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APC Written Assessment study materials
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APC Written Assessment practice questions

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Question 1Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Oral bioavailability is defined as the fraction of an administered dose that:

  1. is absorbed across the intestinal epithelium
  2. undergoes extensive hepatic metabolism
  3. reaches systemic circulation unchanged Correct answer.
  4. binds extensively to plasma albumin

Explanation

Bioavailability measures the extent and rate of unchanged drug reaching the systemic circulation after administration. Absorption is only a prerequisite; drug removed by first-pass metabolism or incomplete absorption does not contribute. Protein binding affects distribution rather than the definition of bioavailability.

Question 2Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

First-pass metabolism of an orally administered drug occurs mainly in the:

  1. renal tubular cells
  2. hepatic tissue Correct answer.
  3. alveolar epithelium
  4. gastric parietal cells

Explanation

After absorption from the gut, drug enters the portal vein and passes through the liver before reaching the systemic circulation. Hepatic enzymes can therefore metabolise a substantial fraction of an oral dose before it is distributed. Other organs contribute minor extrahepatic metabolism.

Question 3Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

The units of drug clearance are best expressed as:

  1. L/h, a volume per unit time Correct answer.
  2. h/L, a time per unit volume
  3. mg/L, a mass per unit volume
  4. mg/h, a mass per unit time

Explanation

Clearance is the volume of plasma from which drug is completely removed per unit time, so its units are volume per time. It is not an amount eliminated per time, which would be a mass rate. Dividing a mass rate by concentration gives volume per time.

Question 4Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A weak acid drug with a pKa of 5 is placed in a solution of pH 2. The drug will be predominantly:

  1. unionised and water-soluble
  2. unionised and lipid-soluble Correct answer.
  3. ionised and lipid-soluble
  4. ionised and water-soluble

Explanation

For a weak acid, the fraction ionised increases as pH rises above the pKa. At pH 2, well below the pKa of 5, the protonated unionised form predominates, and unionised weak acids are generally more lipid-soluble.

Question 5Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Which excipient is added to a tablet formulation primarily to promote tablet breakup in the gastrointestinal tract?

  1. magnesium stearate
  2. lactose monohydrate
  3. colloidal silicon dioxide
  4. starch glycolate Correct answer.

Explanation

Disintegrants such as starch glycolate swell or wick water into the tablet, causing it to break apart and expose drug particles for dissolution. Magnesium stearate is a lubricant, lactose is a filler, and colloidal silicon dioxide is a glidant.

Question 6Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A prodrug is best described as a compound that:

  1. is always more potent than its metabolite
  2. is active as administered without conversion
  3. is converted in the body to an active drug Correct answer.
  4. is unable to cross biological membranes

Explanation

A prodrug is pharmacologically inactive or less active as given and must undergo chemical or enzymatic conversion to release the active species. This strategy can improve absorption, stability, or targeting. Potency and membrane permeability are not defining features.

Question 7Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

After four elimination half-lives, the approximate fraction of a drug remaining in the body is:

  1. 6.25% Correct answer.
  2. 25%
  3. 3.125%
  4. 12.5%

Explanation

Each half-life reduces the amount remaining by half, so after n half-lives the remaining fraction is (1/2)^n. For four half-lives, (1/2)^4 = 1/16, or 6.25%. This assumes first-order elimination.

Question 8Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

An enteric coating on a tablet is designed to:

  1. mask the drug's bitter taste in the mouth
  2. dissolve rapidly in gastric acid
  3. resist gastric acid Correct answer.
  4. provide immediate drug release in the stomach

Explanation

Enteric polymers are insoluble at the low pH of the stomach but dissolve at the higher pH of the intestine. This protects acid-labile drugs and prevents gastric irritation. It is not intended for immediate gastric release.

Question 9Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A 500 mg intravenous bolus dose produces an initial plasma concentration of 25 mg/L. The apparent volume of distribution is:

  1. 5 L
  2. 12,500 L
  3. 20 L Correct answer.
  4. 12.5 L

Explanation

Volume of distribution is calculated as amount of drug in the body divided by plasma concentration: Vd = Dose/C0. Here 500 mg ÷ 25 mg/L = 20 L. The value is a proportionality constant, not a real anatomical volume.

Question 10Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A drug has a clearance of 4 L/h and a volume of distribution of 20 L. Its elimination rate constant is:

  1. 5 h⁻¹
  2. 80 h⁻¹
  3. 0.2 h⁻¹ Correct answer.
  4. 0.05 h⁻¹

Explanation

For first-order elimination, clearance equals the elimination rate constant multiplied by volume of distribution, so ke = CL/Vd. Here 4 L/h ÷ 20 L = 0.2 h⁻¹. The rate constant has units of reciprocal time.

Question 11Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

An oral 100 mg dose gives an AUC of 20 mg·h/L; the same 100 mg dose given intravenously gives an AUC of 50 mg·h/L. The oral bioavailability is:

  1. 25%
  2. 250%
  3. 40% Correct answer.
  4. 20%

Explanation

When doses are equal, bioavailability is the ratio of oral AUC to intravenous AUC: F = AUCoral/AUCIV. Here 20/50 = 0.40, or 40%. Dose correction is needed only when the compared doses differ.

Question 12Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

In zero-order elimination, the amount of drug eliminated per unit time is:

  1. proportional to plasma concentration
  2. constant until the drug is exhausted Correct answer.
  3. exponentially decreasing with time
  4. dependent on the dose administered

Explanation

Zero-order elimination occurs when eliminating processes are saturated, so a fixed amount of drug is removed per unit time regardless of concentration. This contrasts with first-order elimination, where the amount eliminated is proportional to concentration. The rate remains constant until substrate is depleted.

Question 13Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A drug has a volume of distribution of 50 L and a target plasma concentration of 10 mg/L. The intravenous loading dose is:

  1. 50 mg
  2. 5000 mg
  3. 5 mg
  4. 500 mg Correct answer.

Explanation

A loading dose is calculated as Vd multiplied by the target concentration: LD = Vd × Ctarget. Here 50 L × 10 mg/L = 500 mg. Bioavailability would be included only for a non-intravenous route.

Question 14Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A weak base with a pKa of 8 is present in urine at pH 6 and in plasma at pH 7.4. Compared with plasma, the drug in urine will be:

  1. less ionised and more readily reabsorbed
  2. more ionised and more readily reabsorbed
  3. less ionised and less readily reabsorbed
  4. more ionised and less readily reabsorbed Correct answer.

Explanation

For a weak base, lowering pH below the pKa increases protonation and therefore ionisation. Ionised drug is poorly lipid-soluble and is not readily reabsorbed across the tubular epithelium, so acidic urine increases renal elimination of weak bases.

Question 15Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

In medicinal chemistry, a bioisostere is best described as:

  1. an enantiomer with opposite pharmacological action
  2. a metabolite produced by phase II conjugation
  3. a group mimicking another group's properties Correct answer.
  4. a drug that binds to the same receptor protein

Explanation

Bioisosteric replacement substitutes one functional group for another with similar size, electronics, or hydrogen-bonding properties, aiming to preserve activity while improving properties. The replacement is not necessarily a drug, enantiomer, or metabolite.

Question 16Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

An ester prodrug is typically activated in the body by:

  1. acetylation by NAT enzymes
  2. glucuronidation by UGT enzymes
  3. oxidation by cytochrome P450
  4. hydrolysis by esterases Correct answer.

Explanation

Ester prodrugs are commonly hydrolysed by esterases in plasma, liver, or intestinal tissue to release the parent drug and an acid or alcohol. Oxidation, glucuronidation, and acetylation are phase I or phase II metabolic reactions, not the typical activation route for an ester.

Question 17Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Magnesium stearate is included in tablet formulations mainly as a:

  1. glidant
  2. disintegrant
  3. binder
  4. lubricant Correct answer.

Explanation

Magnesium stearate is a hydrophobic lubricant that reduces friction between the tablet and the die wall and prevents sticking to punches. Binders promote granule cohesion, disintegrants break tablets apart, and glidants improve powder flow.

Question 18Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Two polymorphic forms of a drug differ in dissolution rate mainly because they have different:

  1. crystal lattice energies Correct answer.
  2. covalent chemical structures
  3. molecular weights
  4. numbers of chiral centres

Explanation

Polymorphs have the same molecular structure but different arrangements in the crystal lattice, giving different lattice energies and therefore different solubility and dissolution rates. Covalent structure, molecular weight, and chirality are unchanged between polymorphs.

Question 19Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

For a highly protein-bound drug, an increase in plasma protein binding will generally:

  1. increase Vd and filtration rate
  2. lower Vd and filtration Correct answer.
  3. shorten the elimination half-life
  4. increase renal clearance substantially

Explanation

Drug bound to plasma proteins is confined largely to the vascular space, so increased binding tends to lower the apparent volume of distribution. Only unbound drug is filtered at the glomerulus, so increased binding reduces filtration. Changes in clearance depend on the elimination mechanism and are not automatically increased.

Question 20Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A drug with a high hepatic extraction ratio close to 1, given orally, will show:

  1. negligible hepatic metabolism
  2. high oral bioavailability
  3. extensive first-pass loss Correct answer.
  4. predominant renal excretion

Explanation

A high extraction ratio means the liver removes most drug presented to it in a single pass. After oral absorption, portal blood exposes the liver before systemic circulation, so first-pass loss is extensive and oral bioavailability is low. Such drugs often have much higher bioavailability when given intravenously.

Question 21Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

For a drug following saturable Michaelis–Menten metabolism, as plasma concentration rises well above Km, the:

  1. half-life progressively increases Correct answer.
  2. half-life approaches a constant value
  3. clearance increases proportionally
  4. volume of distribution falls

Explanation

When concentration is far above Km, metabolising enzymes are saturated and elimination approaches a constant maximum rate, effectively zero-order. Clearance therefore falls as concentration rises, and the time to eliminate a given fraction lengthens, so half-life progressively increases.

Question 22Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

In a two-compartment model after an intravenous bolus, the distribution phase (alpha phase) is characterised by:

  1. a slower initial decline
  2. an initial rise in concentration
  3. a flat plasma concentration
  4. a steeper initial decline Correct answer.

Explanation

The alpha phase reflects rapid distribution of drug from plasma into tissues, producing a steep early fall in plasma concentration. The later beta phase is slower and reflects elimination from the body. The initial concentration does not rise after an intravenous bolus.

Question 23Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Two enantiomers of a chiral drug most commonly differ in:

  1. optical rotation and often receptor affinity Correct answer.
  2. their molecular formula and molecular weight
  3. their ability to form ionic salts with acids
  4. the number of hydrogen atoms in the molecule

Explanation

Enantiomers have identical connectivity and molecular formula but are non-superimposable mirror images. They rotate plane-polarised light in opposite directions and can differ markedly in binding to chiral receptors or enzymes, and hence in pharmacodynamics and pharmacokinetics.

Question 24Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

According to the Noyes–Whitney equation, which change will increase the dissolution rate of a poorly soluble drug?

  1. decreasing the solubility of the drug
  2. increasing surface area by micronisation Correct answer.
  3. increasing the diffusion layer thickness
  4. increasing the particle size of the drug

Explanation

The Noyes–Whitney equation states that dissolution rate is proportional to surface area and solubility and inversely proportional to diffusion layer thickness. Micronisation increases surface area, so it increases dissolution rate. Reducing particle size also shortens diffusion distance in practice.

Question 25Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A drug has a urine concentration of 200 mg/L, a urine flow rate of 0.5 L/h, and a plasma concentration of 10 mg/L. Its renal clearance is:

  1. 0.025 L/h
  2. 4 L/h
  3. 100 L/h
  4. 10 L/h Correct answer.

Explanation

Renal clearance is calculated as CLr = (U × V)/P, where U is urine concentration, V is urine flow, and P is plasma concentration. Here (200 mg/L × 0.5 L/h) ÷ 10 mg/L = 10 L/h. This value reflects net filtration, secretion, and reabsorption.

Question 26Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

In wet granulation, the primary function of a binder is to:

  1. reduce friction at the die wall
  2. promote particle adhesion Correct answer.
  3. accelerate tablet disintegration
  4. mask an unpleasant taste

Explanation

A binder provides cohesion between powder particles so that granules and tablets retain structure. Disintegration, lubrication and taste masking are separate formulation functions.

Question 27Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A superdisintegrant such as croscarmellose sodium acts mainly by:

  1. forming a hydrophobic film around granules
  2. neutralising gastric acid after swallowing
  3. binding fine particles during granulation
  4. swelling rapidly in water Correct answer.

Explanation

Superdisintegrants swell rapidly when they contact aqueous fluid, breaking the tablet into granules. They do not act by acid neutralisation or by binding particles together.

Question 28Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

In first-order elimination, the fraction of drug eliminated per unit time is:

  1. proportional to the dose administered
  2. increased as plasma concentration rises
  3. equal to the elimination half-life
  4. constant regardless of concentration Correct answer.

Explanation

First-order elimination has a constant elimination rate constant, so a fixed fraction of the remaining drug is removed per unit time. The amount removed per unit time changes with concentration, but the fraction does not.

Question 29Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A very large apparent volume of distribution for a drug usually indicates that the drug:

  1. stays mainly in plasma water
  2. shows little tissue penetration
  3. is cleared unchanged by kidney
  4. enters tissues extensively Correct answer.

Explanation

A large apparent volume of distribution reflects extensive distribution out of plasma into tissues. It is an apparent space, not a real anatomical volume.

Question 30Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Absolute bioavailability is determined by comparing an extravascular dose with:

  1. a repeat extravascular dose
  2. an oral dose of a different drug salt
  3. the same drug given intravenously Correct answer.
  4. a topical dose of the same drug

Explanation

Absolute bioavailability uses an intravenous dose as the reference because intravenous administration is assumed to deliver the full dose to systemic circulation. The same drug must be compared.

Question 31Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

The logarithm of the octanol–water partition coefficient (log P) is used mainly as a measure of a drug's:

  1. lipophilicity Correct answer.
  2. receptor affinity
  3. molecular weight
  4. plasma protein binding

Explanation

Log P quantifies partitioning between octanol and water and is a standard measure of lipophilicity. It does not directly measure receptor affinity, molecular weight or protein binding.

Question 32Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A pharmacophore describes:

  1. the metabolic pathway of a drug
  2. the dose-response slope of an agonist
  3. the spatial features required for activity Correct answer.
  4. the crystalline form of a drug substance

Explanation

A pharmacophore is the arrangement of essential molecular features needed for biological activity. It is an abstract model, not a metabolic route or a crystal form.

Question 33Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Phase II drug metabolism generally involves:

  1. conjugation with polar groups Correct answer.
  2. hydrolysis of ester bonds
  3. oxidation by cytochrome P450 enzymes
  4. reduction of nitro groups

Explanation

Phase II reactions conjugate a drug or metabolite with polar groups, increasing water solubility and usually facilitating excretion. Oxidation, reduction and hydrolysis are typically phase I reactions.

Question 34Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Total body clearance of a drug is best defined as:

  1. fraction of dose reaching circulation
  2. plasma volume cleared per unit time Correct answer.
  3. amount of drug in body at steady state
  4. time for plasma concentration to halve

Explanation

Clearance is the volume of plasma from which drug is completely removed per unit time. It is a primary pharmacokinetic parameter independent of concentration in linear systems.

Question 35Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A drug has a volume of distribution of 40 L and a clearance of 8 L/h. Its elimination half-life is approximately:

  1. 8.0 h
  2. 0.7 h
  3. 3.5 h Correct answer.
  4. 5.0 h

Explanation

For first-order elimination, t1/2 = 0.693 × Vd / CL. Substituting gives 0.693 × 40 / 8 ≈ 3.5 h.

Question 36Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

For a drug with clearance 5 L/h, the infusion rate needed to maintain a steady-state concentration of 4 mg/L is:

  1. 20 mg/h Correct answer.
  2. 0.8 mg/h
  3. 80 mg/h
  4. 1.25 mg/h

Explanation

At steady state, infusion rate equals clearance multiplied by target concentration. 5 L/h × 4 mg/L = 20 mg/h.

Question 37Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Hepatic clearance of a drug with an extraction ratio of 0.6 and hepatic blood flow of 90 L/h is:

  1. 36 L/h
  2. 90 L/h
  3. 60 L/h
  4. 54 L/h Correct answer.

Explanation

Hepatic clearance is blood flow multiplied by extraction ratio when flow is the limiting determinant. 90 L/h × 0.6 = 54 L/h.

Question 38Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Compared with the free acid, the sodium salt of a weakly acidic drug generally has:

  1. much lower dissolution rate
  2. higher volatility
  3. higher aqueous solubility Correct answer.
  4. greater lipid solubility

Explanation

Ionizable salts dissolve more readily in water than the neutral acid because the ionic form hydrates well. Salt formation usually lowers lipid solubility and tends to increase, not decrease, dissolution rate.

Question 39Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

A surfactant increases the apparent aqueous solubility of a poorly soluble drug above its critical micelle concentration mainly by:

  1. forming a covalent drug conjugate
  2. reducing the drug's molecular weight
  3. raising the solution pH
  4. incorporating drug into micelles Correct answer.

Explanation

Above the critical micelle concentration, surfactants self-assemble into micelles that solubilise lipophilic drug within a hydrophobic core. This is a physical partitioning process, not covalent conjugation.

Question 40Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Cyclodextrins improve the aqueous solubility of lipophilic drugs by:

  1. hydrolysing the drug in solution
  2. converting the drug to a salt
  3. increasing plasma protein binding
  4. forming inclusion complexes Correct answer.

Explanation

Cyclodextrins form inclusion complexes in which the lipophilic drug is held within a hydrophilic cavity, improving apparent aqueous solubility. They do not usually act by salt formation or hydrolysis.

Question 41Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

According to Stokes' law, increasing the viscosity of the vehicle in a suspension will:

  1. increase the sedimentation rate
  2. increase particle density
  3. decrease the sedimentation rate Correct answer.
  4. reduce the zeta potential to zero

Explanation

Stokes' law shows sedimentation velocity is inversely proportional to medium viscosity. A more viscous vehicle slows particle settling.

Question 42Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

An oil-in-water emulsion can be diluted with water without phase inversion because its continuous phase is:

  1. oil
  2. air
  3. a solid lipid
  4. water Correct answer.

Explanation

In an oil-in-water emulsion, oil droplets are dispersed in a continuous aqueous phase. Therefore the external phase is water and the emulsion can be diluted with water.

Question 43Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

The Arrhenius equation is used in pharmaceutical stability testing to:

  1. determine pH of maximum solubility
  2. predict microbial growth in a formulation
  3. calculate tablet hardness
  4. predict stability at lower temperature Correct answer.

Explanation

The Arrhenius equation relates degradation rate constant to temperature, allowing accelerated data to estimate stability at lower storage temperatures. It does not measure microbial growth, hardness or solubility.

Question 44Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

For a drug degrading by zero-order kinetics, a plot of concentration against time is:

  1. exponential
  2. parabolic
  3. logarithmic
  4. linear Correct answer.

Explanation

Zero-order degradation proceeds at a constant rate, so concentration declines linearly with time. First-order degradation gives an exponential concentration–time curve.

Question 45Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

According to Lipinski's rule of five, a drug is more likely to have poor oral absorption if its molecular weight exceeds:

  1. 200 Da
  2. 500 Da Correct answer.
  3. 350 Da
  4. 100 Da

Explanation

Lipinski's rule flags poor absorption when molecular weight is greater than 500 Da, along with other thresholds for log P, hydrogen-bond donors and acceptors. The other listed values are below that threshold.

Question 46Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Conformational restriction of a flexible ligand often increases potency because it:

  1. lowers entropic penalty on binding Correct answer.
  2. lowers the compound's molecular weight
  3. prevents ionization at physiological pH
  4. increases the number of rotatable bonds

Explanation

A flexible ligand loses conformational entropy when it binds, which costs free energy. Restricting the active conformation reduces this entropic penalty and can improve potency.

Question 47Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Flip-flop kinetics is said to occur when:

  1. distribution is slower than elimination
  2. metabolism is saturable
  3. absorption is slower than elimination Correct answer.
  4. elimination is slower than absorption

Explanation

In flip-flop kinetics, absorption is the slower process, so the terminal phase reflects absorption rather than elimination. This can make the apparent half-life longer than the true elimination half-life.

Question 48Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

For a weak acid with a pKa of 4.4, the percentage ionized at pH 7.4 is approximately:

  1. 100%
  2. 0.1%
  3. 99.9% Correct answer.
  4. 50%

Explanation

Henderson–Hasselbalch gives pH − pKa = log([A−]/[HA]) = 3, so the ionized-to-unionized ratio is about 1000:1. That corresponds to about 99.9% ionized.

Question 49Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

Enterohepatic recirculation can prolong a drug's half-life because:

  1. it increases hepatic blood flow
  2. reabsorbed drug returns to circulation Correct answer.
  3. it increases renal filtration substantially
  4. it converts drug to a polar metabolite

Explanation

Drug excreted in bile can be reabsorbed from the intestine and re-enter the systemic circulation, effectively recycling it. This recycling lowers apparent clearance and prolongs the half-life.

Question 50Basic pharmaceutical sciences (pharmaceutics, medicinal chemistry, pharmacokinetics)

If active tubular secretion becomes saturated as plasma concentration rises, renal clearance will:

  1. remain exactly equal to GFR
  2. decrease at high concentrations Correct answer.
  3. become independent of concentration
  4. increase proportionally with concentration

Explanation

Once secretory transport is saturated, the secreted amount approaches a maximum while plasma concentration continues to rise. Renal clearance therefore falls toward the filtration component.