Drug Interactions Practice Questions

Q: Which of the following prokinetic drugs has been implicated in causing serious ventricular arrhythmias, particularly in patients concurrently receiving erythromycin or ketoconazole?

Cisapride. **Explanation:** **Cisapride** is the correct answer because it is a substituted benzamide prokinetic that acts as a potent **5-HT₄ receptor agonist**. Its clinical use has been severely restricted or withdrawn in many countries due to its potential to cause **QT interval prolongation** and life-threatening ventricular arrhythmias, specifically **Torsades de Pointes**. The underlying mechanism involves the blockade of **HERG (human ether-a-go-go-related gene) potassium channels** in the heart, which delays repolarization. Cisapride is metabolized primarily by the **CYP3A4** enzyme. Therefore, concurrent administration of CYP3A4 inhibitors like **erythromycin** (macrolide) or **ketoconazole** (azole antifungal) leads to toxic plasma levels of Cisapride, significantly increasing the risk of fatal arrhythmias. **Analysis of Incorrect Options:** * **Domperidone (A):** While it can cause QT prolongation at high doses, it is a peripheral D₂ antagonist and does not carry the same high-risk profile for drug-drug interactions as Cisapride. * **Mosapride (C):** A newer 5-HT₄ agonist that is considered safer because it lacks significant affinity for the HERG potassium channels, thus carrying a lower risk of arrhythmias. * **Metoclopramide (D):** A D₂ antagonist and 5-HT₄ agonist. Its primary side effects are extrapyramidal (dystonias, Parkinsonism) due to central D₂ blockade, not cardiac arrhythmias. **High-Yield Pearls for NEET-PG:** * **Drug of Choice (DOC):** Metoclopramide is often the DOC for diabetic gastroparesis but is contraindicated in pheochromocytoma. * **Prucalopride:** A highly selective 5-HT₄ agonist used for chronic constipation with a superior cardiac safety profile. * **Erythromycin:** Acts as a prokinetic by stimulating **motilin receptors**, but its use is limited by tachyphylaxis.

Q: An asthmatic patient on theophylline suddenly develops an infection. Which of the following antibiotics should be avoided to prevent side effects of theophylline?

Erythromycin. **Explanation:** The correct answer is **Erythromycin**. This question tests the concept of **Enzyme Inhibition** and its impact on drugs with a **Narrow Therapeutic Index (NTI)**. **1. Why Erythromycin is correct:** Theophylline is metabolized primarily by the hepatic cytochrome P450 enzyme system (specifically **CYP1A2** and **CYP3A4**). Erythromycin is a potent **enzyme inhibitor**. When co-administered, Erythromycin inhibits the metabolism of theophylline, leading to increased serum levels of the drug. Since theophylline has a narrow therapeutic window, this elevation can quickly lead to **theophylline toxicity**, manifesting as severe nausea, vomiting, palpitations, arrhythmias, and seizures. **2. Why other options are incorrect:** * **A & C (Ampicillin and Cephalexin):** These are Beta-lactam antibiotics. They are primarily excreted renally and do not significantly inhibit the CYP450 system. They have no clinically significant interaction with theophylline. * **D (Sparfloxacin):** While some fluoroquinolones (like Ciprofloxacin and Enoxacin) are known to inhibit theophylline metabolism, Sparfloxacin does not have a significant inhibitory effect on the CYP enzymes responsible for theophylline clearance. **3. NEET-PG High-Yield Pearls:** * **The "Cimetidine & Macrolide" Rule:** Always look for Enzyme Inhibitors like Cimetidine, Erythromycin, Clarithromycin, and Ciprofloxacin when a patient on Theophylline, Warfarin, or Phenytoin develops toxicity. * **Safe Macrolide:** **Azithromycin** is the macrolide of choice if an interaction must be avoided, as it does not inhibit CYP450 enzymes. * **Theophylline Toxicity:** The earliest sign is usually GI upset, but the most dangerous are cardiac arrhythmias and intractable seizures.

Q: Which of the following diuretics decreases the renal lithium clearance?

Furosemide. **Explanation:** The correct answer is **Furosemide (Option C)**. **Mechanism of Interaction:** Lithium is handled by the kidneys similarly to sodium. It is filtered at the glomerulus and reabsorbed (about 80%) in the proximal convoluted tubule (PCT). When diuretics like **Furosemide** (Loop diuretic) or **Thiazides** cause sodium and water depletion, the body compensates by increasing sodium reabsorption in the PCT. Since the renal tubules cannot distinguish well between sodium and lithium ions, **lithium reabsorption is also increased**. This leads to a **decrease in renal lithium clearance**, resulting in elevated serum lithium levels and a high risk of lithium toxicity. **Analysis of Incorrect Options:** * **A. Acetazolamide:** This is a carbonic anhydrase inhibitor. It actually **increases** lithium excretion (increases clearance) by interfering with its reabsorption in the PCT. * **B. Mannitol:** As an osmotic diuretic, it increases the flow of tubular fluid and inhibits the reabsorption of electrolytes and water throughout the nephron, thereby **increasing** lithium clearance. * **C. Spironolactone:** While potassium-sparing diuretics have a lesser effect on lithium compared to thiazides or loop diuretics, they generally do not significantly decrease clearance to the extent that Furosemide or Thiazides do. However, in the context of this question, Furosemide is the classic established cause of decreased clearance. **High-Yield Clinical Pearls for NEET-PG:** * **Thiazides** are the most notorious for causing lithium toxicity (more so than Loop diuretics). * **NSAIDs** (except Aspirin and Sulindac) and **ACE inhibitors/ARBs** also decrease lithium clearance and can precipitate toxicity. * **Lithium Toxicity Symptoms:** Coarse tremors, ataxia, dysarthria, seizures, and diabetes insipidus (nephrogenic). * **Management of Toxicity:** Hemodialysis is the treatment of choice for severe lithium poisoning.

Q: Rifampicin and ritonavir are examples of which type of drug antagonism?

Pharmacokinetic antagonist. ### Explanation **Pharmacokinetic antagonism** occurs when one drug reduces the concentration of another drug at its site of action by interfering with its absorption, distribution, metabolism, or excretion (ADME) [1]. **Why Option C is correct:** Rifampicin is a potent **inducer of Cytochrome P450 enzymes** (specifically CYP3A4). When co-administered with Ritonavir (a protease inhibitor used in HIV treatment), Rifampicin increases the metabolic breakdown of Ritonavir. This leads to significantly decreased plasma levels of Ritonavir, rendering it sub-therapeutic [1]. Because the antagonism occurs via a metabolic pathway rather than at the receptor site, it is classified as pharmacokinetic antagonism. **Why the other options are incorrect:** * **A. Physical Antagonist:** Based on physical properties (e.g., Charcoal adsorbing alkaloids in the gut). * **B. Chemical Antagonist:** Involves a direct chemical reaction between two substances in solution (e.g., Chelating agents like EDTA binding to heavy metals or Antacids neutralizing gastric acid). * **D. Competitive Antagonist:** Occurs when a drug binds to the same receptor site as the agonist, shifting the dose-response curve to the right (e.g., Atropine vs. Acetylcholine). Rifampicin and Ritonavir do not compete for the same receptor. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin** is the "classic" enzyme inducer. It reduces the efficacy of **Oral Contraceptive Pills (OCPs)**, Warfarin, and Corticosteroids. * **Ritonavir** is actually a potent **enzyme inhibitor**. In HIV therapy, it is often used in low doses as a "pharmacokinetic booster" for other protease inhibitors (like Lopinavir) to *increase* their plasma levels [1]. * **Pharmacodynamic antagonism** (the opposite concept) occurs at the receptor level (e.g., Beta-blockers vs. Adrenaline).

Q: Which drug does not interfere with antacids?

Azithromycin. ### Explanation The core pharmacological concept here is **chelation and pH-dependent absorption**. Antacids contain multivalent cations (e.g., $Al^{3+}$, $Mg^{2+}$, $Ca^{2+}$) which can bind to certain drugs, forming insoluble complexes that cannot be absorbed from the gastrointestinal tract. **Why Azithromycin is the Correct Answer:** Azithromycin is a macrolide antibiotic that does not form chelate complexes with multivalent cations. While its absorption can be slightly delayed by food, its total bioavailability is **not significantly affected** by the co-administration of antacids. Therefore, it does not require the strict spacing intervals necessary for other antibiotics. **Analysis of Incorrect Options:** * **Tetracycline:** This is the classic example of chelation. Tetracyclines bind strongly to $Ca^{2+}$ and $Mg^{2+}$ in antacids, forming non-absorbable chelates, leading to therapeutic failure. * **Norfloxacin:** Fluoroquinolones (like Norfloxacin and Ciprofloxacin) undergo significant chelation with antacids. This can reduce their bioavailability by up to 50–90%. * **Ranitidine:** While Ranitidine is used to reduce acid, taking it simultaneously with potent antacids (especially high-dose magnesium/aluminum hydroxides) can reduce its absorption by approximately 20-30% due to adsorption or pH changes. **NEET-PG High-Yield Pearls:** * **The "2-Hour Rule":** To avoid interactions, patients should be advised to take interacting drugs 2 hours before or 4–6 hours after antacids. * **Other drugs affected by antacids:** Iron salts, Digoxin, Ketoconazole (requires low pH for dissolution), and Phenytoin. * **Macrolide Exception:** Unlike Azithromycin, the absorption of **Erythromycin stearate** is significantly decreased by food and certain gastric conditions, though not primarily through chelation.

Q: Which of the following drugs does NOT induce microsomal enzymes?

Cimetidine. ### Explanation The question focuses on the pharmacological concept of **Cytochrome P450 (CYP450) enzyme modulation**. Microsomal enzymes, primarily located in the liver, are responsible for the metabolism of various drugs. **1. Why Cimetidine is the Correct Answer:** Cimetidine is a potent **Microsomal Enzyme Inhibitor**. It binds to the heme iron of the CYP450 system, reducing the metabolic activity of the liver. This leads to increased plasma concentrations and potential toxicity of co-administered drugs (e.g., Warfarin, Theophylline, Phenytoin). **2. Analysis of Incorrect Options (Enzyme Inducers):** Enzyme inducers increase the synthesis of microsomal enzymes, leading to faster metabolism and decreased efficacy of other drugs. * **Rifampicin:** One of the most potent known inducers of the CYP3A4 isoenzyme. * **Phenobarbitone:** A classic sedative-hypnotic that induces multiple CYP families; it is often used as the prototype for induction. * **Griseofulvin:** An antifungal agent known to induce hepatic enzymes, which can specifically decrease the effectiveness of oral contraceptives and warfarin. **3. High-Yield Clinical Pearls for NEET-PG:** To remember these for the exam, use these common mnemonics: * **Enzyme Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin/Phenobarbitone, **R**ifampicin, **S**moking, **C**arbamazepine. * **Enzyme Inhibitors (VITAMIN K):** **V**alproate, **I**soniazid, **T**erfenadine, **A**miodarone, **M**ethylphenidate, **I**traconazole, **N**ight (Cimetidine), **K**etoconazole. * **Clinical Note:** Cimetidine also has anti-androgenic effects (can cause gynecomastia), a frequent "side-effect" question in NEET-PG.

Question 1

A 40-year-old male with a history of factor V Leiden deficiency and recurrent deep vein thrombosis is well controlled on warfarin. He presents with community-acquired pneumonia and is treated with erythromycin. Three days later, he develops bleeding and his INR is 8.0. Which of the following best explains why this bleeding occurred?

Accepted Answer

The erythromycin inhibited cytochrome P450

Answer

The erythromycin stimulated cytochrome P450

Answer

The causative agent of the pneumonia inhibited vitamin K utilization

Answer

The causative agent of the pneumonia stimulated vitamin K utilization

Question 2

Which of the following prokinetic drugs has been implicated in causing serious ventricular arrhythmias, particularly in patients concurrently receiving erythromycin or ketoconazole?

Accepted Answer

Cisapride

Answer

Domperidone

Answer

Mosapride

Answer

Metoclopramide

Question 3

Failure of oral contraceptives occurs when used with which of the following drugs?

Accepted Answer

Aspirin

Answer

Tetracycline

Answer

Phenytoin

Answer

Rifampicin

Question 4

An asthmatic patient on theophylline suddenly develops an infection. Which of the following antibiotics should be avoided to prevent side effects of theophylline?

Accepted Answer

Erythromycin

Answer

Ampicillin

Answer

Cephalexin

Answer

Sparfloxacin

Question 5

Which of the following diuretics decreases the renal lithium clearance?

Accepted Answer

Furosemide

Answer

Acetazolamide

Answer

Mannitol

Answer

Spironolactone

Question 6

Rifampicin and ritonavir are examples of which type of drug antagonism?

Accepted Answer

Pharmacokinetic antagonist

Answer

Physical antagonist

Answer

Chemical antagonist

Answer

Competitive antagonist

Question 7

Antacids interfere with the absorption of which of the following drugs?

Accepted Answer

Azithromycin

Answer

Ketoconazole

Answer

Oxytetracycline

Answer

Ofloxacin

Question 8

Furosemide should not be administered with NSAIDS because they:

Accepted Answer

Inhibit prostacyclin synthesis

Answer

Prevent platelet aggregation

Answer

Decrease sodium reabsorption

Answer

Increase the secretion of furosemide in urine

Question 9

Which drug does not interfere with antacids?

Accepted Answer

Azithromycin

Answer

Tetracycline

Answer

Norfloxacin

Answer

Ranitidine

Question 10

Which of the following drugs does NOT induce microsomal enzymes?

Accepted Answer

Cimetidine

Answer

Griseofulvin

Answer

Rifampicin

Answer

Phenobarbitone

Drug Interactions — MCQs

Drug Interactions — MCQs

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