Pharmacokinetics and Pharmacodynamics — MCQs
Which of the following statements is true regarding competitive reversible antagonism?
Volume of distribution of a drug is 500 ml and target concentration of drug in blood is 5 g/L. 20% of administered drug is reached to systemic circulation. What will be the loading dose of that drug -
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.
At pKa = pH, what is the relationship between the ionic and non-ionic forms of a drug?
Regarding the concepts of efficacy and potency of a drug, which of the following statements is FALSE?
Lidocaine is used in a loading dose for the treatment of arrhythmias. The loading dose of this drug depends upon which of the following factors?
Major determinant of loading dose of a drug is:
Which of the following statements is true about first-order kinetics?
A 55-year-old man presents with a tremor that occurs when his hands are at rest. He has a slow, shuffling gait and difficulty initiating movement. His symptoms improve with levodopa. What is the most likely diagnosis?
Which one of the following causes low-volume erythrocytosis?
Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs
Question 1: 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 2: Volume of distribution of a drug is 500 ml and target concentration of drug in blood is 5 g/L. 20% of administered drug is reached to systemic circulation. What will be the loading dose of that drug -
- A. 1 gm
- B. 5 gm
- C. 25 gm
- D. 12.5 gm (Correct Answer)
Explanation: ***12.5 gm*** - The formula for loading dose (LD) is: LD = (Target Concentration × Volume of Distribution) / Bioavailability. - Given: Target Concentration = 5 g/L, Volume of Distribution = 500 mL = 0.5 L, Bioavailability = 20% = 0.2. - So, LD = (5 g/L × 0.5 L) / 0.2 = 2.5 g / 0.2 = **12.5 g**. *1 gm* - This value would be obtained if the target concentration was 2 g/L with 100% bioavailability, or if the calculation incorrectly handled the volume or bioavailability factor. - It does not account for the specified **bioavailability of 20%** or the given target concentration and volume of distribution. *5 gm* - This result would be obtained if the bioavailability was assumed to be 50% (LD = 2.5 g / 0.5 = 5 g), or if the volume of distribution was incorrectly used in the calculation. - This option does not correctly factor in the **20% bioavailability** of the administered drug. *25 gm* - This value would result from mistakes such as dividing by bioavailability of 10% instead of 20% (LD = 2.5 g / 0.1 = 25 g), or by multiplying bioavailability instead of dividing by it. - This answer significantly **overestimates** the required dose, which could lead to drug toxicity.
Question 3: 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.
Question 4: At pKa = pH, what is the relationship between the ionic and non-ionic forms of a drug?
- A. Absorption of drug is 50% ionic and 50% non-ionic
- B. Conc. of drug is 25% ionic and 75% non-ionic
- C. Conc. of drug is 75% ionic and 25% non-ionic
- D. Conc. of drug is 50% ionic and 50% non-ionic (Correct Answer)
Explanation: ***Conc. of drug is 50% ionic and 50% non-ionic*** - At **pKa = pH**, the concentrations of the **ionized** and **unionized** forms of a drug are equal as per the **Henderson-Hasselbalch equation**. - This means that exactly **half** of the drug molecules are in their charged (ionic) state, and the other half are in their uncharged (non-ionic) state. *Absorption of drug is 50% ionic and 50% non-ionic* - The amount of drug that is absorbed is dependent on the **non-ionic concentration** available at the absorption site, but this option incorrectly states that the *absorption itself* is 50% ionic. - Absorption primarily occurs for the **non-ionic, lipophilic form** as it can more readily cross cell membranes. *Conc. of drug is 75% ionic and 25% non-ionic* - This ratio would occur when the **pH** is either 0.5 units above the pKa for a weak acid or 0.5 units below the pKa for a weak base. - For example, if **pH = pKa + 0.5** (for a weak acid), approximately 75% would be ionic. *Conc. of drug is 25% ionic and 75% non-ionic* - This ratio would occur when the **pH** is either 0.5 units below the pKa for a weak acid or 0.5 units above the pKa for a weak base. - For example, if **pH = pKa - 0.5** (for a weak acid), approximately 25% would be ionic.
Question 5: Regarding the concepts of efficacy and potency of a drug, which of the following statements is FALSE?
- A. ED50 of the drug corresponds to efficacy (Correct Answer)
- B. Drugs that produce a similar pharmacological effect can have different levels of efficacy
- C. In a clinical setup, efficacy is more important than potency
- D. In the log dose response curve, the height of the curve corresponds with efficacy
Explanation: ***ED50 of the drug corresponds to efficacy*** - **ED50** (median effective dose) is the dose at which 50% of individuals exhibit the specified effect; it quantifies **potency**, not efficacy. - **Efficacy** refers to the maximum effect a drug can produce, while potency refers to the amount of drug needed to produce an effect. *In a clinical setup, efficacy is more important than potency* - **Efficacy** determines the maximal therapeutic benefit a drug can achieve for a patient, making it crucial for clinical outcomes. - While **potency** influences the dose required, a highly potent drug that is not very efficacious may not be clinically useful. *Drugs that produce a similar pharmacological effect can have different levels of efficacy* - Two drugs might act on the same receptor but elicit different maximal responses, indicating varying **efficacy**. - For example, a **partial agonist** and **full agonist** interacting with the same receptor will have different efficacies. *In the log dose response curve, the height of the curve corresponds with efficacy* - The **maximal response** or plateau of the dose-response curve represents the **efficacy** of a drug. - A higher plateau on the curve indicates a drug with greater intrinsic activity achieving a larger effect.
Question 6: Lidocaine is used in a loading dose for the treatment of arrhythmias. The loading dose of this drug depends upon which of the following factors?
- A. Clearance
- B. Volume of distribution (Correct Answer)
- C. Half-life
- D. Bioavailability
- E. Elimination rate constant
Explanation: ***Volume of distribution*** - The **loading dose** of a drug is primarily determined by its **volume of distribution (Vd)** and the **target plasma concentration**. - A larger **Vd** means the drug distributes widely into tissues, requiring a larger loading dose to achieve the desired concentration in the central compartment. *Clearance* - **Clearance** dictates the **maintenance dose** needed to sustain a steady-state concentration once the loading dose has been administered. - It reflects the rate at which the drug is eliminated from the body, not how much is initially needed to fill the distribution volume. *Half-life* - **Half-life** determines the **time required to reach steady-state** and the **dosing interval** for maintaining therapeutic concentrations. - While related to clearance and Vd, it does not directly determine the magnitude of the initial loading dose itself. *Bioavailability* - **Bioavailability** is the fraction of administered drug that reaches the systemic circulation in an unchanged form. - It influences the oral dose required to achieve a certain plasma concentration, but the concept of loading dose is typically considered for the intravenous route where bioavailability is 100%. *Elimination rate constant* - The **elimination rate constant (ke)** describes the rate of drug elimination and is related to clearance and volume of distribution (ke = Cl/Vd). - Like clearance, it determines the **maintenance dose** and dosing frequency, not the initial loading dose required to achieve therapeutic levels.
Question 7: Major determinant of loading dose of a drug is:
- A. Half life
- B. Volume of distribution (Correct Answer)
- C. Clearance
- D. Bioavailability
Explanation: **Volume of distribution** - The **loading dose (LD)** of a drug is calculated using the formula: LD = (Target plasma concentration × **Volume of distribution**) / Bioavailability. - The **volume of distribution (Vd)** is the **major determinant** because it directly determines how much drug is needed to achieve the desired plasma concentration throughout all body compartments. - A larger Vd means more drug must be administered to achieve the same plasma concentration, as the drug distributes extensively into tissues. *Half life* - **Half-life** primarily determines the **time to reach steady-state** and the **dosing interval** for maintenance doses. - It does not directly influence the initial amount of drug required to achieve a target concentration in the loading dose calculation. *Clearance* - **Clearance** is the primary determinant of the **maintenance dose rate**, as it dictates how quickly the drug is eliminated from the body. - Formula for maintenance dose: Maintenance dose rate = Clearance × Target concentration. - It is not the major determinant of the initial loading dose, which aims to achieve a therapeutic level quickly. *Bioavailability* - **Bioavailability (F)** is the fraction of an administered drug that reaches systemic circulation unchanged. - While bioavailability is included in the loading dose formula as a correction factor (especially important for oral drugs), the **volume of distribution** remains the **major determinant** as it defines the fundamental space the drug must fill to achieve target concentration.
Question 8: Which of the following statements is true about first-order kinetics?
- A. The half-life increases with an increase in dose.
- B. The rate of elimination is proportional to the plasma concentration. (Correct Answer)
- C. A constant amount is eliminated in unit time.
- D. The elimination follows zero-order kinetics at therapeutic doses.
Explanation: ***The rate of elimination is proportional to the plasma concentration.*** - In **first-order kinetics**, a **constant fraction** of the drug is eliminated per unit of time, meaning that the higher the plasma concentration, the faster the elimination rate. - This principle ensures that the drug concentration decreases exponentially over time, as the amount eliminated is always a percentage of the remaining drug. - The rate equation is: dC/dt = -kC, where the rate is directly proportional to concentration. *The half-life increases with an increase in dose.* - This statement is incorrect because, for **first-order kinetics**, the **half-life remains constant** regardless of the dose or the initial concentration of the drug. - The time it takes for the plasma concentration to halve is independent of the initial amount. *The elimination follows zero-order kinetics at therapeutic doses.* - This is incorrect. **First-order kinetics** is the most common pattern for drug elimination at **therapeutic doses**. - **Zero-order kinetics** occurs when elimination mechanisms become **saturated**, typically at very high doses (e.g., phenytoin, ethanol, aspirin at high doses). *A constant amount is eliminated in unit time.* - This describes **zero-order kinetics**, where the elimination process is saturated, and the body eliminates a fixed amount of drug per unit of time, regardless of the plasma concentration. - In **first-order kinetics**, a **constant *fraction*** (not amount) is eliminated per unit time.
Question 9: A 55-year-old man presents with a tremor that occurs when his hands are at rest. He has a slow, shuffling gait and difficulty initiating movement. His symptoms improve with levodopa. What is the most likely diagnosis?
- A. Parkinson’s disease (Correct Answer)
- B. Huntington’s disease
- C. Essential tremor
- D. Multiple sclerosis
Explanation: ***Parkinson’s disease*** - The classic triad of symptoms—**resting tremor**, **bradykinesia** (difficulty initiating movement, shuffling gait), and **rigidity**—is highly characteristic of Parkinson's disease [1, 5]. - Significant improvement with **levodopa** is a hallmark of dopaminergic responsiveness seen in Parkinson's disease. *Huntington’s disease* - Characterized by **chorea** (involuntary, jerky movements), cognitive decline, and psychiatric symptoms, which are not described in this patient. - The onset is typically earlier, and the tremor is not primarily a resting tremor. *Essential tremor* - Primarily an **action tremor** [1] that occurs during voluntary movement, unlike the resting tremor described in the patient. - While it can be debilitating, it typically does not present with **bradykinesia** or **shuffling gait**. *Multiple sclerosis* - A demyelinating disease presenting with a wide range of neurological symptoms depending on lesion location, such as sensory disturbances, weakness, visual problems, and **ataxia**. - While tremors can occur (often intention tremors), the clinical presentation of a **resting tremor**, **shuffling gait**, and **bradykinesia** is not typical of MS.
Question 10: Which one of the following causes low-volume erythrocytosis?
- A. High altitude
- B. Polycythemia Rubra Vera
- C. Gaisbock's syndrome (Correct Answer)
- D. Exogenous testosterone therapy
Explanation: ***Gaisbock's syndrome*** - **Gaisbock's syndrome**, also known as **stress erythrocytosis** or **relative polycythemia**, is characterized by a high hematocrit due to reduced plasma volume rather than an absolute increase in red blood cell (RBC) mass. - It is often associated with **hypertension**, **obesity**, and **stress**, predominantly affecting middle-aged men with a normal total erythrocyte mass [1]. *High altitude* - Living at **high altitudes** can cause **secondary erythrocytosis** due to chronic hypoxia, leading to increased erythropoietin production and an absolute increase in red blood cell mass [2]. - This is a **true erythrocytosis**, where both red blood cell count and total blood volume are elevated. *Polycythemia Rubra Vera* - **Polycythemia vera** is a **myeloproliferative neoplasm** causing an absolute increase in the red blood cell mass, resulting from uncontrolled production by the bone marrow. - It is characterized by a **JAK2V617F mutation** and leads to increased total blood volume, not reduced plasma volume [1]. *Exogenous testosterone therapy* - **Exogenous testosterone therapy** can stimulate erythropoiesis, leading to an **increase in red blood cell mass** and hematocrit, which is a form of **secondary erythrocytosis**. - This effect is mediated by increased erythropoietin production and is an absolute increase in RBCs, not a low-volume condition.
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