Pharmacokinetics and Pharmacodynamics — MCQs

A patient named Ram Prasad is admitted to Guru Teg Bahadur Hospital with a respiratory infection. Tobramycin is ordered for treatment. Given that the clearance and volume of distribution of tobramycin in him are 160 mL/min and 40 L, respectively, calculate the intravenous loading dose required to achieve a therapeutic plasma concentration of 4 mg/L.

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 491: Which of the following drugs should be given in a sustained-release oral dosage form?

A. An anti-arrhythmic drug with a plasma half life of 10 seconds used for acute treatment of PSVT
B. Anti inflammatory drugs with the plasma half life of 24 hours
C. Hypnotic drugs with a plasma half life of 2 hours
D. An antihypertensive with a plasma half-life of 3 hours (Correct Answer)

Explanation: *An anti-arrhythmic drug with a plasma half life of 10 seconds used for acute treatment of PSVT* - An extremely short **half-life** (10 seconds) indicates a drug suitable for **rapid-onset, acute interventions**, where the effect is needed immediately and for a very brief duration, making sustained release impractical. - Drugs like **adenosine**, used for acute PSVT, are given intravenously as a rapid bolus due to their ultra-short half-life, not in an oral sustained-release form. *Anti inflammatory drugs with the plasma half life of 24 hours* - A long **half-life** (24 hours) typically means the drug can be administered **once daily** to maintain therapeutic concentrations, rendering a sustained-release formulation unnecessary. - Such drugs already provide **prolonged action** and do not benefit significantly from further extension of release. *Hypnotic drugs with a plasma half life of 2 hours* - While a 2-hour half-life for a hypnotic might suggest potential for sustained release to prolong sleep, the goal of hypnotics is often a **rapid onset and relatively short duration** to avoid hangover effects. - Sustained release might cause **daytime sedation** and interfere with normal wakefulness, which is generally undesirable for this class of drugs. ***An antihypertensive with a plasma half-life of 3 hours*** - A short **half-life** (e.g., 3 hours) often necessitates frequent dosing to maintain therapeutic levels, making a **sustained-release formulation desirable** for patient compliance and consistent drug exposure. - Sustained-release dosage forms are particularly useful for drugs requiring **long-term, stable plasma concentrations**, such as antihypertensives, to manage chronic conditions effectively.

Question 492: Drug X has an affinity for albumin, while drug Y has 150 times greater affinity. Which of the following statements is MOST accurate?

A. Drug X will be more available in tissues
B. Drug Y will be less available in tissues
C. Toxicity of drug Y may be influenced by multiple factors, not just its binding.
D. The free concentration of drug X in blood is higher, facilitating tissue distribution. (Correct Answer)

Explanation: ***Correct: The free concentration of drug X in blood is higher, facilitating tissue distribution.*** - This is the **MOST accurate and complete** answer because it directly addresses the pharmacokinetic mechanism - Drug X has **lower affinity for albumin** → larger proportion remains **unbound (free)** in plasma - Only **free (unbound) drug** can cross capillary membranes to distribute into tissues - This statement precisely explains both the **cause** (higher free concentration) and **effect** (facilitating tissue distribution) *Drug X will be more available in tissues* - This statement is **factually true** and follows logically from drug X's lower protein binding - However, it's **less precise** than the correct answer because it doesn't explicitly explain the **mechanism** (higher free concentration) - The term "available" is less specific than "free concentration," which is the key pharmacokinetic parameter *Drug Y will be less available in tissues* - This statement is also **factually true** - drug Y's **150× higher albumin affinity** means more drug is bound - Higher protein binding → **smaller free fraction** → less tissue distribution - However, like option 1, this doesn't explicitly state the **mechanistic principle** involving free drug concentration - The question asks for the MOST accurate statement, and this focuses on drug Y rather than explaining the core concept *Toxicity of drug Y may be influenced by multiple factors, not just its binding* - While this is a **true general principle**, it's **not directly relevant** to the specific question - This statement doesn't address the **pharmacokinetic implications** of differential albumin binding - It's too vague and doesn't demonstrate understanding of the relationship between protein binding and tissue distribution - The question specifically asks about the affinity differences and their consequences

Question 493: A partial agonist has:

A. High affinity with low intrinsic activity (Correct Answer)
B. High affinity with no intrinsic activity
C. Low affinity with high intrinsic activity
D. Low affinity with low intrinsic activity

Explanation: ***High affinity with low intrinsic activity*** - A **partial agonist** binds to the receptor with **high affinity** [1] but elicits a submaximal response, indicating partial activation [1],[2]. - Its **intrinsic activity** is greater than zero but less than that of a full agonist [2]. *High affinity with no intrinsic activity* - This describes an **antagonist**, which binds to the receptor with **high affinity** but produces no biological effect (zero intrinsic activity). - An antagonist simply blocks the action of other agonists. *Low affinity with high intrinsic activity* - While binding affinity and intrinsic activity are distinct properties, a drug with **high intrinsic activity** typically produces a strong effect, and low affinity would mean a higher concentration is needed for that effect. This combination does not define a partial agonist. - A full agonist would have **high intrinsic activity**, but affinity can vary. *Low affinity with low intrinsic activity* - A drug with both **low affinity** and **low intrinsic activity** would be a very weak partial agonist, requiring high concentrations to produce only a small effect. - While it technically describes a type of partial agonist, the defining characteristic of a partial agonist is often highlighted by its ability to bind effectively (high affinity) but only partially activate (low intrinsic activity) a receptor.

Question 494: Which of the following nonsteroidal anti-inflammatory drugs (NSAIDs) has good tissue penetrability with a concentration in synovial fluid?

A. Ketorolac
B. Diclofenac sodium (Correct Answer)
C. Piroxicam
D. Sulindac

Explanation: ***Diclofenac sodium*** - **Diclofenac** is known for its excellent **tissue penetration**, achieving concentrations in **synovial fluid that approach or exceed plasma levels** within hours of administration. - This **rapid and efficient synovial accumulation** makes it particularly effective for **acute inflammatory joint conditions** such as rheumatoid arthritis and osteoarthritis. - Its **favorable pharmacokinetic profile** combines good penetration with relatively rapid onset of action in joint tissues. *Ketorolac* - While a potent NSAID often used for **acute pain management**, ketorolac does not specifically demonstrate superior synovial fluid penetration compared to other NSAIDs. - It has a relatively **short half-life** and is typically limited to **short-term use** (≤5 days) due to increased risk of adverse effects with prolonged administration. *Piroxicam* - **Piroxicam** does achieve good synovial fluid concentrations with a very **long synovial half-life** due to its overall prolonged elimination. - However, the question specifically asks about "good tissue penetrability with concentration," and **diclofenac** is more characteristically cited for its **rapid synovial penetration** and accumulation. - Piroxicam's main advantage is **once-daily dosing** due to its long plasma half-life, rather than superior initial penetration. *Sulindac* - **Sulindac** is a **prodrug** requiring hepatic conversion to its active sulfide metabolite. - Known for potential **renal-sparing effects** in some patients, but does not demonstrate preferential or superior synovial fluid accumulation compared to diclofenac. - Its prodrug nature may result in less predictable synovial fluid concentrations.

Question 495: Which of the following statements is true regarding competitive reversible antagonism?

A. ED50 remains unchanged in competitive reversible antagonism.
B. Efficacy and Vmax remain unchanged. (Correct Answer)
C. Potency remains unchanged in the presence of a competitive antagonist.
D. Affinity (Kd) remains unchanged in competitive reversible antagonism.

Explanation: ***Efficacy and Vmax remain unchanged.*** - In competitive reversible antagonism, the antagonist binds to the same receptor site as the agonist but can be overcome by increasing the agonist concentration [2]. This means the **maximum effect (efficacy or Vmax)** of the agonist can still be achieved, although a higher dose is needed [2]. - The antagonist does not alter the intrinsic ability of the agonist to produce a full response, only its **apparent affinity** for the receptor. - This is the hallmark of competitive reversible antagonism: **rightward shift of the dose-response curve with no change in maximum response** [2]. *Potency remains unchanged in the presence of a competitive antagonist.* - **Potency** is a measure of the amount of drug needed to produce a given effect (often defined by EC50 or ED50) [3]. - A competitive antagonist requires a **higher concentration of agonist** to achieve the same effect, thus **decreasing the apparent potency** of the agonist [4]. - The dose-response curve shifts to the right (parallel shift) [4]. *ED50 remains unchanged in competitive reversible antagonism.* - **ED50 (effective dose 50)** is the dose that produces 50% of the maximum effect. - Because competitive antagonists shift the dose-response curve to the right, a **higher ED50** is required to achieve 50% of the maximum effect in the presence of an antagonist [4]. *Affinity (Kd) remains unchanged in competitive reversible antagonism.* - The **dissociation constant (Kd)** represents the affinity of a drug for its receptor [1]. - In competitive reversible antagonism, the antagonist increases the **apparent Kd** of the agonist (reduces apparent affinity), requiring more agonist to achieve receptor occupancy. - The **intrinsic Kd** of the agonist doesn't change, but its apparent affinity is reduced due to competition with the antagonist.

Question 496: In treatment of Parkinsonism, L-Dopa is combined with carbidopa mainly to:

A. To decrease dose requirement of L–Dopa
B. To decrease side effects of L–Dopa (Correct Answer)
C. To decrease effectiveness of L–Dopa
D. To increase crossing of L–Dopa through BBB

Explanation: ***To decrease side effects of L–Dopa*** - This is the **primary/main reason** for combining carbidopa with L-Dopa. - Carbidopa is a **peripheral DOPA decarboxylase inhibitor**, preventing the conversion of L-Dopa to dopamine in the periphery. - This reduces side effects like **nausea, vomiting, and cardiac arrhythmias**, which are caused by peripheral dopamine. - Without carbidopa, peripheral side effects make L-Dopa therapy **intolerable at therapeutic doses**. *To decrease dose requirement of L–Dopa* - This is an **important secondary benefit** but not the main reason. - Carbidopa does allow for **75-80% reduction in L-Dopa dose** (from ~5-6g to ~1g daily) by preventing peripheral metabolism. - However, this dose reduction is a **consequence** of preventing peripheral conversion, not the primary therapeutic goal. - The main goal is making L-Dopa therapy **tolerable and safe**, with dose reduction being a beneficial side effect. *To decrease effectiveness of L–Dopa* - Carbidopa **increases the effectiveness** of L-Dopa by ensuring more of it reaches the central nervous system to be converted into dopamine. - By preventing premature peripheral metabolism, carbidopa allows for a greater therapeutic effect on Parkinson's symptoms. *To increase crossing of L–Dopa through BBB* - Carbidopa itself **does not cross the blood-brain barrier (BBB)** and therefore does not directly affect the transport of L-Dopa into the brain. - L-Dopa uses an **active transport system** (large neutral amino acid transporter) to cross the BBB, and carbidopa's role is to prevent its peripheral breakdown before it can utilize this system. - While more L-Dopa reaches the BBB due to reduced peripheral metabolism, carbidopa does not enhance the actual crossing mechanism.

Question 497: Renal threshold of a drug means:

A. Drug concentration above which it appears in urine (Correct Answer)
B. Drug concentration below which it appears in urine
C. Drug concentration above which it appears in blood
D. Drug concentration at which it starts to be metabolized

Explanation: ***Drug concentration above which it appears in urine*** - The **renal threshold** refers to the plasma concentration of a substance that, when exceeded, leads to its excretion in the urine because the renal tubules' reabsorptive capacity is saturated. - For drugs, if their concentration in the blood surpasses this threshold, the kidneys are unable to reabsorb all of it, resulting in its appearance in the urine. *Drug concentration below which it appears in urine* - This statement is incorrect as it contradicts the definition of renal threshold, which implies excretion when concentrations are *high*, not low. - Substances generally do not appear in urine when their concentration is below the renal threshold because the kidneys efficiently reabsorb them. *Drug concentration above which it appears in blood* - This option is flawed because drugs are always present in the blood if administered; their appearance in the blood is not dictated by a threshold in this context. - The renal threshold specifically relates to the kidney's handling of substances and their subsequent excretion into the urine. *Drug concentration at which it starts to be metabolized* - **Metabolism** primarily occurs in the liver, not at a specific renal concentration threshold. - The phrase "renal threshold" is explicitly about kidney function and urinary excretion, not metabolic processes.

Question 498: The longest-acting local anaesthetic drug is?

A. Procaine
B. Prilocaine
C. Lignocaine
D. Bupivacaine (Correct Answer)

Explanation: ***Bupivacaine*** - **Bupivacaine** is an amide-type local anesthetic known for its **longest duration of action** among commonly used local anesthetics, making it suitable for procedures requiring prolonged anesthesia or pain relief. - Its **high lipid solubility** and **high protein binding** (95%) contribute to its extended effect of **3-10 hours**, allowing a slow release from the tissue. - Duration of action: **180-600 minutes** depending on site and use of vasoconstrictors. *Procaine* - **Procaine** is an ester-type local anesthetic and is one of the **shortest-acting** agents (30-60 minutes) due to its rapid metabolism by plasma pseudocholinesterase. - It is rarely used clinically today due to its short duration and higher incidence of allergic reactions. *Prilocaine* - **Prilocaine** is an amide-type local anesthetic with an **intermediate duration of action** (60-120 minutes), longer than procaine but shorter than bupivacaine. - A potential side effect is the formation of **methemoglobinemia** at higher doses due to its metabolite o-toluidine. *Lignocaine* - Also known as **lidocaine**, it is an amide-type local anesthetic with an **intermediate duration of action**, typically lasting **60-120 minutes**. - It is one of the most commonly used local anesthetics and is also used as an **antiarrhythmic drug** (Class Ib).

Question 499: In which of the following conditions is digoxin most likely to accumulate to toxic levels?

A. Renal insufficiency (Correct Answer)
B. Chronic hepatitis
C. Advanced cirrhosis
D. Chronic pancreatitis

Explanation: ***Renal insufficiency*** - **Digoxin** is primarily excreted unchanged by the **kidneys**, so impaired renal function significantly prolongs its half-life and leads to drug accumulation. - Patients with kidney failure require **dose adjustments** or closer monitoring of **digoxin levels** to prevent toxicity. *Chronic hepatitis* - **Chronic hepatitis** primarily affects the **liver's metabolic capacity**, which is not the primary route of **digoxin elimination**. - While severe hepatic dysfunction can subtly impact drug disposition, it's not the main reason for **digoxin accumulation** like **renal insufficiency**. *Advanced cirrhosis* - **Advanced cirrhosis** involves severe liver dysfunction, which can alter drug metabolism and protein binding. - However, **digoxin's elimination** is mainly renal, so liver disease alone does not typically lead to significant accumulation unless accompanied by **renal impairment**. *Chronic pancreatitis* - **Chronic pancreatitis** is a disorder of the pancreas and does not directly impact the **excretion or metabolism** of **digoxin**. - It would not be expected to cause **digoxin accumulation** to toxic levels.

Question 500: A patient named Ram Prasad is admitted to Guru Teg Bahadur Hospital with a respiratory infection. Tobramycin is ordered for treatment. Given that the clearance and volume of distribution of tobramycin in him are 160 mL/min and 40 L, respectively, calculate the intravenous loading dose required to achieve a therapeutic plasma concentration of 4 mg/L.

A. 0.1 mg
B. 10 mg
C. 115.2 mg
D. 160 mg (Correct Answer)

Explanation: ***160 mg*** - The loading dose is calculated using the formula: **Loading Dose = Volume of Distribution (Vd) × Target Plasma Concentration (Cp)**. - Given Vd = 40 L and Cp = 4 mg/L, the calculation is 40 L × 4 mg/L = **160 mg**. *0.1 mg* - This value is significantly too low for a therapeutic loading dose of tobramycin and would not achieve the desired concentration. - It likely results from incorrect units or a miscalculation of the formula. *10 mg* - This dose is too low to reach the therapeutic plasma concentration of 4 mg/L given the patient's volume of distribution. - It suggests a calculation error, possibly dividing Vd by Cp instead of multiplying. *115.2 mg* - This value indicates a calculation error, as it does not correspond to the correct application of the loading dose formula. - It might arise from using an incorrect volume of distribution or target concentration, or an error in multiplication.

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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