pH-partition theory MCQs With Answer

Introduction: pH-Partition Theory MCQs With Answer is a concise question bank designed specifically for M.Pharm students preparing for Advanced Biopharmaceutics & Pharmacokinetics (MPH 202T). This collection focuses on the principles that govern drug ionization, membrane permeation, and pH-dependent distribution — core concepts that determine absorption, distribution, and renal handling of drugs. Questions range from fundamental Henderson–Hasselbalch applications and percent ionization calculations to advanced topics such as ion trapping, apparent partition coefficients (log D), and limitations of the pH-partition theory in biological systems. Each MCQ includes a clear correct answer to reinforce learning and exam readiness.

Q1. What is the central premise of the pH-partition theory in drug absorption?

• Drug absorption is independent of ionization and depends only on molecular size
• Only the ionized form of a drug crosses lipid membranes readily
• Unionized (uncharged) form of a drug crosses lipid membranes more readily, so pH and pKa determine absorption
• Active transporters exclusively determine absorption regardless of pH

Correct Answer: Unionized (uncharged) form of a drug crosses lipid membranes more readily, so pH and pKa determine absorption

Q2. Which Henderson–Hasselbalch equation correctly relates pH, pKa and concentrations for a weak acid (HA)?

• pH = pKa + log([HA]/[A-])
• pH = pKa + log([A-]/[HA])
• pKa = pH + log([A-]/[HA])
• pH = pKa − log([A-]/[HA])

Correct Answer: pH = pKa + log([A-]/[HA])

Q3. For a weak base (B ⇌ BH+), which Henderson–Hasselbalch form is correct?

• pH = pKa + log([BH+]/[B])
• pH = pKa + log([B]/[BH+])
• pKa = pH − log([B]/[BH+])
• pH = pKa − log([B]/[BH+])

Correct Answer: pH = pKa + log([B]/[BH+])

Q4. Which molecular form typically exhibits higher membrane permeability according to pH-partition theory?

• The ionized charged form because it interacts with phospholipids
• The unionized neutral form because it is more lipophilic
• Ionic complexes only when paired with large counter-ions
• Protein-bound drug only

Correct Answer: The unionized neutral form because it is more lipophilic

Q5. If a weak acid has a pKa of 4.0, what is the dominant form at pH 6.0?

• Primarily unionized (HA)
• Primarily ionized (A−)
• Equimolar ionized and unionized
• Completely protonated

Correct Answer: Primarily ionized (A−)

Q6. Which scenario describes “ion trapping” of a weak base?

• A weak base accumulates in alkaline compartments because it becomes unionized there
• A weak base accumulates in acidic compartments because protonation produces a charged form that cannot cross back
• A weak base freely diffuses out of acidic compartments without accumulation
• A weak base is actively transported against the pH gradient without ionization changes

Correct Answer: A weak base accumulates in acidic compartments because protonation produces a charged form that cannot cross back

Q7. Which statement correctly explains why many weak acids are well absorbed from the stomach?

• Stomach pH is high, causing weak acids to ionize and permeate easily
• Stomach pH is low, keeping weak acids largely unionized and favoring lipid membrane permeation
• Gastric motility prevents absorption of weak acids
• Weak acids require active transporters that are abundant in the stomach

Correct Answer: Stomach pH is low, keeping weak acids largely unionized and favoring lipid membrane permeation

Q8. Which limitation of pH-partition theory is most significant for predicting drug distribution into the brain?

• The theory predicts active transport accurately but not passive diffusion
• It ignores active transporters and tight junctions of the blood–brain barrier that can dominate distribution
• It overestimates the role of ionization in lipid solubility always favoring ionized forms
• It requires constant plasma protein binding across compartments

Correct Answer: It ignores active transporters and tight junctions of the blood–brain barrier that can dominate distribution

Q9. How does ion-pairing improve membrane permeation of ionic drugs?

• By converting the drug to a larger charged complex that solubilizes in water
• By forming neutral or less polar paired species that are more lipophilic and cross membranes better
• By increasing drug ionization permanently
• By binding to proteins and reducing free drug concentration

Correct Answer: By forming neutral or less polar paired species that are more lipophilic and cross membranes better

Q10. What does the term “apparent partition coefficient” (distribution coefficient, D) refer to?

• The partitioning of only the unionized species between octanol and water at any pH
• The pH-dependent overall partitioning of both ionized and unionized species between octanol and water
• A constant value equal to intrinsic lipophilicity (P) irrespective of pH
• The ratio of intracellular to extracellular unbound drug only

Correct Answer: The pH-dependent overall partitioning of both ionized and unionized species between octanol and water

Q11. A weak base with pKa 8.0 is placed in plasma at pH 7.4. Which is true about its ionization and expected passive permeability?

• Mostly unionized; high passive permeability
• Mostly ionized (protonated); reduced passive permeability
• Completely unionized; unable to cross membranes
• Ionization is 50:50 and permeability is maximal

Correct Answer: Mostly ionized (protonated); reduced passive permeability

Q12. For a weak base with pKa 7.4 at pH 6.4, what approximate fraction of the drug is in the unprotonated (unionized) form?

• Approximately 90%
• Approximately 50%
• Approximately 9%
• Approximately 0.9%

Correct Answer: Approximately 9%

Q13. Which underlying assumption is fundamental to the classical pH-partition theory?

• Drug transport across membranes is dominated by carrier-mediated active transport
• Equilibrium between ionized and unionized forms is rapidly achieved and permeation is by passive diffusion of the unionized form
• All membranes have the same lipid composition and permeability
• Protein binding totally prevents membrane permeation

Correct Answer: Equilibrium between ionized and unionized forms is rapidly achieved and permeation is by passive diffusion of the unionized form

Q14. Under which circumstance is the pH‑partition theory least reliable for predicting drug absorption?

• When passive diffusion is the only route of permeation
• When drug absorption occurs across simple lipid bilayers without transporters
• When significant carrier-mediated transport, efflux pumps, or irreversible intracellular binding occur
• When the drug is a small, neutral molecule

Correct Answer: When significant carrier-mediated transport, efflux pumps, or irreversible intracellular binding occur

Q15. Which expression correctly gives the fraction of a weak acid present in the unionized (HA) form?

• Fraction unionized = 1 / (1 + 10^(pKa − pH))
• Fraction unionized = 1 / (1 + 10^(pH − pKa))
• Fraction unionized = 10^(pH − pKa) / (1 + 10^(pH − pKa))
• Fraction unionized = 10^(pKa − pH) / (1 + 10^(pKa − pH))

Correct Answer: Fraction unionized = 1 / (1 + 10^(pH − pKa))

Q16. According to pH-partition theory, why are many weak bases better absorbed in the small intestine than in the stomach?

• The small intestine has lower surface area and lower pH than the stomach
• The small intestine has higher pH, making weak bases more unionized and therefore more permeable
• The stomach enzymes actively modify weak bases preventing absorption
• The stomach lining is impermeable to all weak bases

Correct Answer: The small intestine has higher pH, making weak bases more unionized and therefore more permeable

Q17. Where will a weak acid preferentially accumulate due to ion trapping if the two compartments have pH 7.4 (outside) and pH 6.0 (inside)?

• Accumulates in the more acidic compartment (pH 6.0)
• Accumulates in the more basic compartment (pH 7.4)
• No net accumulation because acid is always unionized
• Distribution is determined solely by protein binding, not pH

Correct Answer: Accumulates in the more basic compartment (pH 7.4)

Q18. How does urinary alkalinization affect renal excretion of a weak acidic drug?

• It decreases excretion by increasing unionized fraction and reabsorption
• It increases excretion by increasing ionization and reducing reabsorption
• It has no effect because renal excretion is pH-independent
• It converts the drug into a metabolite that is retained

Correct Answer: It increases excretion by increasing ionization and reducing reabsorption

Q19. What is log D and how does it differ from log P?

• log D is the pH-dependent distribution coefficient including ionized and unionized species; log P is the partition coefficient of the neutral species only
• log D is the partition coefficient of neutral species; log P accounts for ionization at any pH
• log D refers to drug diffusion rate; log P refers to protein binding
• log D and log P are equivalent and interchangeable terms

Correct Answer: log D is the pH-dependent distribution coefficient including ionized and unionized species; log P is the partition coefficient of the neutral species only

Q20. Which of the following best describes a practical limitation when applying pH‑partition theory to predict in vivo drug absorption?

• The theory accounts for active transport and metabolism but not for pH
• Biological membranes are homogeneous and hence the theory overcomplicates predictions
• Local microenvironment pH, membrane heterogeneity, protein binding, and carrier-mediated processes can all invalidate simple pH-based predictions
• The theory accurately predicts all drug absorption phenomena without exception

Correct Answer: Local microenvironment pH, membrane heterogeneity, protein binding, and carrier-mediated processes can all invalidate simple pH-based predictions

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