Drug Interactions Practice Questions

Q: All of the following are inducers of the microsomal enzyme system except?

Ticlopidine. ### Explanation The microsomal enzyme system, primarily the **Cytochrome P450 (CYP450)** family in the liver, is responsible for the metabolism of many drugs. **Enzyme Inducers** increase the synthesis of these enzymes, leading to faster metabolism and decreased plasma concentrations of co-administered drugs. **Why Ticlopidine is the Correct Answer:** Ticlopidine is an antiplatelet drug that acts as a **microsomal enzyme inhibitor**, specifically inhibiting CYP2C19. Unlike inducers, inhibitors decrease enzyme activity, which can lead to toxic levels of other drugs (e.g., phenytoin or theophylline) if administered concurrently. **Analysis of Incorrect Options (Inducers):** * **A. Carbamazepine:** A potent inducer of CYP3A4. It is unique because it is an **auto-inducer**, meaning it induces its own metabolism over time. * **B. Phenytoin:** A classic broad-spectrum inducer that affects multiple CYP isoforms, often leading to significant drug-drug interactions (e.g., reducing the efficacy of oral contraceptives). * **C. Phenobarbitone:** One of the most powerful known inducers. It increases the synthesis of glucuronyl transferase and CYP enzymes, and is clinically used in neonatal jaundice to accelerate bilirubin conjugation. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Mnemonic for Inhibitors (VITAMIN K):** **V**alproate, **I**soniazid, **T**iclopidine/Terfenadine, **A**miodarone, **M**acrolides (except Azithromycin), **I**ndinavir, **N**on-DHP CCBs (Verapamil/Diltiazem), **K**etoconazole (and other Azoles). * **Chronic Alcoholism** induces enzymes, whereas **Acute Alcohol intake** inhibits them. * **St. John’s Wort** is a notable herbal enzyme inducer often tested in exams.

Q: Warfarin therapy is prolonged by all except:

Rifampicin. The core concept behind this question is the modulation of **Cytochrome P450 (CYP450)** enzymes and the role of intestinal flora in Vitamin K synthesis. **1. Why Rifampicin is the correct answer:** Warfarin is metabolized primarily by the CYP2C9 enzyme. **Rifampicin** is a potent **microsomal enzyme inducer** [2]. By inducing these enzymes, it accelerates the metabolism of Warfarin, leading to decreased plasma levels and a **reduced** anticoagulant effect (shortened prothrombin time). Therefore, it does *not* prolong Warfarin therapy; it antagonizes it. **2. Why the other options are incorrect:** * **Erythromycin:** This is a known **enzyme inhibitor** [1]. It inhibits CYP450 enzymes, slowing down Warfarin metabolism, which increases its plasma concentration and **prolongs** its effect (increasing the risk of bleeding). * **Amoxicillin & Tetracycline:** Broad-spectrum antibiotics prolong Warfarin therapy through a different mechanism [3]. They eliminate **intestinal bacterial flora** that normally synthesize Vitamin K2 [3]. Since Warfarin works by inhibiting Vitamin K epoxide reductase, a reduction in endogenous Vitamin K levels potentiates Warfarin’s anticoagulant action. **Clinical Pearls for NEET-PG:** * **S-Warfarin** is 3-5 times more potent than R-Warfarin and is metabolized by **CYP2C9** [2]. * **Enzyme Inducers (Reduce Warfarin effect):** Phenytoin, Carbamazepine, Phenobarbitone, Rifampicin, Griseofulvin (Mnemonic: **GPRS Cell Phone**). * **Enzyme Inhibitors (Increase Warfarin effect):** Valproate, Ketoconazole, Cimetidine, Erythromycin, Isoniazid (Mnemonic: **Vitamins K, C, E, I**). * **Monitoring:** Warfarin efficacy is monitored using **PT/INR** (Extrinsic pathway) [3].

Question 1

Nitrates are contraindicated with which of the following drug classes?

Accepted Answer

Phosphodiesterase-5 (PDE-5) inhibitors

Answer

Sulphonylureas

Answer

Immunomodulators

Answer

Selective Serotonin Reuptake Inhibitors (SSRIs)

Question 2

A diabetic female on Isoniazid (INH) and Rifampicin for tuberculosis developed Deep Vein Thrombosis (DVT). She was started on Warfarin, but her Prothrombin Time (PT) is not elevated. What is the next appropriate step in management?

Accepted Answer

Use Low Molecular Weight Heparin (LMWH)

Answer

Increase the dose of Warfarin

Answer

Replace Warfarin with Acenocoumarol

Answer

Switch Ethambutol for Rifampicin

Question 3

All of the following are inducers of the microsomal enzyme system except?

Accepted Answer

Ticlopidine

Answer

Carbamazepine

Answer

Phenytoin

Answer

Phenobarbitone

Question 4

Which drug may inhibit the cytochrome P450 metabolism of warfarin?

Accepted Answer

Cimetidine

Answer

Ethanol

Answer

Rifampicin

Answer

Procainamide

Question 5

Warfarin therapy is prolonged by all except:

Accepted Answer

Rifampicin

Answer

Amoxicillin

Answer

Erythromycin

Answer

Tetracycline

Drug Interactions — MCQs

Drug Interactions — MCQs

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