In Zero-order kinetics of elimination:

Appeared in: HP CHO -9 October 2022

Explanation

Zero-order kinetics describes a scenario where the rate of drug elimination is constant and independent of the drug's concentration in the plasma.
This happens because the metabolic enzymes or transport systems responsible for elimination are saturated, meaning they are working at their maximum capacity.
As a result, a fixed amount or quantity of the drug (e.g., 10 mg) is eliminated per unit of time (e.g., per hour), regardless of how much drug is in the body.

Option B: This describes first-order kinetics, not zero-order. In first-order kinetics, the rate of elimination is proportional to the drug concentration, meaning a constant fraction or percentage of the drug is removed per unit time.
Option C: This statement defines drug clearance. While related to elimination, it is not the definition of zero-order kinetics. In zero-order kinetics, clearance is not constant; it decreases as the drug concentration increases.
Option D: This is incorrect. In zero-order kinetics, the drug is actively being eliminated, but at a fixed rate. A zero amount of excretion would mean the drug is not being removed from the body at all.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Pharmacokinetics: Zero-Order Elimination as background academic context rather than a clinical decision trigger.
Nurses must be extremely cautious when administering drugs that follow zero-order kinetics, such as phenytoin and high-dose aspirin.
Because the elimination system is saturated, small increases in dosage can lead to a disproportionately large rise in plasma levels, quickly reaching toxic concentrations.
What if a patient on phenytoin (zero-order) is prescribed another drug that inhibits its metabolism? The elimination rate would decrease further, causing a rapid and dangerous accumulation of phenytoin, significantly increasing the risk of toxicity (e.g., nystagmus, ataxia, confusion).

How to Approach the Question

First, identify the core concept in the question stem: 'Zero-order kinetics of elimination'.
This is a factual recall question that tests your understanding of a fundamental pharmacokinetic principle.
Recall the key difference between zero-order and first-order kinetics: 'zero-order' means a constant AMOUNT is eliminated, while 'first-order' means a constant FRACTION is eliminated.
Evaluate the options based on this core definition.
Option A states 'a fixed quantity' (amount), which directly matches the definition of zero-order kinetics.
Option B states 'a fixed fraction', which is the definition of first-order kinetics, making it the primary distractor.

Concept Tested & Keywords

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E6 Pharmacology KD Tripathi Essentials 9e Part 1 pp. 57-59, 56-58

E6 Pharmacology Katzung 16e p. 76-78

Practise the full HP CHO -9 October 2022

Attempt every question from this paper in a timed mock, then review the full solution for each one.