Introduction: Osmotic diuretics, especially Mannitol, are essential topics for B.Pharm students studying clinical pharmacology and therapeutics. Mannitol is a low‑molecular‑weight sugar alcohol that produces osmotic diuresis by increasing plasma and tubular fluid osmolarity, reducing intracranial and intraocular pressure, and promoting renal perfusion. Key points include mechanism of action, site of action (proximal tubule and thin descending limb), pharmacokinetics (not metabolized, renally excreted), common formulations (10–25%), indications, contraindications, adverse effects (pulmonary edema, electrolyte imbalance), dosing and monitoring (serum osmolality). This focused review will prepare you to apply concepts clinically. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary mechanism of action of Mannitol as an osmotic diuretic?
- Inhibition of Na+/K+/2Cl− cotransporter in thick ascending limb
- Blocking aldosterone receptors in collecting duct
- Increasing tubular fluid osmolarity to inhibit water reabsorption
- Inhibition of carbonic anhydrase in proximal tubule
Correct Answer: Increasing tubular fluid osmolarity to inhibit water reabsorption
Q2. Which nephron segments are primarily affected by osmotic diuretics like Mannitol?
- Thick ascending limb only
- Collecting duct only
- Proximal tubule and descending limb of loop of Henle
- Distal convoluted tubule only
Correct Answer: Proximal tubule and descending limb of loop of Henle
Q3. Mannitol reduces intracranial pressure effectively when which condition is present?
- Blood–brain barrier is intact
- Large intracerebral hemorrhage with disrupted barrier
- Severe hypotension
- When renal function is absent
Correct Answer: Blood–brain barrier is intact
Q4. Which of the following is a common indication for intravenous Mannitol?
- Chronic hypertension management
- Acute reduction of raised intracranial pressure
- Maintenance fluid therapy in surgery
- Long‑term edema control in heart failure
Correct Answer: Acute reduction of raised intracranial pressure
Q5. Mannitol is eliminated from the body primarily by which route?
- Hepatic metabolism to sorbitol
- Renal excretion unchanged
- Metabolism by intestinal flora
- Pulmonary exhalation as CO2
Correct Answer: Renal excretion unchanged
Q6. Which lab parameter should be monitored closely during Mannitol therapy to avoid toxicity?
- Serum bicarbonate only
- Serum osmolality and electrolytes
- Liver function tests exclusively
- Platelet count only
Correct Answer: Serum osmolality and electrolytes
Q7. A commonly recommended upper limit for serum osmolality during Mannitol treatment is:
- 200 mOsm/kg
- 320 mOsm/kg
- 420 mOsm/kg
- 500 mOsm/kg
Correct Answer: 320 mOsm/kg
Q8. Which adverse effect is most likely with Mannitol use in a patient with poor cardiac function?
- Hypoglycemia
- Pulmonary edema due to ECF expansion
- Severe constipation
- Agranulocytosis
Correct Answer: Pulmonary edema due to ECF expansion
Q9. Mannitol is contraindicated in which of the following conditions?
- Oliguria with adequate perfusion
- Anuria or severe renal failure
- Acute glaucoma with intact BBB
- Preventive therapy for contrast nephropathy with good urine output
Correct Answer: Anuria or severe renal failure
Q10. The typical concentration of Mannitol commonly used for IV bolus to reduce intracranial pressure is:
- 0.9% solution
- 5% solution
- 20% solution
- 50% solution
Correct Answer: 20% solution
Q11. Which property of Mannitol explains its inability to cross an intact blood–brain barrier rapidly?
- High lipid solubility
- Low molecular weight
- Poor lipid permeability and polar nature
- Strong protein binding
Correct Answer: Poor lipid permeability and polar nature
Q12. Mannitol lowers intraocular pressure primarily by:
- Blocking aqueous humor production at ciliary body
- Increasing trabecular outflow only
- Creating an osmotic gradient drawing water from vitreous
- Stimulating pupil constriction to open drainage angle
Correct Answer: Creating an osmotic gradient drawing water from vitreous
Q13. Which statement about Mannitol pharmacokinetics is correct?
- It is extensively metabolized by the liver.
- It is rapidly absorbed orally and undergoes first‑pass metabolism.
- It is not metabolized and is excreted unchanged by the kidney.
- It is converted to urea in the kidney.
Correct Answer: It is not metabolized and is excreted unchanged by the kidney.
Q14. The initial effect of Mannitol on extracellular fluid volume is:
- Marked chronic contraction of ECF only after several days
- Transient expansion of ECF due to intravascular fluid shift
- Permanent increase in intracellular fluid volume
- No change in fluid compartments
Correct Answer: Transient expansion of ECF due to intravascular fluid shift
Q15. When Mannitol is given to prevent acute renal failure after toxin exposure, the mechanism is primarily:
- Direct detoxification of the toxin
- Increasing renal blood flow and urine flow to flush tubules
- Blocking tubular secretion of the toxin
- Stimulating hepatic elimination
Correct Answer: Increasing renal blood flow and urine flow to flush tubules
Q16. Which monitoring parameter indicates effective osmotic diuresis after Mannitol administration?
- Urine osmolality increases above plasma osmolality
- Urine output increases and urine osmolality decreases
- Serum potassium rises above normal immediately
- Serum glucose falls below fasting level
Correct Answer: Urine output increases and urine osmolality decreases
Q17. Mannitol crystals may form if stored at low temperature. What is the recommended precaution before administration?
- Freeze and thaw the solution rapidly
- Warm the solution and inspect for crystals; use a filter if needed
- Dilute the solution 1:10 with ethanol
- Shake vigorously to dissolve crystals
Correct Answer: Warm the solution and inspect for crystals; use a filter if needed
Q18. Which electrolyte disturbance can occur with prolonged Mannitol‑induced diuresis?
- Hyperkalemia only
- Hypokalemia and hyponatremia or hypernatremia depending on net water loss
- Isolated hypercalcemia
- No change in electrolytes
Correct Answer: Hypokalemia and hyponatremia or hypernatremia depending on net water loss
Q19. A contraindication specific to use of Mannitol in brain injury is:
- Presence of intact skull
- Patients with established pulmonary embolism
- Severe dehydration with circulatory collapse
- Intracranial hemorrhage with destroyed blood–brain barrier causing rebound edema
Correct Answer: Intracranial hemorrhage with destroyed blood–brain barrier causing rebound edema
Q20. Typical IV bolus dosing range of Mannitol for acute elevated intracranial pressure is approximately:
- 0.01–0.05 g/kg
- 0.25–1 g/kg
- 5–10 g/kg
- 10–20 mg/kg
Correct Answer: 0.25–1 g/kg
Q21. Mannitol’s effect on renal hemodynamics includes which immediate action?
- Decreasing renal blood flow markedly
- Increasing renal blood flow and glomerular filtration transiently
- Vasoconstriction of afferent arteriole only
- Permanent damage to glomeruli
Correct Answer: Increasing renal blood flow and glomerular filtration transiently
Q22. Which of the following best describes Mannitol’s chemical classification?
- A loop diuretic sulfonamide
- A carbonic anhydrase inhibitor derivative
- A sugar alcohol (polyol) osmotic agent
- A thiazide‑type benzothiadiazine
Correct Answer: A sugar alcohol (polyol) osmotic agent
Q23. In patients at risk for acute tubular necrosis, Mannitol may be used for renal protection because it:
- Causes intracellular swelling in tubular cells
- Precipitates toxins in tubular lumen
- Prevents tubular obstruction by maintaining urine flow
- Inhibits cytochrome P450 enzymes
Correct Answer: Prevents tubular obstruction by maintaining urine flow
Q24. Which of the following statements about Mannitol and the blood–brain barrier is true?
- Mannitol freely crosses an intact BBB and accumulates in brain parenchyma
- Mannitol cannot enter the cerebrospinal fluid even with high doses
- Mannitol does not cross an intact BBB, creating an osmotic gradient to draw water out
- Mannitol is actively transported across the BBB by GLUT transporters
Correct Answer: Mannitol does not cross an intact BBB, creating an osmotic gradient to draw water out
Q25. Which of the following is a potential interaction concern when using Mannitol?
- Concomitant lithium therapy leading to lithium retention
- Enhanced renal excretion of drugs eliminated by glomerular filtration
- Inactivation of aminoglycosides in plasma
- Marked increase in warfarin anticoagulant effect
Correct Answer: Enhanced renal excretion of drugs eliminated by glomerular filtration
Q26. For safe infusion, Mannitol should ideally be administered via:
- Oral route mixed with saline
- Slow IV infusion into a large vein with monitoring and filter if needed
- Subcutaneous bolus
- Intraosseous only
Correct Answer: Slow IV infusion into a large vein with monitoring and filter if needed
Q27. Which physicochemical property of Mannitol is most relevant to its diuretic action?
- High degree of ionization at physiological pH
- Low water solubility
- High osmotic activity per unit mass
- Strong protein binding in plasma
Correct Answer: High osmotic activity per unit mass
Q28. In traumatic brain injury with disrupted BBB, Mannitol may be ineffective or harmful because:
- It increases cerebrospinal fluid production
- Mannitol can cross into brain tissue and cause rebound intracranial hypertension
- It causes permanent vasodilation of cerebral vessels
- It converts into glucose in brain tissue
Correct Answer: Mannitol can cross into brain tissue and cause rebound intracranial hypertension
Q29. Which of the following is true regarding Mannitol dosing in renal impairment?
- No dose adjustment is necessary in all renal failure stages
- Mannitol is contraindicated in anuria and should be used cautiously with impaired renal function
- Dose should be doubled in renal impairment
- Mannitol is cleared by the liver so renal impairment has no effect
Correct Answer: Mannitol is contraindicated in anuria and should be used cautiously with impaired renal function
Q30. Which monitoring sign would most quickly indicate developing pulmonary congestion during Mannitol therapy?
- Rising serum creatinine only
- Sudden increase in respiratory rate and crackles on lung auscultation
- Decreased urine output without respiratory changes
- Improved level of consciousness only
Correct Answer: Sudden increase in respiratory rate and crackles on lung auscultation
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