Drug Interactions — MCQs

Drug Interactions Indian Medical PG Practice Questions and MCQs

Question 91: A patient with glaucoma and bronchial asthma presents with status asthmaticus. What causative agent might have precipitated this condition?

A. Pilocarpine eye drop
B. Timolol eye drop (Correct Answer)
C. Betaxolol eye drop
D. Levobunolol eye drop

Explanation: **Explanation:** **1. Why Timolol is the Correct Answer:** Timolol is a **non-selective beta-blocker** ($\beta_1$ and $\beta_2$ antagonist) commonly used as first-line therapy for glaucoma to decrease aqueous humor production. Even when administered topically as eye drops, significant systemic absorption occurs via the nasolacrimal duct, bypassing first-pass metabolism. In patients with pre-existing bronchial asthma, the blockade of $\beta_2$ receptors in the bronchial smooth muscle leads to bronchoconstriction, which can precipitate life-threatening **status asthmaticus**. **2. Analysis of Incorrect Options:** * **Pilocarpine (Option A):** A direct-acting cholinergic agonist (miotic). While it can cause bronchoconstriction in theory, it is not a beta-blocker and is rarely associated with precipitating status asthmaticus compared to $\beta$-blockers. * **Betaxolol (Option C):** This is a **cardioselective ($\beta_1$) blocker**. Because it lacks significant $\beta_2$ antagonist activity, it is the safest beta-blocker for glaucoma patients who also have respiratory diseases (though it should still be used with caution). * **Levobunolol (Option D):** Like timolol, it is a non-selective beta-blocker. However, Timolol is the classic prototype and the most frequently cited agent in exam questions regarding this specific adverse interaction. **3. NEET-PG High-Yield Pearls:** * **Systemic Absorption:** To minimize systemic side effects of eye drops, patients should be taught **nasolacrimal occlusion** (pressing the inner canthus) for 1–2 minutes after instillation. * **Drug of Choice:** For a glaucoma patient with asthma, **Betaxolol** is the preferred beta-blocker, or alternative classes like Prostaglandin analogues (Latanoprost) should be used. * **Contraindications for Timolol:** Asthma, COPD, Bradycardia, and Second/Third-degree heart block.

Question 92: Theophylline levels in blood are increased by which of the following?

A. Barbiturates
B. Methotrexate
C. Cimetidine (Correct Answer)
D. All of the above

Explanation: ### Explanation Theophylline is a methylxanthine with a **narrow therapeutic index**, meaning small changes in its serum concentration can lead to toxicity (seizures, arrhythmias). It is primarily metabolized in the liver by the **Cytochrome P450 (CYP1A2 and CYP3A4)** enzyme system. **1. Why Cimetidine is Correct:** Cimetidine is a potent **enzyme inhibitor**. It binds to the heme iron of the CYP450 system, reducing the metabolic clearance of theophylline. This leads to an increase in theophylline blood levels, significantly raising the risk of toxicity. **2. Why the Other Options are Incorrect:** * **Barbiturates (e.g., Phenobarbitone):** These are classic **enzyme inducers**. They increase the synthesis of CYP450 enzymes, thereby accelerating the metabolism of theophylline and **decreasing** its blood levels. * **Methotrexate:** While methotrexate has many drug interactions (especially with NSAIDs), it does not significantly inhibit the specific CYP enzymes responsible for theophylline metabolism. Therefore, it does not typically cause a rise in theophylline levels. **Clinical Pearls for NEET-PG:** * **The "G-PACMAN" Mnemonic for Enzyme Inhibitors:** **G**rapefruit juice, **P**rotease inhibitors, **A**zoles, **C**imetidine, **M**acrolides (except Azithromycin), **A**miodarone, **N**on-DHP CCBs (Verapamil/Diltiazem). These all can increase levels of drugs like theophylline, warfarin, and phenytoin. * **Smoking Effect:** Cigarette smoking **induces CYP1A2**, which *decreases* theophylline levels. If a patient stops smoking abruptly while on theophylline, their blood levels may rise to toxic levels. * **Alternative H2 Blocker:** If an H2 blocker is needed for a patient on theophylline, **Famotidine** or **Ranitidine** are preferred as they have minimal inhibitory effects on CYP450 compared to Cimetidine.

Question 93: Probenecid interacts with which of the following antibiotics?

A. Streptomycin
B. Ampicillin (Correct Answer)
C. Vancomycin
D. Erythromycin

Explanation: ### Explanation **Correct Answer: B. Ampicillin** **Mechanism of Interaction:** The interaction between Probenecid and Ampicillin is based on **renal tubular secretion**. Probenecid is a uricosuric agent that inhibits the **Organic Anion Transporter (OAT)** in the proximal convoluted tubule of the kidney. Many beta-lactam antibiotics, including **Penicillins (like Ampicillin)** and most Cephalosporins, are actively secreted into the renal tubule via these same OAT transporters. When co-administered, Probenecid competitively inhibits the secretion of Ampicillin, leading to: 1. **Decreased renal clearance** of the antibiotic. 2. **Increased plasma concentration** and a prolonged half-life. Clinically, this interaction is often used therapeutically to enhance the efficacy of penicillins in treating infections like neurosyphilis or pelvic inflammatory disease. **Analysis of Incorrect Options:** * **A. Streptomycin:** This is an aminoglycoside. Aminoglycosides are primarily excreted via **glomerular filtration**, not active tubular secretion; therefore, Probenecid does not significantly affect their levels. * **C. Vancomycin:** This glycopeptide is also primarily eliminated by **glomerular filtration**. It does not utilize the OAT system for secretion. * **D. Erythromycin:** This macrolide is primarily metabolized by the **liver** and excreted in the bile. Renal excretion plays a minor role in its clearance. **High-Yield Clinical Pearls for NEET-PG:** * **Therapeutic Use:** Probenecid is co-administered with **Cidofovir** (antiviral) to prevent nephrotoxicity by blocking its entry into tubular cells. * **Oseltamivir:** Probenecid also increases the plasma levels of Oseltamivir (Tamiflu) by inhibiting its renal secretion. * **Contraindication:** Probenecid should be avoided in patients taking **Methotrexate**, as it can lead to toxic levels of the drug by inhibiting its renal excretion. * **Uricosuric Action:** At low doses, salicylates (Aspirin) can block the uricosuric effect of Probenecid.

Question 94: Tacrolimus level is increased by all of the following drugs except?

A. Erythromycin
B. Itraconazole
C. Danazol
D. Rifampicin (Correct Answer)

Explanation: **Explanation:** The core concept behind this question is the **Cytochrome P450 (CYP3A4) enzyme system**. Tacrolimus, a potent calcineurin inhibitor used in organ transplantation, is primarily metabolized by the CYP3A4 isoenzyme in the liver and intestines. **1. Why Rifampicin is the correct answer:** Rifampicin is a classic, potent **enzyme inducer**. It increases the synthesis and activity of CYP3A4 enzymes. When co-administered with Tacrolimus, Rifampicin accelerates its metabolism, leading to **decreased** plasma levels of Tacrolimus. This can result in sub-therapeutic drug concentrations and an increased risk of graft rejection. **2. Why the other options are incorrect:** Options A, B, and C are all **enzyme inhibitors**: * **Erythromycin (Macrolide):** A well-known inhibitor of CYP3A4. * **Itraconazole (Azole antifungal):** One of the most potent inhibitors of the CYP3A4 system. * **Danazol (Androgen):** Also acts as a CYP3A4 inhibitor. These drugs decrease the metabolism of Tacrolimus, thereby **increasing** its serum concentration and increasing the risk of toxicity (e.g., nephrotoxicity, neurotoxicity). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Big" Inducers (Decrease Tacrolimus levels):** Rifampicin, Phenytoin, Carbamazepine, Phenobarbitone, and St. John’s Wort. * **The "Big" Inhibitors (Increase Tacrolimus levels):** Ketoconazole/Itraconazole, Erythromycin/Clarithromycin, Verapamil/Diltiazem, and **Grapefruit juice**. * **Monitoring:** Because Tacrolimus has a narrow therapeutic index, Therapeutic Drug Monitoring (TDM) is mandatory when starting or stopping any of the above medications. * **Cyclosporine** follows the same metabolic pathway and interaction profile as Tacrolimus.

Question 95: All of the following drugs can precipitate Tacrolimus toxicity, except:

A. Gentamicin
B. Cisplatin
C. Vancomycin
D. Rifampicin (Correct Answer)

Explanation: **Explanation:** The core concept in this question involves distinguishing between **pharmacokinetic** and **pharmacodynamic** drug interactions. **Why Rifampicin is the correct answer:** Tacrolimus is a calcineurin inhibitor primarily metabolized by the hepatic enzyme **CYP3A4**. **Rifampicin** is a potent **CYP450 inducer**. When co-administered, Rifampicin accelerates the metabolism of Tacrolimus, leading to *decreased* plasma levels and potential graft rejection, rather than toxicity. To cause toxicity, a drug would need to be a CYP3A4 inhibitor (e.g., Ketoconazole, Erythromycin, or Grapefruit juice). **Why the other options are incorrect:** Options A, B, and C (Gentamicin, Cisplatin, and Vancomycin) do not necessarily increase the blood concentration of Tacrolimus, but they precipitate toxicity through **additive pharmacodynamic effects**. * **Tacrolimus** is inherently **nephrotoxic** (causing afferent arteriolar vasoconstriction). * **Gentamicin (Aminoglycoside)**, **Cisplatin**, and **Vancomycin** are also well-known nephrotoxins. * Co-administration of these agents with Tacrolimus significantly increases the risk of acute kidney injury (AKI), thereby "precipitating" clinical toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Tacrolimus Toxicity Profile:** Nephrotoxicity (most common), Neurotoxicity (tremors, seizures), and New-onset Diabetes After Transplantation (NODAT). * **Drug of Choice:** Tacrolimus is preferred over Cyclosporine because it has a lower incidence of hirsutism and gum hyperplasia. * **Monitoring:** Therapeutic Drug Monitoring (TDM) is mandatory for Tacrolimus due to its narrow therapeutic index. * **Rule of Thumb:** Enzyme *Inducers* (Rifampicin, Phenytoin, Carbamazepine) decrease levels; Enzyme *Inhibitors* (Macrolides, Azoles) increase levels.

Question 96: A 60-year-old patient with chronic liver disease and rheumatoid arthritis is to be treated with procainamide. He is already taking digoxin, hydrochlorothiazide, and potassium supplementation. Which of the following statements is relevant to this patient's treatment?

A. A possible drug interaction with digoxin suggests that digoxin blood levels should be obtained before and after starting procainamide.
B. Hyperkalemia should be avoided to reduce the likelihood of procainamide toxicity. (Correct Answer)
C. Procainamide cannot be used if the patient has asthma because it has a beta-blocking effect.
D. Procainamide is not active by the oral route.

Explanation: **Explanation** **1. Why Option B is Correct:** Procainamide is a **Class IA antiarrhythmic** drug that works primarily by blocking voltage-gated sodium channels and, to a lesser extent, potassium channels. The efficacy and toxicity of Class I antiarrhythmics are significantly influenced by extracellular potassium levels. **Hyperkalemia** increases the resting membrane potential (making it less negative) and enhances the sodium-channel-blocking effect of procainamide. This can lead to excessive slowing of conduction, potentially causing severe arrhythmias or cardiac arrest. Since this patient is on potassium supplements and a diuretic (HCTZ), careful monitoring is essential to avoid hyperkalemia-induced toxicity. **2. Why the Other Options are Incorrect:** * **Option A:** Unlike Quinidine (another Class IA drug), **Procainamide does not significantly increase digoxin levels.** Quinidine reduces the renal clearance of digoxin, but procainamide does not share this pharmacokinetic interaction. * **Option C:** Procainamide does **not** possess beta-blocking activity. It has mild ganglion-blocking properties but is not contraindicated in asthma. (Note: Propranolol or Sotalol would be concerns in asthmatics). * **Option D:** Procainamide is well-absorbed and **highly active via the oral route**, with a bioavailability of approximately 75-85%. **3. NEET-PG High-Yield Pearls:** * **Metabolism:** Procainamide is acetylated in the liver by **N-acetyltransferase** to **N-acetylprocainamide (NAPA)**. NAPA is an active metabolite with Class III properties. * **Side Effect:** Long-term use is famously associated with **Drug-Induced Lupus Erythematosus (DILE)**, especially in "slow acetylators." Unlike systemic lupus, DILE usually spares the kidneys and reverses upon drug discontinuation. * **ECG Changes:** Class IA drugs typically increase the QRS duration and the QT interval.

Question 97: Antiepileptic effect of phenytoin is increased by all of the following except:

A. Isoniazid
B. Sucralfate (Correct Answer)
C. Cimetidine
D. Warfarin

Explanation: ### Explanation The antiepileptic effect of Phenytoin is directly related to its plasma concentration. Since Phenytoin has a narrow therapeutic index and follows zero-order kinetics at high doses, drugs that interfere with its metabolism or absorption significantly impact its clinical efficacy and toxicity. **Why Sucralfate is the Correct Answer:** Sucralfate is an aluminum-containing mucosal protective agent. It acts locally in the GI tract and can **decrease the absorption** of Phenytoin by binding to it or physically hindering its uptake. Reduced absorption leads to lower plasma levels, thereby **decreasing** (not increasing) the antiepileptic effect. **Analysis of Incorrect Options:** * **Isoniazid (INH):** It is a potent microsomal enzyme inhibitor. It inhibits the metabolism of Phenytoin, leading to increased plasma levels and enhanced antiepileptic effects (and potential toxicity). * **Cimetidine:** A well-known H2-receptor blocker that inhibits Cytochrome P450 enzymes (specifically CYP2C9 and CYP3A4). This reduces Phenytoin clearance, increasing its effect. * **Warfarin:** This interaction is complex. While both drugs compete for protein binding (displacement), Warfarin can also inhibit the metabolism of Phenytoin, leading to an initial increase in Phenytoin levels. **Clinical Pearls for NEET-PG:** * **Phenytoin Metabolism:** Primarily metabolized by **CYP2C9** and **CYP2C19**. Any inhibitor of these enzymes will increase Phenytoin levels. * **Zero-Order Kinetics:** Phenytoin shifts from first-order to zero-order kinetics within the therapeutic range (Capacity-limited elimination). Small dose increases can lead to disproportionately large increases in plasma concentration. * **Absorption Rule:** To avoid the interaction with Sucralfate or Antacids, Phenytoin should be administered at least **2 hours apart** from these medications. * **Enzyme Inducers:** Remember that Phenytoin itself is a potent enzyme inducer, often reducing the efficacy of drugs like Oral Contraceptive Pills (OCPs) and Steroids.

Question 98: Oral contraceptive pills fail when used with the following drugs, EXCEPT:

A. Aspirin (Correct Answer)
B. Phenytoin
C. Rifampin
D. Tetracycline

Explanation: **Explanation:** The failure of Oral Contraceptive Pills (OCPs) occurs when their plasma concentration falls below the therapeutic threshold, usually due to increased metabolism or decreased absorption. **1. Why Aspirin is the Correct Answer:** Aspirin is a non-steroidal anti-inflammatory drug (NSAID) that does not significantly induce hepatic enzymes or interfere with the enterohepatic circulation of estrogens. Therefore, it has no clinically significant interaction with OCPs and does not reduce their contraceptive efficacy. **2. Why the Other Options are Wrong:** * **Rifampin (Option C):** This is the most potent inducer of the **CYP3A4** enzyme. It significantly increases the metabolism of estrogen and progesterone, leading to a high risk of contraceptive failure. It is a classic "high-yield" example of this interaction. * **Phenytoin (Option B):** An anti-epileptic drug that acts as a potent microsomal enzyme inducer. Like Rifampin, it accelerates the breakdown of OCP components. Other similar drugs include Carbamazepine and Phenobarbitone. * **Tetracycline (Option D):** Broad-spectrum antibiotics can disrupt the normal intestinal flora. These bacteria are responsible for the hydrolysis of estrogen conjugates, which allows for the reabsorption of free estrogen (enterohepatic circulation). By killing these bacteria, tetracyclines decrease estrogen levels. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Inducers (Failure):** Rifampin, Phenytoin, Carbamazepine, Griseofulvin, and Chronic Alcoholism. * **Antibiotics (Failure):** Tetracyclines, Ampicillin (via disruption of enterohepatic circulation). * **Management:** Patients on enzyme-inducing drugs should be advised to use an alternative or additional method of contraception (e.g., barrier methods) or a higher dose of estrogen (though the latter is less preferred). * **Note:** Rifampin is the only antibiotic proven to consistently lower OCP levels via enzyme induction; for others, the clinical significance is often debated but still tested in exams.

Question 99: Oral contraceptives are not given with which of the following medications?

A. Streptomycin
B. Grisiofulvin (Correct Answer)
C. Pyraziniamide
D. Ethambutol

Explanation: **Explanation:** The correct answer is **Griseofulvin**. **1. Why Griseofulvin is correct:** Oral contraceptive pills (OCPs) are primarily metabolized in the liver by the **Cytochrome P450 (CYP450)** enzyme system. Griseofulvin is a potent **microsomal enzyme inducer**. When co-administered, it increases the rate of metabolism of estrogen and progesterone components of the OCP, leading to decreased plasma concentrations of these hormones. This results in **contraceptive failure** and an increased risk of unintended pregnancy. **2. Why other options are incorrect:** * **Streptomycin:** This is an aminoglycoside that is not metabolized by the liver and does not induce or inhibit CYP450 enzymes. * **Pyrazinamide and Ethambutol:** These are first-line antitubercular drugs. Unlike Rifampicin (which is a powerful enzyme inducer and a classic cause of OCP failure), Pyrazinamide and Ethambutol do not significantly affect the hepatic metabolism of other drugs. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Rifampicin Rule":** Among antitubercular drugs, **Rifampicin** is the most notorious enzyme inducer causing OCP failure. * **Other Enzyme Inducers (Mnemonic: GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. All these can decrease OCP efficacy. * **Antibiotic Interaction:** Broad-spectrum antibiotics (like Ampicillin or Tetracycline) were traditionally thought to decrease OCP efficacy by interfering with **enterohepatic circulation** (killing gut flora that deconjugate estrogens), though clinical evidence for this is less robust than for enzyme inducers. * **Management:** Patients on enzyme-inducing drugs should be advised to use an alternative or additional method of contraception (e.g., barrier methods).

Question 100: Which of the following drugs interacts with cefotaxime?

A. Digoxin
B. Paracetamol
C. Loop diuretics (Correct Answer)
D. Nifedipine

Explanation: ### Explanation **Correct Option: C. Loop diuretics** The interaction between **Cefotaxime** (a 3rd generation cephalosporin) and **Loop diuretics** (like Furosemide) is a classic example of additive **nephrotoxicity**. **Mechanism:** While cephalosporins are generally safe, they are primarily excreted via the kidneys. Loop diuretics can cause dehydration and reduce renal blood flow, which decreases the clearance of cephalosporins [2]. This leads to higher concentrations of the antibiotic in the renal tubules, increasing the risk of acute tubular necrosis. This interaction is particularly significant in elderly patients or those with pre-existing renal impairment [1]. **Analysis of Incorrect Options:** * **A. Digoxin:** Digoxin primarily interacts with drugs that affect potassium levels (like diuretics) [2] or P-glycoprotein inhibitors (like Verapamil/Amiodarone). It does not have a clinically significant interaction with cefotaxime. * **B. Paracetamol:** This is a safe analgesic/antipyretic frequently co-administered with antibiotics. There is no documented pharmacokinetic or pharmacodynamic interaction between the two. * **D. Nifedipine:** As a Calcium Channel Blocker, its interactions usually involve CYP3A4 inhibitors/inducers. It does not interfere with the renal excretion or toxicity profile of cefotaxime. **High-Yield Clinical Pearls for NEET-PG:** * **Cephalosporin + Aminoglycosides:** This combination also significantly increases the risk of nephrotoxicity. * **Disulfiram-like Reaction:** Remember that certain cephalosporins (Cefoperazone, Cefotetan) contain a **Methylthiotetrazole (MTT) side chain** which causes a reaction with alcohol. * **Probenecid Interaction:** Probenecid inhibits the renal tubular secretion of most cephalosporins, leading to increased and prolonged plasma levels (often used therapeutically to extend drug action).

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Clinically Significant Drug Interactions

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Management of Drug Interactions

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Role of P-glycoprotein in Drug Interactions

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