**Question:** Zero order kinetics occur in following drug with high dose

A. Lidocaine
B. Morphine
C. Penicillin
D. Digoxin

**Core Concept:** Zero order kinetics is a model of drug kinetics that describes a drug's rate of absorption or elimination being proportional to the concentration of the drug in the body, independent of the drug's concentration. This means that the rate remains constant regardless of the amount of drug present.

**Why the Correct Answer is Right:**

Correct Answer: **D. Digoxin**

Digoxin is a cardiac glycoside used to treat heart failure and atrial fibrillation. Its pharmacokinetics follow zero order kinetics because the rate of absorption and elimination is constant, regardless of the administered dose. This is because digoxin works by inhibiting the Na+/K+-ATPase pump in cardiac cells, thereby increasing the intracellular sodium and potassium concentrations. This leads to increased calcium influx, which is essential for cardiac contraction. The effect is independent of the drug concentration, explaining zero order kinetics.

**Why Each Wrong Option is Incorrect:**

A. **Lidocaine**: Lidocaine is a local anesthetic that works by inhibiting voltage-gated sodium channels in nerve cells, preventing the propagation of an action potential. Its pharmacokinetics are first order kinetics, as the rate of absorption and elimination depends on the concentration of the drug, not its dose.

B. **Morphine**: Morphine is an opioid analgesic that binds to mu opioid receptors to produce analgesia. Its pharmacokinetics are first order kinetics, as the rate of absorption and elimination depends on the concentration of the drug, not its dose.

C. **Penicillin**: Penicillin is an antibiotic that inhibits bacterial cell wall synthesis by binding to specific penicillin-binding proteins (PBPs). Its pharmacokinetics are first order kinetics, as the rate of absorption and elimination depends on the concentration of the drug, not its dose.

**Clinical Pearl:** In clinical practice, understanding the pharmacokinetic properties of drugs is crucial for predicting drug concentrations and adjusting doses to achieve therapeutic effects and minimize side effects. Zero order kinetics can be useful for drugs like digoxin, where the desired effect is maintained regardless of the administered dose. First order kinetics are more common for rapidly absorbed and eliminated drugs like opiates and antibiotics.