Drug Interactions — MCQs

Drug Interactions Indian Medical PG Practice Questions and MCQs

Question 31: Oral contraceptive pills (OCPs) are contraindicated in patients receiving which of the following medications?

A. Rifampicin (Correct Answer)
B. Ethambutol
C. Streptomycin
D. Pyrazinamide

Explanation: **Explanation:** **1. Why Rifampicin is the Correct Answer:** Rifampicin is a potent **inducer of hepatic microsomal enzymes**, specifically the Cytochrome P450 system (notably CYP3A4). When a patient takes Rifampicin alongside Oral Contraceptive Pills (OCPs), the drug-metabolizing enzymes in the liver are upregulated. This leads to the **accelerated metabolism** of the estrogen and progestogen components of the OCP, significantly reducing their plasma concentration. Consequently, the drug levels fall below the therapeutic threshold required to suppress ovulation, leading to **contraceptive failure** and unwanted pregnancy. **2. Why the Other Options are Incorrect:** * **Ethambutol, Streptomycin, and Pyrazinamide:** These are first-line antitubercular drugs (ATT) that do not possess enzyme-inducing or enzyme-inhibiting properties. They do not interfere with the metabolic pathway of steroid hormones; therefore, they do not reduce the efficacy of OCPs. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Rifampicin Rule":** Rifampicin is one of the most powerful enzyme inducers. Patients on OCPs starting Rifampicin must be advised to use an **alternative/barrier method** of contraception (e.g., condoms) during the treatment and for 4 weeks after stopping Rifampicin. * **Other Enzyme Inducers (Mnemonic: GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. All of these can cause OCP failure. * **Broad-spectrum Antibiotics:** Drugs like Ampicillin or Tetracycline can also decrease OCP efficacy, but via a different mechanism: they inhibit gut flora, preventing the **enterohepatic circulation** of estrogens.

Question 32: The action of theophylline is reduced by which of the following?

A. Smoking (Correct Answer)
B. Erythromycin
C. Cimetidine
D. Lithium

Explanation: **Explanation:** Theophylline is a methylxanthine bronchodilator with a narrow therapeutic index, primarily metabolized by the hepatic cytochrome P450 enzyme system, specifically the **CYP1A2** isoenzyme. **Why Smoking is Correct:** Cigarette smoke contains **polycyclic aromatic hydrocarbons**, which act as potent **inducers of CYP1A2**. Enzyme induction increases the rate of theophylline metabolism, leading to decreased plasma concentrations and reduced therapeutic action. Consequently, chronic smokers often require significantly higher doses of theophylline (up to 50-100% more) to achieve the same clinical effect as non-smokers. **Why Other Options are Incorrect:** * **Erythromycin & Cimetidine:** These are classic **enzyme inhibitors**. They inhibit CYP1A2 and CYP3A4, decreasing the clearance of theophylline. This leads to increased plasma levels and a high risk of theophylline toxicity (nausea, seizures, arrhythmias). * **Lithium:** Theophylline actually increases the renal excretion of Lithium by increasing the glomerular filtration rate (GFR). Therefore, theophylline reduces the action of Lithium, not the other way around. **High-Yield Clinical Pearls for NEET-PG:** 1. **Narrow Therapeutic Window:** The therapeutic range for theophylline is 10–20 µg/mL. Toxicity often starts above 20 µg/mL. 2. **Other Inducers (Reduce levels):** Phenytoin, Rifampicin, Phenobarbitone, and Carbamazepine. 3. **Other Inhibitors (Increase levels):** Ciprofloxacin, Clarithromycin, and Allopurinol. 4. **Smoking Cessation:** If a patient on theophylline stops smoking, the dose must be reduced immediately to prevent toxicity as the enzyme induction effect wears off.

Question 33: Which of the following drugs, when used with carbonic anhydrase inhibitors, can lead to the development of renal stones?

A. Valproic acid
B. Carbamazepine
C. Gabapentin
D. Topiramate (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Topiramate** **Mechanism and Rationale:** The development of renal stones (nephrolithiasis) is a known side effect of both **Carbonic Anhydrase Inhibitors (CAIs)** like acetazolamide and the antiepileptic drug **Topiramate**. Topiramate possesses weak carbonic anhydrase inhibitory activity. When CAIs are used, they inhibit the enzyme in the proximal renal tubule, leading to: 1. **Alkalinization of urine:** Reduced hydrogen ion secretion increases urinary pH. 2. **Hypercalciuria and Hypocitraturia:** Citrate normally keeps calcium in solution; its reduction, combined with alkaline urine, promotes the precipitation of **calcium phosphate stones**. Using Topiramate concurrently with other CAIs creates a synergistic effect, significantly increasing the risk of stone formation. **Analysis of Incorrect Options:** * **A. Valproic acid:** Primarily associated with hepatotoxicity, weight gain, and neural tube defects. It does not inhibit carbonic anhydrase. * **B. Carbamazepine:** Known for causing SIADH (hyponatremia), blood dyscrasias, and Stevens-Johnson Syndrome. It has no significant effect on urinary pH or stone formation. * **C. Gabapentin:** Primarily excreted unchanged by the kidneys. Its main side effects are sedation and peripheral edema; it does not influence renal stone pathophysiology. **NEET-PG High-Yield Pearls:** * **Zonisamide:** Another antiepileptic drug that inhibits carbonic anhydrase and carries a similar risk of renal stones. * **Ketogenic Diet:** Often used for refractory epilepsy, this diet also increases the risk of nephrolithiasis; combining it with Topiramate requires cautious monitoring. * **Clinical Advice:** Patients on Topiramate should be advised to maintain **vigorous hydration** to decrease the concentration of stone-forming salts in the urine.

Question 34: Which of the following drugs may enhance the effect of warfarin and increase the risk of bleeding?

A. Phenobarbitone
B. Ketoconazole (Correct Answer)
C. Rifampicin
D. Carbamazepine

Explanation: **Explanation:** The interaction between drugs and **Warfarin** is a high-yield topic for NEET-PG, primarily revolving around the **Cytochrome P450 (CYP450)** enzyme system. Warfarin is metabolized mainly by CYP2C9. **1. Why Ketoconazole is Correct:** Ketoconazole is a potent **enzyme inhibitor**. By inhibiting the CYP450 enzymes responsible for Warfarin metabolism, it leads to decreased clearance and increased plasma concentrations of Warfarin. This enhances its anticoagulant effect, prolongs the Prothrombin Time (PT)/INR, and significantly increases the clinical risk of bleeding. **2. Analysis of Incorrect Options:** * **Phenobarbitone, Rifampicin, and Carbamazepine:** These are all classic **enzyme inducers**. They increase the synthesis of CYP450 enzymes, which accelerates the metabolism of Warfarin. This leads to *decreased* therapeutic levels of Warfarin, potentially causing treatment failure and increasing the risk of thrombosis (clotting) rather than bleeding. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Inhibitors (Increase Warfarin effect):** **VITAMINS K** – **V**erapamil, **I**soniazid, **T**rimethoprim, **A**miodarone, **M**etronidazole, **I**ndinavir, **N**eomycin, **S**ulfonamides, **K**etoconazole (and other azoles). * **Mnemonic for Enzyme Inducers (Decrease Warfarin effect):** **G P R S Cell Phone** – **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Key Monitoring:** Always monitor **INR** (International Normalized Ratio) when adding or stopping any drug in a patient on Warfarin. * **Broad-spectrum antibiotics** can also enhance Warfarin's effect by killing gut flora that synthesize Vitamin K.

Question 35: A patient being treated with ketoconazole develops Gastroesophageal Reflux Disease (GERD). Which of the following drugs should not be prescribed to him?

A. Cisapride (Correct Answer)
B. Itopride
C. Metoclopramide
D. Domperidone

Explanation: ### Explanation **1. Why Cisapride is the Correct Answer:** The core concept here is **Cytochrome P450 (CYP3A4) inhibition**. Cisapride is a prokinetic agent that is primarily metabolized by the CYP3A4 enzyme. Ketoconazole is a potent **CYP3A4 inhibitor**. When administered together, ketoconazole inhibits the metabolism of cisapride, leading to toxic plasma levels of the drug. High levels of cisapride cause a dangerous prolongation of the QT interval, which can progress to **Torsades de Pointes** (a life-threatening ventricular tachycardia). Due to this risk of fatal cardiac arrhythmias, cisapride has been withdrawn or strictly restricted in many markets. **2. Why the Other Options are Incorrect:** * **B. Itopride:** This is a newer prokinetic with a dual mechanism (D2 antagonism and AChE inhibition). It is metabolized by **flavin monooxygenase (FMO3)**, not the CYP450 system, making it safe to use with ketoconazole. * **C. Metoclopramide:** This drug is primarily metabolized by CYP2D6 and glucuronidation. While it has central side effects (Extrapyramidal symptoms), it does not share the significant CYP3A4-mediated cardiotoxicity risk seen with cisapride. * **D. Domperidone:** Although domperidone is metabolized by CYP3A4 and can prolong the QT interval, the interaction is significantly less hazardous than with cisapride. In clinical practice, cisapride is the "classic" contraindication tested in exams regarding fatal CYP3A4 interactions. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Terrible Trio":** Remember that **Cisapride, Astemizole, and Terfenadine** are the three classic drugs that cause Torsades de Pointes when combined with CYP3A4 inhibitors (like Macrolides or Azole antifungals). * **Mechanism of Arrhythmia:** These drugs block the **delayed rectifier K+ channels (hERG gene)** in the heart, leading to delayed repolarization. * **Safe Alternatives:** Itopride and Prucalopride are generally preferred prokinetics when CYP3A4 interactions are a concern.

Question 36: Which of the following drugs is not associated with myasthenia gravis?

A. Clofibrate
B. Polymyxin B (Correct Answer)
C. Penicillin
D. All of the above

Explanation: ### Explanation The question focuses on drugs that can **induce or unmask Myasthenia Gravis (MG)** or cause a myasthenic-like syndrome. **1. Why Polymyxin B is the Correct Answer:** Polymyxin B is associated with **neuromuscular blockade**, but it does not cause or exacerbate autoimmune Myasthenia Gravis itself. Instead, it acts directly at the neuromuscular junction (NMJ) by interfering with the release of acetylcholine or blocking the post-synaptic receptors. While it can cause respiratory paralysis (especially in patients with renal failure), it is categorized as a drug causing **neuromuscular weakness** rather than an immunological trigger for MG. **2. Analysis of Incorrect Options:** * **Clofibrate (Option A):** This lipid-lowering agent is a known trigger that can unmask or aggravate MG. It is associated with myopathic side effects and can exacerbate pre-existing neuromuscular transmission defects. * **Penicillin (Option C):** Specifically, **D-Penicillamine** (a derivative) is a classic high-yield trigger for drug-induced MG. It induces the production of anti-acetylcholine receptor (AChR) antibodies, leading to a condition clinically indistinguishable from idiopathic MG. While "Penicillin" is broad, in the context of MG questions, it often refers to this association or rare reports of hypersensitivity-related neuromuscular weakness. **3. Clinical Pearls for NEET-PG:** * **D-Penicillamine** is the most common drug to cause *de novo* autoimmune MG. * **Aminoglycosides** (e.g., Gentamicin, Neomycin) are the most notorious for worsening MG by inhibiting presynaptic ACh release. * **Fluoroquinolones** (e.g., Ciprofloxacin) carry a Black Box Warning for MG exacerbation. * **Beta-blockers** and **Magnesium salts** can also impair neuromuscular transmission and should be used with caution in myasthenic patients.

Question 37: Co-administration of which of the following drugs with Zidovudine is not preferred?

A. Indomethacin
B. Aspirin
C. Trimethoprim
D. All of the above (Correct Answer)

Explanation: **Explanation:** The primary concern when co-administering drugs with **Zidovudine (AZT)** is the potential for additive toxicity, specifically **bone marrow suppression** and interference with its metabolic pathway. Zidovudine is metabolized in the liver via **glucuronidation** (by the enzyme UDP-glucuronosyltransferase). * **Aspirin and Indomethacin (NSAIDs):** These drugs compete with Zidovudine for the same glucuronidation pathway in the liver. This competition inhibits the metabolism of Zidovudine, leading to increased plasma levels and a significantly higher risk of systemic toxicity, particularly severe anemia and neutropenia. * **Trimethoprim:** While often used in HIV patients for PCP prophylaxis (as Co-trimoxazole), Trimethoprim can cause additive hematological toxicity. Both Zidovudine and Trimethoprim are known to cause bone marrow suppression; their combined use increases the risk of leucopenia and megaloblastic changes. **Why "All of the above" is correct:** Each of these drugs either pharmacokinetically interferes with Zidovudine’s clearance (Aspirin, Indomethacin) or pharmacodynamically enhances its bone marrow toxicity (Trimethoprim). Therefore, their co-administration is generally avoided or monitored very closely. **High-Yield NEET-PG Pearls:** * **Dose-limiting toxicity of Zidovudine:** Anemia and Neutropenia. * **Other drugs to avoid with AZT:** Ganciclovir, Pyrimethamine, and Amphotericin B (due to additive myelosuppression or nephrotoxicity). * **Stavudine (d4T) Interaction:** Never combine Zidovudine with Stavudine because they compete for the same intracellular phosphorylation (activation) pathway, leading to antagonism. * **Drug of choice for preventing vertical transmission (Mother-to-child) of HIV:** Zidovudine (though now often part of HAART regimens).

Question 38: What is the effect of concomitant use of hydrochlorothiazide on serum lithium levels?

A. Serum lithium levels rise (Correct Answer)
B. Serum lithium levels decrease
C. Serum hydrochlorothiazide levels rise
D. Serum hydrochlorothiazide levels fall

Explanation: ### Explanation **1. Why the correct answer is right (Mechanism):** Lithium is a monovalent cation that is handled by the kidneys in a manner very similar to sodium. Approximately 80% of filtered lithium is reabsorbed in the **proximal convoluted tubule (PCT)**. When a patient takes **Hydrochlorothiazide (HCTZ)**, it inhibits the Na+/Cl- symporter in the distal tubule, leading to increased excretion of sodium and water. This mild volume depletion triggers a compensatory mechanism in the proximal tubule: the body attempts to conserve volume by increasing the reabsorption of sodium and water. Because the PCT cannot distinguish well between sodium and lithium, it **co-reabsorbs lithium** along with sodium. This leads to decreased renal clearance of lithium, causing its serum levels to rise, potentially reaching toxic thresholds. **2. Why the incorrect options are wrong:** * **Option B:** Serum lithium levels do not decrease; rather, the compensatory proximal reabsorption prevents its excretion. (Note: Osmotic diuretics like mannitol or carbonic anhydrase inhibitors like acetazolamide actually *decrease* lithium levels by increasing its excretion). * **Options C & D:** While pharmacokinetic interactions can occur, the clinically significant interaction here is the effect of the diuretic on the narrow therapeutic index drug (Lithium), not the change in the diuretic's own concentration. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Lithium-Safe" Diuretic:** Amiloride is the diuretic of choice for treating Lithium-induced Nephrogenic Diabetes Insipidus because it blocks the ENaC channels in the collecting duct, preventing lithium from entering and damaging those cells. * **Drugs that INCREASE Lithium levels:** **T**hiazides, **N**SAIDs (except aspirin/sulindac), and **A**CE inhibitors/ARBs (Mnemonic: **TNA**). * **Therapeutic Index:** Lithium has a very narrow therapeutic index (0.6–1.2 mEq/L). Any drug decreasing its clearance can quickly lead to toxicity (tremors, ataxia, seizures).

Question 39: Why should a patient on verapamil not be given a beta-blocker?

A. Conduction block (Correct Answer)
B. Bronchospasm
C. Neurogenic shock
D. Anaphylaxis

Explanation: ### Explanation **1. Why "Conduction Block" is Correct:** Verapamil is a non-dihydropyridine Calcium Channel Blocker (CCB) with significant **negative inotropic, chronotropic, and dromotropic** effects. It primarily acts on the SA and AV nodes. Beta-blockers (e.g., Propranolol, Metoprolol) also exert potent negative effects on the heart by blocking sympathetic stimulation. When used together, their effects on the **AV node** are synergistic. This leads to a profound slowing of electrical impulses, which can result in severe **bradycardia** or a complete **AV conduction block**. Furthermore, the combined negative inotropic effect can precipitate acute heart failure in susceptible patients. **2. Analysis of Incorrect Options:** * **B. Bronchospasm:** While beta-blockers (especially non-selective ones) can cause bronchospasm, verapamil does not exacerbate this effect. In fact, CCBs are generally safe in asthmatics. * **C. Neurogenic shock:** This is typically caused by spinal cord injury or severe CNS trauma, leading to loss of sympathetic tone. It is not a direct pharmacological consequence of this drug-drug interaction. * **D. Anaphylaxis:** This is an IgE-mediated Type I hypersensitivity reaction. While any drug can cause an allergy, the specific interaction between verapamil and beta-blockers is hemodynamic/electrophysiological, not immunological. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diltiazem:** Like verapamil, diltiazem should also be used with extreme caution alongside beta-blockers due to similar risks. * **Dihydropyridines (e.g., Amlodipine):** These are safer to combine with beta-blockers because they primarily cause peripheral vasodilation rather than direct nodal suppression. * **Antidote:** In cases of severe toxicity from this combination, **Intravenous Glucagon** or **High-dose Insulin-Euglycemia Therapy (HIET)** may be used. * **Contraindication:** This combination is strictly contraindicated in patients with pre-existing Sick Sinus Syndrome or 2nd/3rd-degree heart block.

Question 40: Which of the following interacts with Digitalis maximally?

A. Furosemide (Correct Answer)
B. Triamterene
C. Amiloride
D. Spironolactone

Explanation: The interaction between **Digitalis (Digoxin)** and diuretics is primarily mediated by changes in serum potassium levels. Digoxin works by inhibiting the **Na+/K+-ATPase pump** on cardiac myocytes [2]. Since potassium competes with Digoxin for the same binding site on this pump, **hypokalemia** (low potassium) increases Digoxin binding, leading to digitalis toxicity [2]. **Why Furosemide is the correct answer:** Furosemide is a potent **Loop Diuretic**. It inhibits the Na+-K+-2Cl- symporter in the thick ascending limb of the Loop of Henle, leading to significant excretion of potassium in the urine [1]. This profound hypokalemia sensitizes the myocardium to Digoxin, "maximally" increasing the risk of life-threatening arrhythmias [1]. **Analysis of Incorrect Options:** * **B, C, and D (Triamterene, Amiloride, Spironolactone):** These are all **Potassium-Sparing Diuretics**. Unlike Furosemide, they increase serum potassium levels. While hyperkalemia can actually *decrease* the effectiveness of Digoxin (by outcompeting it for the pump) [2], it does not carry the same immediate risk of acute toxicity as the hypokalemia induced by Furosemide. In fact, Spironolactone is often used alongside Digoxin in heart failure management to prevent hypokalemia. **NEET-PG High-Yield Pearls:** * **Electrolyte Triad of Digoxin Toxicity:** Hypokalemia, Hypomagnesemia, and Hypercalcemia all increase the risk of toxicity. * **ECG Hallmark:** The most common ECG finding in Digoxin toxicity is **PVCs (Premature Ventricular Contractions)**, while the most characteristic/specific is **Atrial Tachycardia with AV block**. * **Antidote:** Digoxin-specific antibody fragments (**DigiFab/Digibind**). * **Drug of Choice:** For Digoxin-induced arrhythmias, the drug of choice is **Lidocaine** or **Phenytoin**.

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Mechanisms of Drug Interactions

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Pharmacokinetic Interactions

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Pharmacodynamic Interactions

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Drug-Food Interactions

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Drug-Disease Interactions

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Clinically Significant Drug Interactions

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Computer Systems for Detecting Drug Interactions

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Management of Drug Interactions

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Drug Interactions in Special Populations

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Role of P-glycoprotein in Drug Interactions

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