**Question:** Rate of elimination with first order kinetics is proportional to -

A. Inhibition constant (Ki)
B. Concentration of the drug
C. Inducer constant (Km)
D. Maximum velocity (Vmax)

**Core Concept:** First order kinetics describes the relationship between the rate of an enzyme-catalyzed reaction and the concentration of one of the reactants. In the context of pharmacology, first order elimination represents a single elimination pathway where the rate of elimination is directly proportional to the concentration of the drug.

**Why the Correct Answer is Right:** In first order elimination, the rate of elimination (R) is directly proportional to the concentration of the drug (C), according to the formula: R = k * C, where k is the rate constant. This means that as the concentration of the drug increases, so does the rate of elimination, which is why option B is correct.

**Why Each Wrong Option is Incorrect:**

A. Inhibition constant (Ki) is related to the ability of a drug to inhibit an enzyme. It describes the concentration of the inhibitor that halves the enzyme's activity, not the rate of elimination.

C. Inducer constant (Km) is related to the enzyme's affinity for its substrate. It describes the substrate concentration at which the enzyme's reaction rate is half-maximal, not the rate of elimination.

D. Maximum velocity (Vmax) is the maximum reaction rate an enzyme can achieve when all substrate is converted to product. It is not directly related to the rate of elimination in the context of pharmacokinetics.

**Clinical Pearl:** Understanding first order kinetics is crucial for pharmacokineticists, as it helps them predict the elimination half-life of a drug and its clearance from the body, which in turn influences the dosing regimen.

**Correct Answer:** B. Concentration of the drug