Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 501: Which of the following describes the first-pass metabolism?

A. Drug given orally is metabolized by the liver before entering the circulation. (Correct Answer)
B. Drug given intravenously bypasses the liver initially.
C. Gastric acids primarily affect the stability of drugs.
D. Absorption of a drug occurs in the intestines.

Explanation: ***Drug given orally is metabolized by the liver before entering the circulation.*** - **First-pass metabolism**, also known as **presystemic metabolism**, refers to the phenomenon where a drug is extensively metabolized in the **gastrointestinal tract** and **liver** before it reaches systemic circulation. - This process significantly reduces the **bioavailability** of orally administered drugs, as a substantial portion of the drug is inactivated or converted to metabolites before it can exert its therapeutic effect. *Drug given intravenously bypasses the liver initially.* - While intravenous (IV) administration does bypass **first-pass metabolism** in the liver and gastrointestinal tract, this statement describes what happens with IV drugs, not the first-pass metabolism itself. - IV drugs enter the **systemic circulation** directly, achieving 100% bioavailability, unlike orally administered drugs affected by first-pass metabolism. *Gastric acids primarily affect the stability of drugs.* - **Gastric acids** primarily affect the **chemical stability** and degradation of certain drugs, but this is a separate phenomenon from first-pass metabolism. - While acid degradation can reduce drug absorption, first-pass effect specifically refers to metabolic transformation in the gut wall and liver. *Absorption of a drug occurs in the intestines.* - The **small intestine** is indeed the primary site for drug absorption due to its large surface area and rich blood supply. - However, this statement describes **drug absorption** in general, not specifically the process of first-pass metabolism, which occurs *after* absorption and involves metabolism before systemic circulation.

Question 502: What is the term used to describe the phenomenon where individuals with liver damage experience heightened effects from smaller doses of alcohol?

A. Withdrawal
B. Mellanby phenomenon
C. Reverse tolerance (Correct Answer)
D. Cross tolerance

Explanation: ***Reverse tolerance*** - This term describes the phenomenon where individuals with **liver damage**, particularly due to chronic alcohol use, become more sensitive to the effects of alcohol. - The damaged liver is less efficient at metabolizing alcohol, leading to higher and longer-lasting blood alcohol concentrations, even with smaller doses. - This represents a **decrease in tolerance** (increased sensitivity), where smaller amounts of alcohol produce heightened effects due to impaired hepatic clearance. *Withdrawal* - **Withdrawal** refers to the set of symptoms that occur when a person who is physically dependent on a substance, like alcohol, stops or significantly reduces their intake. - It is characterized by symptoms such as tremors, seizures, and delirium, and is distinct from the **heightened effects of alcohol** from a small dose. *Mellanby phenomenon* - The **Mellanby phenomenon** describes the observation that the effects of alcohol are more pronounced when blood alcohol levels are rising compared to when they are falling, even if the absolute blood alcohol concentration is the same. - This relates to the acute dynamics of alcohol's effect on the brain, not to chronic liver damage increasing sensitivity to small doses. *Cross tolerance* - **Cross tolerance** occurs when an individual develops tolerance to one drug, and this tolerance extends to another, pharmacologically similar drug, often due to shared metabolic pathways or receptor systems. - It does not describe an increased sensitivity to the original substance due to organ damage, but rather a reduced response to a different substance.

Question 503: At toxic doses, zero-order kinetics is seen in which of the following substances?

A. Phenytoin (Correct Answer)
B. Valproate
C. Carbamazepine
D. Penicillin

Explanation: ***Phenytoin*** - **Phenytoin** exhibits **zero-order kinetics** at toxic (saturating) doses because its metabolic enzymes become saturated, leading to a constant amount of drug eliminated per unit time rather than a constant fraction - This property makes its plasma concentration disproportionately increase with small dose adjustments once the enzymes are saturated, greatly increasing the risk of **toxicity** - Phenytoin is the **classic example** of capacity-limited metabolism due to saturation of hepatic enzymes (CYP2C9 and CYP2C19) *Penicillin* - Penicillin generally follows **first-order kinetics**, meaning a constant fraction of the drug is eliminated per unit time, and its elimination rate is proportional to its concentration - It is not commonly associated with zero-order kinetics even at higher doses, as its elimination pathways (renal excretion and metabolism) are typically not saturated within therapeutic or moderately toxic ranges *Valproate* - Valproate primarily follows **first-order kinetics** within its therapeutic range, with its elimination rate dependent on the drug concentration - While it can exhibit non-linear kinetics at very high concentrations due to protein binding saturation and enzyme saturation, it is less commonly cited as a classic example of zero-order kinetics compared to phenytoin *Carbamazepine* - Carbamazepine follows **first-order kinetics** within its therapeutic window - It undergoes **autoinduction** of its own metabolism, meaning that with continued dosing, its metabolic enzymes become more active, leading to increased elimination over time rather than saturation-induced zero-order kinetics

Question 504: What does the term 'tolerance' refer to in pharmacology?

A. Increased effect of drug on the same dose
B. Decreased effect of drug on the same dose (Correct Answer)
C. Same effect at lower doses
D. No effect

Explanation: ***Decreased effect of drug on the same dose*** - **Tolerance** is a state in which the body's response to a drug is **reduced** over time, requiring higher doses to achieve the same effect. - This phenomenon often develops with **repeated exposure** to a drug, leading to a need for dose escalation. *Increased effect of drug on the same dose* - This describes **sensitization** or **reverse tolerance**, where the body becomes more responsive to the drug over time, which is the opposite of tolerance. - It is not a characteristic feature of pharmacological tolerance. *Same effect at lower doses* - This would imply an **increased sensitivity** to the drug, meaning that less drug is needed to achieve the desired effect. - This is contrary to the definition of tolerance, which requires higher doses for the same effect. *No effect* - While extreme tolerance can lead to a point where a drug has minimal or no clinical effect at standard doses, "no effect" itself is not the primary definition of tolerance. - Tolerance refers to the gradual **reduction in effect**, rather than an immediate absence of effect.

Question 505: Alkaline diuresis is done for which of the following drugs?

A. Morphine
B. Amphetamine
C. Phenobarbitone (Correct Answer)
D. Atropine

Explanation: ***Phenobarbitone*** - **Phenobarbitone** is a weak acid, and its elimination can be enhanced by **alkaline diuresis**. - By increasing the pH of urine, **ionization** of the drug increases, reducing its reabsorption in the renal tubules and promoting excretion. *Morphine* - **Morphine** is an opioid analgesic that is largely metabolized in the liver and primarily excreted as **glucuronide conjugates** in the urine; alkaline diuresis is not a primary method for its elimination. - Although morphine is a **weak base**, its excretion is not significantly affected by alteration of urine pH because it is primarily eliminated through hepatic metabolism and conjugation rather than pH-dependent renal excretion. *Amphetamine* - **Amphetamine** is a weak base, and its elimination is enhanced by **acidic diuresis**, which increases its ionization in the urine. - **Alkaline diuresis** would *decrease* its excretion by promoting its reabsorption in the renal tubules. *Atropine* - **Atropine** is an anticholinergic drug with both renal and hepatic elimination, but its excretion is not significantly altered by **alkaline diuresis**. - Its elimination is influenced by its metabolism and renal excretion of unchanged drug, but not substantially by urinary pH manipulation for therapeutic purposes.

Question 506: A patient stabilized on a selective serotonin reuptake inhibitor (SSRI) for depression may experience withdrawal symptoms when stopped. Which of the following drugs has the minimum risk of causing drug discontinuation symptoms?

A. Paroxetine
B. Fluoxetine (Correct Answer)
C. Fluvoxamine
D. Sertraline

Explanation: ***Fluoxetine*** - **Fluoxetine** (Prozac) has the **longest half-life** among the SSRIs, leading to a much slower decrease in plasma concentration upon discontinuation. - This gradual reduction in drug levels lessens the severity and incidence of **discontinuation syndrome** symptoms. *Paroxetine* - **Paroxetine** (Paxil) has one of the **shortest half-lives** among the SSRIs, making it associated with a higher risk of severe discontinuation symptoms. - Its rapid elimination from the body leads to a quicker onset of withdrawal effects if stopped abruptly. *Sertraline* - **Sertraline** (Zoloft) has an intermediate half-life among SSRIs. - While generally better tolerated than paroxetine during discontinuation, it still carries a higher risk of withdrawal symptoms compared to fluoxetine. *Fluvoxamine* - **Fluvoxamine** (Luvox) has a relatively short half-life, similar to paroxetine, contributing to a higher risk of **discontinuation syndrome**. - It is particularly known for its potential for significant interactions due to potent **CYP450 inhibition**.

Question 507: A 70 kg man was given a drug with a dose of 100 mg/kg body weight, twice daily. The half-life (t1/2) is 10 hours, the plasma concentration is 1.9 mg/mL, and the clearance is unknown. What is the clearance of this drug?

A. 20 liter/hr
B. K is 0.0693
C. 0.22 L/hr (Correct Answer)
D. 0.02 L/hr

Explanation: ***0.22 L/hr*** - To calculate clearance at steady state, we use the formula: **Clearance (Cl) = Dose Rate / Css** (steady-state plasma concentration). - **Dose rate calculation**: 100 mg/kg × 70 kg × 2 doses/day = 14,000 mg/day = 583.33 mg/hr - **Converting plasma concentration**: 1.9 mg/mL = 1900 mg/L - **Clearance calculation**: Cl = 583.33 mg/hr ÷ 1900 mg/L = **0.307 L/hr** - **Note**: The calculated value (0.307 L/hr) does not exactly match any option. The marked answer (0.22 L/hr) is the closest approximation among the given choices. This discrepancy may arise from rounding in the original question parameters or implicit assumptions about bioavailability/volume of distribution. *0.02 L/hr* - This value is approximately 15 times lower than the calculated clearance. - Such low clearance would result in much higher plasma concentrations or require significantly lower dosing. *20 liter/hr* - This clearance is approximately 65 times higher than calculated, representing an unrealistically high value for this scenario. - Such high clearance would result in very low plasma concentrations unless extremely high doses were administered. *K is 0.0693* - This represents the **elimination rate constant (k)**, calculated as k = 0.693/t1/2 = 0.693/10 hr = 0.0693 hr⁻¹. - While mathematically correct for k, the question specifically asks for **clearance**, not the elimination rate constant. - Clearance is related to k by: Cl = k × Vd (volume of distribution).

Question 508: An acidic drug with a pKa value of 4.5 has what percentage of ionized drug at a pH of 3.5?

A. 1%
B. 90%
C. 99%
D. 10% (Correct Answer)

Explanation: ***Correct: 10%*** - For an acidic drug, use the **Henderson-Hasselbalch equation**: **pH = pKa + log([ionized]/[unionized])** - Rearranging: **pH - pKa = log([ionized]/[unionized])** - Given **pH = 3.5** and **pKa = 4.5**: 3.5 - 4.5 = **-1** - Therefore: **log([ionized]/[unionized]) = -1** - This means: **[ionized]/[unionized] = 10^-1 = 0.1 = 1/10** - Percentage ionized = **1/(1+10) × 100% = 1/11 × 100% = 9.09%**, which rounds to **~10%** - **Key principle**: When pH < pKa for an acidic drug, the drug is predominantly **unionized** (protonated, HA form) *Incorrect: 1%* - This percentage would occur if **pH - pKa = -2** (e.g., pH 2.5 and pKa 4.5) - At this difference, [ionized]/[unionized] = 10^-2 = **1/100**, giving **~1% ionized** - Does not match the given values where the difference is only **-1** *Incorrect: 90%* - This represents the percentage of **unionized drug** in this scenario, not ionized - For an acidic drug to be **90% ionized**, the pH must be **significantly higher than pKa** (pH - pKa ≈ +1) - Since pH < pKa here, the drug is predominantly **unionized**, not ionized *Incorrect: 99%* - This high ionization would occur when **pH >> pKa** (specifically when **pH - pKa ≥ +2**) - For example, at pH 6.5 and pKa 4.5, the drug would be **~99% ionized** - The given **pH of 3.5 is below the pKa**, so the drug is predominantly **unionized** (~91%), not ionized

Question 509: Which of the following statements about the rectal route of drug administration is MOST clinically significant?

A. Diazepam can be effectively administered via the rectal route. (Correct Answer)
B. It can be used in unconscious patients.
C. It can be used for irritant and unpleasant drugs.
D. Absorption of drugs can vary significantly.

Explanation: ***Diazepam can be effectively administered via the rectal route.*** - This is the **MOST clinically significant** statement as rectal **diazepam** is a **life-saving intervention** specifically indicated for **acute seizure management**, particularly **status epilepticus in children** when IV access is not immediately available. - The rectal route for diazepam represents a **well-established, evidence-based clinical practice** with standardized formulations (diazepam rectal gel/solution) specifically designed for emergency use. - This is a **specific therapeutic application** with proven efficacy, making it the most clinically relevant answer. *It can be used in unconscious patients.* - While this statement is **technically true**, it describes a general characteristic rather than a specific clinical advantage. - The rectal route can be used when patients cannot take oral medications, but this is **not unique** to the rectal route (IV, IM, and sublingual routes can also be used). - This is more of a **practical consideration** than a clinically significant feature. *It can be used for irritant and unpleasant drugs.* - This statement is **incorrect** regarding irritant drugs, as they can cause **proctitis and rectal mucosal damage**. - While unpleasant-tasting drugs may be given rectally to avoid oral administration issues, **irritant drugs are generally contraindicated** by this route. - The combination makes this statement misleading and incorrect. *Absorption of drugs can vary significantly.* - While this statement is **factually true**, it describes a **disadvantage** of the rectal route rather than a clinically significant advantage. - Factors such as **presence of fecal matter**, **rectal blood flow variability**, **first-pass metabolism** (lower rectum bypasses portal circulation, upper rectum does not), and **patient positioning** all contribute to unpredictable absorption. - This variability is why the rectal route is **not preferred** for most drugs, making this the least clinically significant correct statement.

Question 510: MDR gene acts by what mechanism?

A. Causes efflux of drug (Correct Answer)
B. Inhibit drug activation
C. Inhibit intracellular DNA synthesis
D. Inhibit DNA repair

Explanation: ***Causes efflux of drug*** - The **MDR gene** (multidrug resistance gene) encodes for **P-glycoprotein**, an ATP-dependent efflux pump. - This pump actively **transports drugs out of the cell**, reducing their intracellular concentration and effectiveness. *Inhibit drug activation* - This mechanism is associated with enzymes like **CYP450 isoenzymes** or **esterases** that metabolize prodrugs into their active forms. - The MDR gene and its product, P-glycoprotein, are efflux pumps and do not directly inhibit drug activation. *Inhibit intracellular DNA synthesis* - This is the mechanism of action for certain classes of drugs, such as **antimetabolites** (e.g., methotrexate) or **nucleoside analogs**. - The MDR gene's role is in drug transport, not in interfering with DNA synthesis itself. *Inhibit DNA repair* - Some chemotherapeutic agents, like **PARP inhibitors**, work by preventing cells from repairing DNA damage, leading to apoptosis. - While related to drug action, this is not the primary mechanism by which the MDR gene confers resistance; instead, it reduces drug exposure to DNA targets.

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