Drug Interactions Practice Questions

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

Rifampicin. **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.

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

Smoking. **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.

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

Topiramate. ### 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.

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

Ketoconazole. **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.

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

Polymyxin B. ### 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.

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

All of the above. **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).

Q: Which of the following interacts with Digitalis maximally?

Furosemide. 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**.

Question 1

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

Accepted Answer

Rifampicin

Answer

Ethambutol

Answer

Streptomycin

Answer

Pyrazinamide

Question 2

The action of theophylline is reduced by which of the following?

Accepted Answer

Smoking

Answer

Erythromycin

Answer

Cimetidine

Answer

Lithium

Question 3

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

Accepted Answer

Topiramate

Answer

Valproic acid

Answer

Carbamazepine

Answer

Gabapentin

Question 4

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

Accepted Answer

Ketoconazole

Answer

Phenobarbitone

Answer

Rifampicin

Answer

Carbamazepine

Question 5

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

Accepted Answer

Cisapride

Answer

Itopride

Answer

Metoclopramide

Answer

Domperidone

Question 6

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

Accepted Answer

Polymyxin B

Answer

Clofibrate

Answer

Penicillin

Answer

All of the above

Question 7

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

Accepted Answer

All of the above

Answer

Indomethacin

Answer

Aspirin

Answer

Trimethoprim

Question 8

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

Accepted Answer

Serum lithium levels rise

Answer

Serum lithium levels decrease

Answer

Serum hydrochlorothiazide levels rise

Answer

Serum hydrochlorothiazide levels fall

Question 9

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

Accepted Answer

Conduction block

Answer

Bronchospasm

Answer

Neurogenic shock

Answer

Anaphylaxis

Question 10

Which of the following interacts with Digitalis maximally?

Accepted Answer

Furosemide

Answer

Triamterene

Answer

Amiloride

Answer

Spironolactone

Drug Interactions — MCQs

Drug Interactions — MCQs

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