Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 511: What are the therapeutic levels of phenytoin?

A. 0-9 mg/ml
B. 20-29 mg/ml
C. 30-39 mg/ml
D. 10-20 mg/ml (Correct Answer)

Explanation: ***10-20 mg/ml*** - This range is generally considered the **therapeutic concentration** for phenytoin in most adult patients to achieve adequate seizure control. - Concentrations below this level may be **subtherapeutic**, leading to poor seizure control, while levels above can cause significant **toxicity**. *0-9 mg/ml* - This range is typically **subtherapeutic**, meaning it is too low to effectively control seizures in most patients. - Patients within this range may experience breakthrough seizures, indicating a need to **increase the dose** of phenytoin. *20-29 mg/ml* - This range is considered **toxic** for phenytoin, leading to various dose-related side effects. - Clinical manifestations of toxicity at these levels can include **nystagmus**, ataxia, and slurred speech. *30-39 mg/ml* - This range represents significantly **toxic levels** of phenytoin, associated with severe adverse effects. - At these concentrations, patients can experience pronounced **ataxia**, confusion, lethargy, and potentially even seizure exacerbation.

Question 512: Which of the following anesthetics is known for its rapid redistribution from the central nervous system to peripheral tissues?

A. Thiopentone (Correct Answer)
B. Halothane
C. Ether
D. Propofol

Explanation: ***Thiopentone*** - This **barbiturate** is highly **lipid-soluble**, allowing for rapid passage across the **blood-brain barrier**, leading to quick onset of action [2]. - Its short duration of action is primarily due to **rapid redistribution** from the brain to less well-perfused peripheral tissues, not by quick metabolism [1], [3]. - Classic example of an **ultra-short acting intravenous anesthetic** where redistribution is the primary mechanism terminating drug effect [4]. *Propofol* - **Propofol** also undergoes redistribution, but its clinical duration is significantly influenced by **rapid hepatic metabolism** and **high clearance**. - While redistribution contributes to offset, thiopentone is the **classic textbook example** specifically emphasized for redistribution as the primary mechanism [3]. *Halothane* - **Halothane** is an **inhalational anesthetic** whose effects are terminated by elimination via exhalation, not primarily by redistribution. - While some redistribution occurs with all anesthetics, it's not the dominant mechanism for offset of inhalational agents. *Ether* - **Diethyl ether** is an **inhalational anesthetic** with a slow onset and offset due to its high blood-gas solubility and prolonged elimination via the lungs. - Its clinical use has largely been superseded by newer agents due to issues like flammability and slower recovery.

Question 513: Which of the following is an example of a prodrug?

A. Primidone
B. Digitoxin
C. Amitriptyline
D. Levodopa (Correct Answer)

Explanation: ***Levodopa is a prodrug that converts to dopamine.*** - A **prodrug** is an inactive precursor of a drug that is converted into its active form within the body. - **Levodopa** is converted to the active neurotransmitter **dopamine** in the brain, making it effective for Parkinson's disease. *Primidone is an anticonvulsant with active metabolites.* - While Primidone has **active metabolites** (phenobarbital and phenylethylmalonamide), the parent drug itself also possesses significant **anticonvulsant activity**, meaning it is not an inactive precursor. - A true prodrug is therapeutically inactive until metabolized. *Digitoxin is a cardiac glycoside that acts directly.* - **Digitoxin** is a cardiac glycoside that directly inhibits the **Na+/K+-ATPase pump**, exerting its therapeutic effect without requiring conversion to an active metabolite. - It does not undergo enzymatic conversion to an active form to produce its primary action. *Amitriptyline is an antidepressant that acts directly.* - **Amitriptyline** is a **tricyclic antidepressant** that directly inhibits the reuptake of norepinephrine and serotonin. - While it is metabolized to an active metabolite (nortriptyline), Amitriptyline itself is **pharmacologically active** as the parent drug.

Question 514: Which of the following drugs does not concentrate in bile?

A. Erythromycin
B. Tetracycline
C. Oral contraceptives
D. Alpha methyl dopa (Correct Answer)

Explanation: ***Alpha methyl dopa*** - **Alpha methyl dopa** is primarily excreted by the kidneys and does not undergo significant biliary excretion or concentration in bile. - Its concentration in bile is negligible compared to other drugs known for biliary excretion. *Erythromycin* - **Erythromycin** is well-known for its significant biliary excretion and concentration in bile. - This characteristic can lead to drug interactions and cholestasis in some patients due to its processing in the liver. *Tetracycline* - **Tetracycline** antibiotics, including tetracycline itself, are excreted extensively in bile. - **Enterohepatic recirculation** is a common phenomenon with tetracyclines, contributing to their prolonged half-life. *Oral contraceptives* - Many components of **oral contraceptives**, particularly estrogen metabolites, undergo extensive hepatic metabolism and enterohepatic recirculation, leading to their concentration in bile. - The biliary excretion of these compounds is a key factor in their pharmacokinetic profile and drug interactions.

Question 515: What is the primary purpose of xenobiotic metabolism?

A. Increase water solubility (Correct Answer)
B. Increase lipid solubility
C. Make them nonpolar
D. None of the above

Explanation: ***Increase water solubility*** - The primary goal of xenobiotic metabolism is to make these foreign compounds more **hydrophilic** (water-soluble). - This increased water solubility facilitates their **excretion** from the body via urine or bile. *Increase lipid solubility* - Increasing **lipid solubility** would make xenobiotics more likely to accumulate in **adipose tissue** and pass through cell membranes, hindering their excretion. - This is the opposite of the desired outcome for xenobiotic elimination. *Make them nonpolar* - Making xenobiotics **nonpolar** would be equivalent to increasing their lipid solubility, as nonpolar molecules tend to be lipid-soluble. - This would impede excretion and potentially lead to **bioaccumulation**, which is harmful. *None of the options* - This option is incorrect because xenobiotic metabolism specifically aims to increase **water solubility** for elimination.

Question 516: Which drug exhibits zero-order kinetics at high doses?

A. Phenytoin (Correct Answer)
B. Propranolol
C. Amiloride
D. Lithium

Explanation: ***Phenytoin*** - At **high doses**, the metabolic enzymes for phenytoin become saturated, leading to **zero-order kinetics** where a constant amount of drug is eliminated per unit time, regardless of concentration. - This saturation can result in a disproportionate increase in plasma concentration with small dose increases, making **phenytoin toxicity** a significant concern. *Propranolol* - Generally follows **first-order kinetics** within its therapeutic dose range, meaning a constant *fraction* of the drug is eliminated per unit time. - Its elimination rate is **concentration-dependent** at typical doses. *Amiloride* - This diuretic is primarily eliminated unchanged by the kidneys and follows **first-order kinetics**. - Its elimination is proportional to its plasma concentration. *Lithium* - Primarily eliminated renally and exhibits **first-order kinetics**, with its elimination rate proportional to its concentration. - It has a **narrow therapeutic index** but its elimination profile does not switch to zero-order at high doses.

Question 517: Which of the following drugs is known for its long-acting effects due to enterohepatic circulation?

A. Phenylbutazone (Correct Answer)
B. Sulindac
C. Piroxicam
D. Aspirin

Explanation: ***Phenylbutazone*** - Phenylbutazone is a **long-acting NSAID** with a prolonged half-life of **50-100 hours**. - It undergoes significant **enterohepatic circulation**, where it is excreted in bile, reabsorbed from the intestine, and returned to the systemic circulation. - This enterohepatic recirculation, combined with **extensive protein binding** and slow metabolism, contributes significantly to its prolonged duration of action. - Due to serious adverse effects (bone marrow suppression, aplastic anemia), it is now rarely used in clinical practice. *Sulindac* - Sulindac is a prodrug that does undergo **enterohepatic circulation**. - However, despite this circulation, sulindac has a relatively **short half-life of 7-8 hours** and requires **twice-daily dosing**. - While enterohepatic circulation occurs, it does NOT make sulindac a long-acting NSAID. *Piroxicam* - Piroxicam is a **long-acting NSAID** with a half-life of approximately **50 hours**, allowing for once-daily dosing. - However, its prolonged duration of action is primarily due to its very **slow elimination** and extensive protein binding, NOT significant enterohepatic circulation. *Aspirin* - Aspirin has a relatively **short half-life** (15-20 minutes for the parent drug) and does not undergo significant enterohepatic circulation. - Its prolonged effects on platelets are due to **irreversible COX-1 inhibition**, not extended plasma half-life.

Question 518: Assuming the half-life of gentamicin is 3 hours in this patient, what percentage of the initial dose is most likely to remain in the body 6 hours later?

A. 12.5%
B. 25% (Correct Answer)
C. 33%
D. 50%

Explanation: ***25%*** - After **one half-life** (3 hours), 50% of the initial dose remains. - After **two half-lives** (6 hours), 25% of the initial dose remains (50% of the remaining 50%). *12.5%* - This percentage would remain after **three half-lives** (9 hours), not 6 hours. - Each half-life reduces the remaining drug by 50%, so 50% -> 25% -> 12.5%. *33%* - This value does not correspond to a direct calculation based on the given half-life. - The drug concentration would decrease by a factor of 4 (100% to 25%) over two half-lives. *50%* - This percentage would remain after **one half-life** (3 hours), not 6 hours. - The question asks for the amount remaining after two half-lives have passed.

Question 519: Drugs that cross the placenta are:

A. Isoniazid
B. Rifampicin
C. Pyrazinamide
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - **Isoniazid**, **rifampicin**, and **pyrazinamide** are all antitubercular drugs that cross the placenta. - This is the **most comprehensive answer** as it correctly identifies that all three listed drugs have placental transfer. - Important consideration for treating **tuberculosis in pregnancy**, as all three drugs are part of standard TB regimens and fetal exposure must be monitored. *Isoniazid (Incomplete answer)* - While **isoniazid** does cross the placenta, selecting only this option is incomplete as the other drugs also cross. - Can cause fetal **hepatotoxicity** and **peripheral neuropathy**; vitamin B6 supplementation is recommended. - Generally considered safe in pregnancy when treating active TB. *Rifampicin (Incomplete answer)* - **Rifampicin** crosses the placenta, but this option alone is incomplete. - May cause **neonatal hemorrhage** due to vitamin K deficiency; prophylactic vitamin K should be given to the neonate. - Safe for use in pregnancy for TB treatment. *Pyrazinamide (Incomplete answer)* - **Pyrazinamide** also crosses the placenta, making this option incomplete when selected alone. - Previously avoided in pregnancy due to limited data, but **WHO now recommends** its use in pregnancy as part of standard TB regimens. - Current evidence supports its safety profile in pregnancy.

Question 520: Which of the following drugs is metabolized by CYP2D6?

A. Propranolol (Correct Answer)
B. Warfarin
C. Statins
D. Amiodarone

Explanation: ***Correct Answer: Propranolol*** - **Propranolol** is a non-selective beta-blocker that undergoes extensive **first-pass metabolism**, primarily via the **CYP2D6** and CYP1A2 enzymes. - Genetic variations in **CYP2D6** can significantly affect propranolol's metabolism, leading to altered drug levels and therapeutic responses. *Incorrect: Warfarin* - **Warfarin** is predominantly metabolized by **CYP2C9**, with minor contributions from other CYP enzymes. - Genetic polymorphisms in **CYP2C9** are a major factor in determining individual warfarin dose requirements. *Incorrect: Statins* - Most **statins** (e.g., simvastatin, lovastatin, atorvastatin) are primarily metabolized by **CYP3A4**. - **Fluvastatin** is an exception, being mainly metabolized by CYP2C9, while **rosuvastatin** is largely unmetabolized. *Incorrect: Amiodarone* - **Amiodarone** is primarily metabolized by **CYP3A4** and to a lesser extent by CYP2C8. - Due to its **long half-life** and extensive metabolism, amiodarone has numerous drug interactions, often involving CYP3A4 inhibition.

Practice by Chapter

Absorption and Bioavailability

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Drug Distribution and Protein Binding

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Biotransformation and Metabolism Pathways

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Renal and Non-renal Excretion

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Compartment Models

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Dose-Response Relationships

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Drug Efficacy and Potency

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Drug Tolerance and Tachyphylaxis

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Population Pharmacokinetics

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Pharmacokinetic Variability

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