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Question 1General Principles of Toxicology

The median lethal dose (LD50) of a chemical is best defined as the dose that:

  1. produces a measurable adverse effect in 50% of exposed animals
  2. represents the lowest dose at which any mortality is observed
  3. kills 50% of a defined test population under specified conditions Correct answer.
  4. is lethal to every member of a susceptible subpopulation

Explanation

The LD50 is a statistically derived single dose expected to kill half of a defined animal population under stated conditions; it is a population estimate and not a threshold for individual lethality.

Question 2General Principles of Toxicology

Which reaction is classified as a phase I biotransformation?

  1. Conjugation of a carboxylic acid with glycine
  2. Glucuronidation of a phenolic hydroxyl group
  3. Sulfate conjugation of a primary alcohol
  4. Hydroxylation of an aromatic ring by cytochrome P450 Correct answer.

Explanation

Phase I reactions are functionalization reactions such as oxidation, reduction, and hydrolysis that add or expose a polar group; cytochrome P450 hydroxylation is the classic example. Phase II reactions are conjugations with endogenous cofactors.

Question 3General Principles of Toxicology

The principal organ of xenobiotic biotransformation in mammals is the:

  1. kidney
  2. small intestine
  3. lung
  4. liver Correct answer.

Explanation

The liver receives portal blood carrying absorbed xenobiotics and contains the highest concentrations of cytochrome P450 and conjugating enzymes, making it the principal site of biotransformation.

Question 4General Principles of Toxicology

The no-observed-adverse-effect level (NOAEL) is defined as the:

  1. lowest dose producing a statistically significant response
  2. highest dose tested with no significant adverse effect Correct answer.
  3. dose estimated to produce a 10% increase over background
  4. dose at which adverse effects are first detected

Explanation

The NOAEL is the highest dose in a study at which no statistically or biologically significant increase in adverse effects is found relative to controls, and it is the traditional starting point for deriving reference values.

Question 5General Principles of Toxicology

A very large apparent volume of distribution (for example, greater than 5 L/kg) usually indicates that the compound:

  1. is extensively distributed into tissues Correct answer.
  2. is confined largely to the plasma compartment
  3. has negligible binding to tissue proteins
  4. is excreted unchanged by the kidney very rapidly

Explanation

Apparent volume of distribution is the proportionality constant between amount of drug in the body and plasma concentration; a large value means the compound leaves plasma and concentrates in tissues.

Question 6General Principles of Toxicology

After oral administration, extensive first-pass metabolism in the liver typically causes:

  1. an increase in systemic bioavailability
  2. a reduction in systemic bioavailability Correct answer.
  3. complete avoidance of hepatic clearance
  4. an increase in the absorption rate constant

Explanation

Absorbed compound travels via the portal vein to the liver before reaching the systemic circulation, so hepatic extraction during this first pass lowers the fraction of dose that reaches systemic blood.

Question 7General Principles of Toxicology

Enterohepatic recirculation of a xenobiotic or its conjugate generally:

  1. shortens the plasma half-life
  2. increases renal clearance
  3. prevents biliary excretion
  4. prolongs the plasma half-life Correct answer.

Explanation

Conjugates excreted in bile can be hydrolyzed by gut flora and reabsorbed, cycling the compound back to the liver and prolonging its persistence in the body.

Question 8General Principles of Toxicology

The blood-brain barrier restricts entry of many xenobiotics primarily because of:

  1. active efflux pumps located only in the kidney
  2. tight junctions between capillary endothelial cells Correct answer.
  3. absence of blood flow to the brain parenchyma
  4. highly fenestrated endothelial cells in brain capillaries

Explanation

Brain capillary endothelial cells are joined by tight junctions and lack fenestrations, forming the anatomical basis of the blood-brain barrier; small lipid-soluble, unbound molecules cross most readily.

Question 9General Principles of Toxicology

Which statement describes zero-order elimination of a xenobiotic?

  1. Elimination rate is proportional to the square of the concentration
  2. Half-life is independent of the initial concentration
  3. A constant amount is eliminated per unit time Correct answer.
  4. A constant fraction of the dose is eliminated per unit time

Explanation

In zero-order (saturable) kinetics the eliminating enzymes are saturated, so a fixed amount is removed per unit time and the apparent half-life lengthens as dose increases.

Question 10General Principles of Toxicology

A competitive antagonist, in the presence of a fixed concentration of agonist, will typically:

  1. shifts the dose-response curve leftward and increases potency
  2. reduce the maximal response without changing the EC50
  3. shifts the curve right without lowering the maximum Correct answer.
  4. produce an insurmountable block at all agonist concentrations

Explanation

A competitive antagonist binds reversibly at the same site as the agonist, so higher agonist concentrations can surmount the block; the curve shifts right while the maximal response is unchanged.

Question 11General Principles of Toxicology

A noncompetitive antagonist that binds irreversibly to the receptor will:

  1. be overcome by increasing agonist concentration
  2. reduce the maximal agonist response Correct answer.
  3. shift the agonist curve to the left
  4. increase agonist potency

Explanation

Irreversible or noncompetitive antagonism removes functional receptors from the pool, lowering the maximal attainable response; added agonist cannot restore the lost receptor capacity.

Question 12General Principles of Toxicology

When a tissue has a large receptor reserve, the EC50 for an agonist is typically:

  1. lower than the Kd for receptor binding Correct answer.
  2. unrelated to receptor occupancy
  3. greater than the Kd for receptor binding
  4. equal to the Kd for receptor binding

Explanation

With spare receptors, near-maximal response is achieved when only a small fraction of receptors is occupied, so half-maximal response occurs at a concentration well below the Kd.

Question 13General Principles of Toxicology

The default 100-fold uncertainty factor applied when deriving a reference dose from an animal NOAEL consists of:

  1. 10-fold for species differences and 10-fold for human variability Correct answer.
  2. 3-fold for pharmacokinetics and 3-fold for pharmacodynamics
  3. 10-fold for study duration and 10-fold for database gaps
  4. 10-fold for LOAEL-to-NOAEL and 10-fold for subchronic-to-chronic

Explanation

The default 100-fold factor is the product of 10 for animal-to-human extrapolation and 10 for variation among humans; additional modifying factors may be applied for other deficiencies.

Question 14General Principles of Toxicology

The benchmark dose lower confidence limit (BMDL) is often preferred over the NOAEL because it:

  1. eliminates the need for uncertainty factors
  2. always yields a value higher than the observed LOAEL
  3. requires no dose-response modeling
  4. is fit to all dose-response data Correct answer.

Explanation

The BMDL is derived by fitting a model to the full dose-response dataset, so it is not tied to a single tested dose and is less sensitive to dose spacing; uncertainty factors are still applied afterward.

Question 15General Principles of Toxicology

The Ames Salmonella reverse mutation assay is designed primarily to detect:

  1. point mutations that reverse histidine auxotrophy Correct answer.
  2. aneuploidy induced by spindle poisons
  3. chromosomal aberrations in mammalian cells
  4. DNA adduct formation measured by mass spectrometry

Explanation

The Ames test uses histidine-dependent Salmonella strains and scores reversion to prototrophy caused by point mutations, typically with and without an exogenous metabolic activation system.

Question 16General Principles of Toxicology

In the two-stage model of carcinogenesis, tumor promoters are characterized as agents that:

  1. enhance tumor formation without being mutagenic themselves Correct answer.
  2. act only when administered before the initiator
  3. induce heritable genetic changes in a single exposure
  4. are mutagenic and bind covalently to DNA

Explanation

Promoters expand the population of initiated cells but are generally nonmutagenic and must be applied repeatedly after the initiating event to produce tumors.

Question 17General Principles of Toxicology

The hepatotoxicity of a high acetaminophen dose is attributed mainly to:

  1. direct glucuronidation in the intestinal mucosa
  2. renal tubular secretion of unchanged acetaminophen
  3. bioactivation by CYP2E1 to N-acetyl-p-benzoquinone imine Correct answer.
  4. inhibition of mitochondrial fatty acid oxidation by the parent drug

Explanation

At high doses the sulfation and glucuronidation pathways become saturated, and CYP2E1 oxidizes acetaminophen to the reactive intermediate NAPQI, which depletes glutathione and binds cellular proteins, producing centrilobular necrosis.

Question 18General Principles of Toxicology

Repeated exposure to an enzyme inducer such as phenobarbital typically:

  1. increases the volume of distribution substantially
  2. increases hepatic clearance and shortens half-life Correct answer.
  3. decreases hepatic clearance and prolongs half-life
  4. increases plasma protein binding

Explanation

Induction increases the amount of metabolizing enzyme, raising intrinsic clearance so that steady-state concentrations fall and half-life shortens when clearance is the dominant determinant.

Question 19General Principles of Toxicology

Only the unbound fraction of a drug in plasma is available for:

  1. storage in adipose tissue
  2. binding to plasma albumin
  3. formation of a circulating protein complex
  4. glomerular filtration and receptor binding Correct answer.

Explanation

Free drug can cross membranes and interact with receptors, transporters, and eliminating organs, so the unbound fraction governs both pharmacologic effect and availability for filtration.

Question 20General Principles of Toxicology

Potentiation is best described as an interaction in which:

  1. the combined effect equals the sum of individual effects
  2. the combined effect is less than either agent alone
  3. two chemicals with the same mechanism produce a greater-than-additive effect
  4. a chemical with no effect of its own increases the toxicity of another Correct answer.

Explanation

Potentiation occurs when an agent that produces no effect alone enhances the toxicity of another chemical; synergism, by contrast, involves two active agents producing more than an additive effect.

Question 21General Principles of Toxicology

Increasing the octanol-water partition coefficient (log P) of a small organic molecule generally:

  1. increases permeation until solubility limits absorption Correct answer.
  2. lowers passive permeation across lipid bilayers
  3. always increases both aqueous solubility and the rate of absorption
  4. has no bearing on passive diffusion across membranes

Explanation

Passive diffusion is favored at intermediate lipophilicity; beyond an optimum, very high log P reduces aqueous solubility and dissolution, which can limit absorption despite high membrane affinity.

Question 22General Principles of Toxicology

In developmental toxicity, the period of greatest susceptibility to major structural malformations is:

  1. the preimplantation period
  2. the fetal growth period
  3. organogenesis Correct answer.
  4. the perinatal period

Explanation

Organogenesis is when the major organ systems form, so exposure during this window produces structural malformations; earlier exposures tend to be all-or-none and later ones produce growth or functional deficits.

Question 23General Principles of Toxicology

After an intravenous bolus, a drug that distributes rapidly to tissues and is eliminated slowly will show a plasma concentration-time curve in which:

  1. a single exponential describes the entire curve
  2. the terminal slope directly reflects distribution into tissues
  3. terminal phase is elimination, early phase is distribution Correct answer.
  4. the alpha phase equals the elimination rate constant

Explanation

In a two-compartment model the early alpha decline reflects distribution into the peripheral compartment, while the later beta phase reflects elimination and determines the terminal half-life.

Question 24General Principles of Toxicology

A drug has a clearance of 5 L/h and a volume of distribution of 50 L. Its elimination half-life is approximately:

  1. 5 hours
  2. 7 hours Correct answer.
  3. 0.7 hours
  4. 20 hours

Explanation

Half-life is 0.693 multiplied by volume of distribution divided by clearance, so 0.693 times 50 L divided by 5 L/h equals about 6.9 hours.

Question 25General Principles of Toxicology

In the framework for mode of action analysis, the key question after a plausible animal mode of action is established is whether:

  1. the key events of the mode of action can occur in humans Correct answer.
  2. the NOAEL is below the LOAEL
  3. the chemical produces a positive result in the Ames test
  4. the chemical is absorbed by the oral route

Explanation

The framework asks whether the key events are biologically plausible in humans and whether the dose-response is consistent with the proposed mode of action, which supports a conclusion of human relevance or non-relevance.

Question 26General Principles of Toxicology

Bioavailability of an orally administered xenobiotic is best defined as:

  1. the total amount of parent compound excreted unchanged in the urine
  2. the fraction of the dose reaching the systemic circulation unchanged Correct answer.
  3. the proportion of the administered dose that is bound to plasma albumin
  4. the rate at which the compound is absorbed from the gastrointestinal tract

Explanation

Bioavailability (F) expresses the extent of systemic exposure after extravascular dosing relative to the dose given, integrating both the fraction absorbed and any loss during first pass through gut wall and liver. It is an extent term, whereas absorption rate and protein binding are separate pharmacokinetic descriptors.

Question 27General Principles of Toxicology

Which reaction is characteristic of phase II biotransformation?

  1. Glucuronidation of a hydroxyl group Correct answer.
  2. Cytochrome P450-catalyzed epoxidation of an aromatic ring
  3. Reduction of a nitro group by NADPH-cytochrome P450 reductase
  4. Hydrolysis of an ester bond by carboxylesterase

Explanation

Phase II reactions are conjugation reactions that attach an endogenous moiety such as glucuronate, sulfate, glutathione, or acetate to a functional group, generally increasing aqueous solubility and excretability. Oxidation, reduction, and hydrolysis are grouped as phase I because they introduce or expose functional groups.

Question 28General Principles of Toxicology

The median lethal dose (LD50) of a chemical is:

  1. the lowest dose at which death occurs in any individual animal tested
  2. the dose at which 50% of animals show clinical signs of toxicity
  3. the dose that kills all animals in a treated test group under the study conditions
  4. the dose expected to kill 50% of an exposed population Correct answer.

Explanation

The LD50 is a quantal point estimate read from a fitted dose-response curve, defined as the single dose expected to produce death in half of the exposed population under specified conditions. It is a population statistic, not a threshold and not a measure of morbidity.

Question 29General Principles of Toxicology

A no-observed-adverse-effect level (NOAEL) is defined as:

  1. the highest dose tested at which no adverse effect is observed Correct answer.
  2. the dose that produces a 50% reduction in a measured endpoint
  3. the highest dose administered in a study, regardless of the outcome observed
  4. the lowest dose at which a statistically significant adverse effect first appears

Explanation

The NOAEL is the greatest administered dose or exposure at which no statistically or biologically significant adverse effect is detected relative to control. The next higher tested dose showing an effect is the LOAEL.

Question 30General Principles of Toxicology

The proximal tubule of the kidney is a common site of toxic injury mainly because it:

  1. contains high concentrations of transport proteins and biotransforming enzymes Correct answer.
  2. is exposed to the lowest concentration of filtered xenobiotics along the nephron
  3. is lined by cells that lack mitochondria and do not require ATP for solute transport
  4. lacks active transport capacity and expresses no xenobiotic-metabolizing enzymes

Explanation

The proximal tubule reabsorbs the bulk of filtered water and solute and is richly endowed with organic anion and cation transporters plus cytochrome P450 and conjugating enzymes, so xenobiotics are concentrated and often bioactivated there. Its high metabolic and transport activity also makes it vulnerable to energy-dependent injury.

Question 31General Principles of Toxicology

With repeated dosing at intervals equal to one elimination half-life, plasma concentrations approach steady state after approximately:

  1. ten to twelve half-lives
  2. one half-life
  3. four to five half-lives Correct answer.
  4. twenty half-lives

Explanation

On repeated dosing, each half-life closes half of the remaining gap to the plateau concentration, so roughly 94% of steady state is reached after four half-lives and about 97% after five. Accumulation beyond that is clinically negligible.

Question 32General Principles of Toxicology

A compound with a very large apparent volume of distribution (for example greater than 10 L/kg) is most likely to be:

  1. restricted to the extracellular fluid space with poor cell penetration
  2. confined largely to the plasma volume with essentially no tissue uptake
  3. widely distributed into tissues with low plasma levels Correct answer.
  4. highly bound to plasma albumin, with negligible distribution into tissues

Explanation

Apparent volume of distribution is a proportionality constant relating the amount of drug in the body to its plasma concentration, so a value exceeding body water signals extensive tissue partitioning and correspondingly low circulating concentrations. It is not a real anatomical space.

Question 33General Principles of Toxicology

Inhibition of a cytochrome P450 enzyme by a co-administered drug most typically:

  1. leaves the metabolism of other substrates of that enzyme essentially unaffected
  2. increases systemic exposure to co-administered substrates of the enzyme Correct answer.
  3. lowers the steady-state plasma concentration of the co-administered drug
  4. shortens the elimination half-life of the co-administered victim drug substantially

Explanation

Metabolic inhibition lowers intrinsic clearance of co-substrates, so their area under the curve rises and half-life lengthens, with the largest effects seen for high-extraction drugs given orally because first-pass loss is reduced. The interaction is enzyme-specific and dose-dependent.

Question 34General Principles of Toxicology

Compared with a full agonist, a partial agonist at the same receptor is characterized by:

  1. higher intrinsic efficacy that shifts the dose-response curve leftward and upward
  2. lower binding affinity but full intrinsic efficacy once bound to the receptor
  3. lower intrinsic efficacy, so full occupancy gives a submaximal response Correct answer.
  4. irreversible receptor blockade that cannot be overcome by any agonist concentration

Explanation

Intrinsic efficacy describes the ability of an occupied receptor to generate a response; a partial agonist stabilizes a conformation that produces less than the full response even at saturating occupancy. Affinity and efficacy are independent properties, so a partial agonist may bind with high affinity.

Question 35General Principles of Toxicology

In noncancer risk assessment, the default 10-fold interspecies uncertainty factor is intended to account for:

  1. limitations in the duration or design of the critical toxicity study used
  2. the use of a lowest-observed-adverse-effect level rather than a NOAEL
  3. toxicokinetic and toxicodynamic differences between animals and humans Correct answer.
  4. variability in susceptibility among individual humans in the population

Explanation

The interspecies factor covers differences in both the disposition of the chemical and the responsiveness of the target tissue between laboratory animals and humans, and it is often subdivided into kinetic and dynamic components. Human-to-human variability is covered by a separate intraspecies factor.

Question 36General Principles of Toxicology

An in vivo micronucleus assay in rodent bone marrow or peripheral blood is designed to detect:

  1. covalent DNA adducts formed by reactive metabolites in the target tissue
  2. chromosome fragments or whole chromosomes lost during cell division Correct answer.
  3. point mutations in a bacterial reporter gene used for mutagenicity testing
  4. unscheduled DNA synthesis in primary hepatocyte cultures after genotoxic insult

Explanation

Micronuclei arise from acentric chromosome fragments or from whole chromosomes that lag during anaphase, so the assay reports clastogenic and aneugenic events in dividing cells. It does not measure base-pair substitution or adduct formation directly.

Question 37General Principles of Toxicology

A non-genotoxic carcinogen is best characterized as an agent that:

  1. forms covalent DNA adducts in the target tissue after metabolic bioactivation
  2. induces chromosomal aberrations in cultured cells at low concentrations
  3. acts through indirect mechanisms without direct DNA reactivity Correct answer.
  4. produces reverse mutations in the Ames test without exogenous metabolic activation

Explanation

Non-genotoxic carcinogens increase tumor incidence through secondary processes such as sustained cytotoxicity and compensatory proliferation, receptor-mediated trophic signaling, immunosuppression, or hormonal imbalance, and they typically show threshold-like dose-response behavior. They are not directly DNA-reactive and usually test negative in standard genotoxicity batteries.

Question 38General Principles of Toxicology

Lipid peroxidation initiated by reactive oxygen species is best described as:

  1. irreversible binding of molecular oxygen to cytochrome c oxidase in mitochondria
  2. enzymatic conjugation of fatty acid carboxyl groups with glucuronic acid
  3. direct enzymatic hydrolysis of membrane phospholipids by phospholipase A2
  4. a chain reaction in which polyunsaturated fatty acids are oxidized Correct answer.

Explanation

A hydroxyl radical abstracts a hydrogen from a polyunsaturated fatty acid to form a lipid radical, which adds oxygen to give a peroxyl radical that propagates damage to neighboring lipids and generates reactive aldehydes such as 4-hydroxynonenal. The process is nonenzymatic and self-propagating until terminated by antioxidants.

Question 39General Principles of Toxicology

Which of the following is a biomarker of exposure rather than a biomarker of effect?

  1. an increased frequency of micronuclei in peripheral blood lymphocytes
  2. elevated serum alanine aminotransferase activity reflecting hepatocellular injury
  3. the concentration of a xenobiotic or its metabolite in blood or urine Correct answer.
  4. a reduced glomerular filtration rate, measured as inulin clearance, indicating renal impairment

Explanation

Exposure biomarkers document that a chemical has entered the body, either as an internal dose measurement in a biological matrix or as an adduct, and do not by themselves indicate harm. Effect biomarkers describe a biological or functional response to the agent.

Question 40General Principles of Toxicology

A physiologically based pharmacokinetic (PBPK) model is most useful for:

  1. identifying which metabolic pathway is responsible for bioactivation
  2. predicting tissue dosimetry across species and routes Correct answer.
  3. replacing the need for any in vivo toxicity testing in risk assessment
  4. estimating a chemical's octanol-water partition coefficient from its structure

Explanation

PBPK models represent the organism as organ compartments connected by blood flow, parameterized with physiological volumes, flows, partition coefficients, and biochemical rates, so they can extrapolate internal target-tissue dose across species, dose levels, and exposure routes. They inform interpretation of toxicity data but do not by themselves identify hazard.

Question 41General Principles of Toxicology

A dose-response curve that is J-shaped, with low doses producing effects opposite in direction to those at high doses, is most consistent with:

  1. saturable elimination that alters systemic clearance
  2. a linear no-threshold carcinogenic risk model
  3. hormesis, with low-dose adaptive stimulation Correct answer.
  4. a simple threshold model with a single inflection point

Explanation

Hormesis denotes a biphasic dose-response in which low doses elicit adaptive, protective, or stimulatory responses while high doses produce inhibition or toxicity, generating a J or inverted-U shape. A monotonic threshold or linear model cannot produce a change in the direction of the response.

Question 42General Principles of Toxicology

Which observation best demonstrates a species difference in toxicokinetics rather than toxicodynamics?

  1. two species show different degrees of injury despite identical tissue concentrations
  2. two species differ in the affinity of the target receptor for the same chemical
  3. two species achieve different plasma concentrations at the same mg/kg dose Correct answer.
  4. two species differ markedly in their capacity to repair identical DNA damage

Explanation

Toxicokinetic differences change the concentration-time profile delivered to the target, through differences in absorption, distribution, metabolism, or excretion, so equal external doses yield unequal internal doses. Toxicodynamic differences appear as divergent responses at equal target-site concentrations.

Question 43General Principles of Toxicology

For a compound negligibly bound to plasma proteins, a renal clearance of 200 mL/min when glomerular filtration rate is 125 mL/min and urine flow is 1 mL/min indicates that the compound undergoes:

  1. glomerular filtration only
  2. net tubular secretion Correct answer.
  3. extensive tubular reabsorption
  4. no renal elimination at all

Explanation

When clearance exceeds the filtered load (approximately GFR for a freely filtered solute), the excess amount excreted per unit time must have been added by the tubules, indicating net secretion. Reabsorption would drive clearance below the filtered value.

Question 44General Principles of Toxicology

In a mode-of-action analysis, showing that the key events seen in animals cannot plausibly occur in humans primarily addresses which consideration?

  1. statistical significance and magnitude of the tumor response in the bioassay
  2. concordance of the dose-response relationship between animals and humans
  3. temporal concordance of the key events in the proposed pathway
  4. human biological plausibility of the animal mode of action Correct answer.

Explanation

The human relevance framework asks whether the animal mode of action can operate in humans given anatomical, physiological, and biochemical similarities and differences in kinetics and dynamics. Dose concordance and temporal ordering are separate but complementary criteria evaluated alongside it.

Question 45Toxicokinetics and Biotransformation

Which pharmacokinetic parameter relates the amount of drug in the body to its measured plasma concentration?

  1. Oral bioavailability
  2. Systemic clearance
  3. Volume of distribution Correct answer.
  4. Elimination half-life

Explanation

Volume of distribution is an apparent volume calculated as the amount of drug in the body divided by the plasma concentration. It reflects the extent of distribution rather than a real anatomical space. Clearance, half-life, and bioavailability describe different kinetic properties.

Question 46Toxicokinetics and Biotransformation

Which reaction is classified as a Phase II conjugation reaction in xenobiotic biotransformation?

  1. Oxidation by CYP450
  2. Nitro reduction
  3. Ester hydrolysis
  4. Glucuronidation Correct answer.

Explanation

Phase II reactions conjugate a functional group on the xenobiotic with an endogenous molecule such as glucuronic acid, sulfate, or glutathione. These reactions generally increase water solubility and promote excretion. Oxidation, hydrolysis, and reduction are Phase I reactions.

Question 47Toxicokinetics and Biotransformation

A drug is completely absorbed from the gut but has low oral bioavailability because it is metabolized in enterocytes and liver before reaching systemic blood. What is this phenomenon called?

  1. Renal tubular secretion
  2. Enterohepatic recirculation
  3. First-pass metabolism Correct answer.
  4. Plasma protein binding

Explanation

First-pass metabolism removes drug during its initial passage through the gut wall and liver before it reaches the systemic circulation. This reduces oral bioavailability even when absorption is complete. Enterohepatic recirculation and renal tubular secretion involve different routes and processes.

Question 48Toxicokinetics and Biotransformation

Oral bioavailability of a drug is defined as the...

  1. Fraction of dose reaching systemic circulation Correct answer.
  2. Rate of drug transfer across the blood-brain barrier
  3. Fraction of dose excreted unchanged in urine
  4. Ratio of renal clearance to total clearance

Explanation

Bioavailability describes the extent and rate at which an administered dose reaches the systemic circulation unchanged. For an intravenous dose, bioavailability is defined as 100%. The other options describe excretion, clearance ratios, or barrier transfer.

Question 49Toxicokinetics and Biotransformation

For a drug following first-order elimination, the elimination half-life is the time required for...

  1. Plasma level to fall by half Correct answer.
  2. Volume of distribution to double
  3. Clearance to decrease by one-half
  4. Absorbed fraction to reach 50%

Explanation

In first-order elimination, a constant fraction of the drug is removed per unit time, so plasma concentration declines exponentially with a constant half-life. Clearance and volume of distribution are not halved during this process. The absorbed fraction relates to bioavailability, not elimination half-life.

Question 50Toxicokinetics and Biotransformation

Which statement describes zero-order elimination kinetics?

  1. Clearance increases as concentration increases
  2. Half-life is independent of dose
  3. A constant amount of drug is eliminated per unit time Correct answer.
  4. A constant fraction of the drug is eliminated per unit time

Explanation

Zero-order kinetics occurs when eliminating enzymes or transporters are saturated, so the elimination rate is constant and independent of concentration. A constant fraction eliminated per unit time describes first-order kinetics. In zero-order kinetics, half-life increases as dose or concentration increases.