yes; it binds toxins in the stomach
• No; activated charcoal is always superior for whole-bowel irrigation

Correct Answer: No; polyethylene glycol-electrolyte lavage solutions are preferred for whole-bowel irrigation

Q39. Clinical signs of symptomatic hypocalcemia after sodium orthophosphate include:

• Perioral numbness, paresthesias and muscle cramps or tetany
• Excessive sweating and hyperactivity
• Jaundice and abdominal swelling
• Visual hallucinations only

Correct Answer: Perioral numbness, paresthesias and muscle cramps or tetany

Q40. Which organ is primarily responsible for excreting excess phosphate absorbed after sodium orthophosphate use?

• Liver
• Kidneys
• Spleen
• Lungs

Correct Answer: Kidneys

Q41. Sodium orthophosphate is contraindicated in patients with suspected or known:

• Bowel obstruction or ileus
• Seasonal allergic rhinitis
• Mild acne vulgaris
• Recovering minor skin abrasions

Correct Answer: Bowel obstruction or ileus

Q42. Before administering sodium orthophosphate for colonoscopy prep, which pre-procedure test is most important?

• Serum creatinine and electrolyte panel
• Chest X-ray
• Skin prick allergy test
• Hearing test

Correct Answer: Serum creatinine and electrolyte panel

Q43. The expected pharmacodynamic effect of sodium orthophosphate on stool characteristics is:

• Decrease in stool volume and constipation
• Increase in stool water content, volume and frequency
• No change in stool frequency
• Formation of hard, dry pellets

Correct Answer: Increase in stool water content, volume and frequency

Q44. Elderly patients are at increased risk when using sodium orthophosphate because they are more likely to have:

• Robust renal function and overhydration
• Reduced renal function, polypharmacy and dehydration risk
• Faster intestinal transit negating the drug effect
• Increased bone density preventing hypocalcemia

Correct Answer: Reduced renal function, polypharmacy and dehydration risk

Q45. The FDA has issued safety communications regarding oral sodium phosphate products primarily because of:

• Risk of acute phosphate nephropathy and significant electrolyte disturbances
• Excessive abuse as a recreational drug
• Carcinogenic impurities found in production
• Inadequate marketing materials

Correct Answer: Risk of acute phosphate nephropathy and significant electrolyte disturbances

Q46. Acute phosphate nephropathy typically presents as which of the following clinical outcomes?

• Irreversible sensorineural hearing loss
• Acute or chronic renal impairment often with persistent decreased glomerular filtration
• Acute pancreatitis only
• Transient dermatitis that resolves spontaneously

Correct Answer: Acute or chronic renal impairment often with persistent decreased glomerular filtration

Q47. Which gastrointestinal condition is a contraindication to sodium orthophosphate use due to increased mucosal injury risk?

• Active inflammatory bowel disease (ulcerative colitis or Crohn’s disease)
• Hemorrhoids without systemic disease
• Prior appendectomy decades ago
• Seasonal constipation

Correct Answer: Active inflammatory bowel disease (ulcerative colitis or Crohn’s disease)

Q48. After sodium orthophosphate use, which of the following is true about potassium levels in most patients?

• Potassium always rises dramatically to dangerous levels
• Significant potassium changes are less consistent, but monitoring is prudent; hyperkalemia may occur with renal dysfunction
• Potassium is unaffected and never needs monitoring
• Potassium is transformed into sodium by intestinal enzymes

Correct Answer: Significant potassium changes are less consistent, but monitoring is prudent; hyperkalemia may occur with renal dysfunction

Q49. Which of the following is true about sodium orthophosphate and pregnancy?

• It is routinely used as first-line bowel prep in pregnancy without restriction
• Use only if benefits outweigh risks; alternatives such as PEG may be preferred
• It guarantees improved fetal bone mineralization
• It is completely contraindicated and never used in any circumstance

Correct Answer: Use only if benefits outweigh risks; alternatives such as PEG may be preferred

Q50. What should patients be warned to expect after taking sodium orthophosphate for bowel preparation?

• No change in bowel habits
• Multiple watery bowel movements, abdominal cramping, and the need for frequent restroom access
• Immediate relief of chronic diarrhea symptoms
• Persistent constipation for 1–2 weeks

Correct Answer: Multiple watery bowel movements, abdominal cramping, and the need for frequent restroom access

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com