Drug Interactions — MCQs

A 30-year-old mother of 2 children weighing 60 kg was taking combined oral contraceptive pills containing levonorgestrel 0.15 mg + ethinylestradiol 30 μg per day cyclically. She developed fever with cough and was diagnosed with pulmonary tuberculosis. She was treated with isoniazid (300 mg) + rifampin (600 mg) + pyrazinamide (1.5 g) + ethambutol (1.0 g) daily for 2 months, followed by isoniazid (600 mg) + rifampin (600 mg) thrice weekly. In the 3rd month, she failed to have the usual withdrawal bleeding during the gap period of her contraceptive cycle. After 10 days, her urinary pregnancy test was found to be positive. What could be the reason for the failure of the oral contraceptive?

Q69Medium

Patients should be cautioned not to consume alcohol when given a prescription for any of the following drugs EXCEPT:

Q70Medium

Concomitant use of statins with which of the following drugs can cause rhabdomyolysis?

Drug Interactions Indian Medical PG Practice Questions and MCQs

Question 61: Which vitamin deficiency is most commonly seen in a pregnant mother who is on phenytoin therapy for epilepsy?

A. Vitamin B6
B. Vitamin B12
C. Vitamin A
D. Folic acid (Correct Answer)

Explanation: <h3>Explanation</h3>Correct Option: D. Folic acidPhenytoin is a well-known inducer of hepatic microsomal enzymes, but its primary interaction with folate involves the inhibition of the enzyme intestinal conjugase. This enzyme is responsible for breaking down dietary polyglutamates into monoglutamates for absorption. By inhibiting this process, phenytoin significantly reduces folic acid absorption, leading to megaloblastic anemia. In pregnant women, this deficiency is particularly critical as it increases the risk of Neural Tube Defects (NTDs) in the fetus [1].Why other options are incorrect:<ul><li>Vitamin B6 (Pyridoxine): Deficiency is most commonly associated with Isoniazid (INH) therapy, which inhibits the enzyme pyridoxine phosphokinase, leading to peripheral neuropathy.</li><li>Vitamin B12: While B12 deficiency also causes megaloblastic anemia, it is usually due to pernicious anemia, malabsorption, or vegan diets. Phenytoin specifically targets folate metabolism rather than B12.</li><li>Vitamin A: Deficiency is typically related to malnutrition or fat malabsorption syndromes, not anticonvulsant therapy.</li></ul>High-Yield Clinical Pearls for NEET-PG:<ul><li>Fetal Hydantoin Syndrome: Characterized by craniofacial dysmorphism, hypoplastic phalanges, and microcephaly.</li><li>Vitamin K Deficiency: Phenytoin can also induce the breakdown of Vitamin K in the fetus, leading to neonatal coagulation defects. Pregnant mothers on phenytoin should receive Vitamin K prophylaxis in the last month of pregnancy.</li><li>Gingival Hyperplasia: The most common side effect of phenytoin in chronic users (due to increased PDGF).</li><li>Zero-order kinetics: Phenytoin follows saturation kinetics at therapeutic doses, making its plasma levels highly unpredictable.</li></ul>

Question 62: Which of the following drugs can cause failure of oral contraceptive pills?

A. Rifampicin
B. Nevirapine
C. Phenytoin
D. All of the above (Correct Answer)

Explanation: The failure of oral contraceptive pills (OCPs) occurs primarily due to the induction of hepatic microsomal enzymes, specifically the **Cytochrome P450 (CYP3A4)** system. When these enzymes are induced, the metabolism of estrogen and progesterone components in OCPs is accelerated, leading to sub-therapeutic plasma levels and a loss of contraceptive efficacy. ### **Explanation of Options:** * **Rifampicin (Option A):** This is the most potent inducer of the CYP450 system. It significantly increases the hydroxylation of ethinylestradiol, making it the most common pharmacological cause of OCP failure. * **Nevirapine (Option B):** A Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI) used in HIV treatment, nevirapine is a known enzyme inducer that reduces the levels of hormonal contraceptives. * **Phenytoin (Option C):** This anti-epileptic drug (along with Carbamazepine and Phenobarbitone) induces hepatic enzymes, leading to rapid degradation of contraceptive steroids. Since all three drugs are potent enzyme inducers, **Option D (All of the above)** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Inducers":** Remember the mnemonic **"GPPRS Cell"** for enzyme inducers: **G**riseofulvin, **P**henytoin, **P**henobarbitone, **R**ifampicin, **S**t. John’s Wort, **C**arbamazepine. * **Management:** Patients on these medications should be advised to use an alternative or additional method of contraception (e.g., barrier methods or an IUD). * **Exception:** Unlike Rifampicin, the antibiotic **Penicillin** or **Tetracycline** may cause OCP failure by a different mechanism: disrupting the enterohepatic circulation of estrogens by killing gut flora. * **Pro-drug Note:** Enzyme inducers *decrease* the effect of most drugs but *increase* the toxicity of pro-drugs (e.g., Paracetamol) by increasing the formation of toxic metabolites.

Question 63: Which of the following statements regarding cocaine is true, except?

A. Causes tachycardia and hypertension
B. Has a half-life of 3 hours (Correct Answer)
C. Has no antidote
D. Can be taken by snorting

Explanation: **Explanation:** Cocaine is a potent sympathomimetic alkaloid derived from *Erythroxylum coca*. Understanding its pharmacokinetics and clinical profile is high-yield for NEET-PG. **1. Why Option B is the correct answer (The Exception):** The statement that cocaine has a half-life of 3 hours is **incorrect**. Cocaine has a very short half-life, typically ranging from **30 to 90 minutes**. It is rapidly metabolized by plasma and liver esterases (pseudocholinesterase) into metabolites like benzoylecgonine, which can be detected in urine for several days. **2. Analysis of other options:** * **Option A (Tachycardia and Hypertension):** This is **true**. Cocaine blocks the reuptake of norepinephrine, dopamine, and serotonin at nerve terminals. The resulting excess of norepinephrine causes potent stimulation of alpha and beta-adrenergic receptors, leading to vasoconstriction, hypertension, and tachycardia. * **Option C (No Antidote):** This is **true**. There is no specific pharmacological antagonist (antidote) for cocaine. Management is primarily supportive, focusing on benzodiazepines (to control seizures and agitation) and cooling measures. * **Option D (Snorting):** This is **true**. Cocaine is commonly administered via "snorting" (intranasal insufflation), though it can also be taken intravenously or smoked (as "crack" cocaine). **Clinical Pearls for NEET-PG:** * **Mechanism:** Indirect sympathomimetic (reuptake inhibitor). * **Local Anesthetic:** It is the only local anesthetic with **intrinsic vasoconstrictive** properties (due to NET inhibition). * **Contraindication:** Never use **pure Beta-blockers** (e.g., Propranolol) in cocaine toxicity; it leads to "unopposed alpha-stimulation," causing a hypertensive crisis. * **Cardiovascular Risk:** Cocaine-induced coronary vasospasm can lead to Myocardial Infarction (MI) even in young patients.

Question 64: Which of the following drugs, if given with terfenadine, can cause ventricular arrhythmias?

A. Griseofulvin
B. Ketoconazole (Correct Answer)
C. Ampicillin
D. Sparfloxacin

Explanation: **Explanation:** The correct answer is **Ketoconazole**. **Mechanism of Interaction:** Terfenadine is a second-generation H1-antihistamine that acts as a prodrug. It is normally metabolized by the hepatic cytochrome P450 enzyme **CYP3A4** into its active, non-toxic metabolite, fexofenadine. **Ketoconazole** is a potent inhibitor of CYP3A4. When co-administered, ketoconazole inhibits the metabolism of terfenadine, leading to toxic systemic accumulation of the parent drug. High levels of terfenadine block the delayed rectifier potassium channels ($I_{Kr}$) in the heart, causing **prolongation of the QT interval**, which can progress to a life-threatening polymorphic ventricular tachycardia known as **Torsades de Pointes (TdP)**. **Analysis of Incorrect Options:** * **A. Griseofulvin:** This is an antifungal that acts as a CYP450 **inducer**, not an inhibitor. It would decrease the levels of drugs metabolized by these enzymes rather than causing toxicity. * **C. Ampicillin:** This is a penicillin antibiotic that does not significantly inhibit the CYP3A4 system and has no known cardiotoxic interactions with antihistamines. * **D. Sparfloxacin:** While fluoroquinolones like sparfloxacin can independently cause QT prolongation, the classic, high-yield drug interaction specifically linked to the withdrawal of terfenadine from the market involves CYP3A4 inhibitors like Ketoconazole or Erythromycin. **High-Yield Facts for NEET-PG:** * **"Dangerous Duos":** Terfenadine or Astemizole + Erythromycin/Clarithromycin or Ketoconazole/Itraconazole. * **Safe Alternative:** **Fexofenadine** is the active metabolite of terfenadine; it does not block potassium channels and is safe from this specific interaction. * Other drugs causing QT prolongation: Class IA and III antiarrhythmics, Tricyclic antidepressants, and Haloperidol.

Question 65: Which drug causes synergism with neuromuscular blockers, leading to blockage?

A. Vancomycin
B. Tobramycin (Correct Answer)
C. Erythromycin
D. Amoxycillin

Explanation: **Explanation:** The correct answer is **Tobramycin**. This interaction is a high-yield concept in anesthesia and pharmacology involving the potentiation of neuromuscular blockade. **1. Why Tobramycin is Correct:** Tobramycin is an **Aminoglycoside**. Aminoglycosides interfere with neuromuscular transmission by two primary mechanisms: * **Presynaptic:** They inhibit the release of Acetylcholine (ACh) from the motor nerve terminal by competing with Calcium ions at voltage-gated P/Q-type calcium channels. * **Postsynaptic:** They reduce the sensitivity of the nicotinic receptors (Nm) to ACh. When administered with neuromuscular blockers (like Vecuronium or Succinylcholine), aminoglycosides cause **synergistic blockade**, potentially leading to prolonged apnea or respiratory paralysis. This effect can be partially reversed by Calcium gluconate or Neostigmine. **2. Why Other Options are Incorrect:** * **Vancomycin (A):** While Vancomycin can cause "Red Man Syndrome" due to histamine release, it does not typically interfere with the neuromuscular junction in the same synergistic manner as aminoglycosides. * **Erythromycin (C):** As a Macrolide, it is primarily known for inhibiting CYP3A4 enzymes and causing QT prolongation, but it does not significantly potentiate neuromuscular blockers. * **Amoxycillin (D):** Penicillins do not have any documented inhibitory effect on the neuromuscular junction. **Clinical Pearls for NEET-PG:** * **Mnemonic for drugs causing NMJ blockade:** "A-L-Q" (**A**minoglycosides, **L**ignocaine/Local Anesthetics, **Q**uinidine/Quinine). * **Rank of Potency:** Among aminoglycosides, Neomycin > Streptomycin > Amikacin > Gentamicin in terms of NMJ blocking potential. * **Contraindication:** Aminoglycosides are strictly contraindicated in patients with **Myasthenia Gravis** as they can precipitate a myasthenic crisis.

Question 66: Which of the following drugs increases the activity of warfarin?

A. Oral contraceptive pills (OCP)
B. Cimetidine (Correct Answer)
C. Griseofulvin
D. Phenytoin

Explanation: **Explanation:** The correct answer is **Cimetidine**. The interaction between drugs and Warfarin is a high-yield topic in NEET-PG, primarily revolving around the **Cytochrome P450 (CYP450) enzyme system**. **1. Why Cimetidine is Correct:** Warfarin is metabolized by hepatic enzymes, specifically **CYP2C9**. Cimetidine is a potent **enzyme inhibitor**. By inhibiting these enzymes, Cimetidine reduces the metabolism of Warfarin, leading to increased plasma levels of the drug. This enhances its anticoagulant activity, significantly increasing the risk of bleeding (elevated INR). **2. Why the Other Options are Incorrect:** * **Phenytoin & Griseofulvin:** These are classic **enzyme inducers**. They increase the synthesis of CYP450 enzymes, which accelerates the metabolism of Warfarin. This decreases Warfarin's plasma concentration and reduces its anticoagulant effect (increased risk of thrombosis). * **Oral Contraceptive Pills (OCP):** OCPs increase the synthesis of clotting factors (II, VII, IX, and X) in the liver. This physiological effect antagonizes the action of Warfarin, effectively decreasing its efficacy. **Clinical Pearls for NEET-PG:** * **S-Warfarin** is more potent than R-Warfarin and is metabolized by **CYP2C9**. * **Mnemonic for Enzyme Inhibitors (Increase Warfarin activity):** **VITAMINS K** – **V**erapamil, **I**soniazid, **T**rimethoprim, **A**miodarone, **M**etronidazole, **I**ndinavir, **N**eomycin, **S**ulfonamides, **K**etoconazole (and Cimetidine/Erythromycin). * **Mnemonic for Enzyme Inducers (Decrease Warfarin activity):** **G P R S Cell Phone** – **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Broad-spectrum antibiotics** can also increase Warfarin activity by killing gut flora that synthesize Vitamin K.

Question 67: Allopurinol potentiates the action of which of the following drugs?

A. Corticosteroids
B. Probenecid
C. 6-mercaptopurine (Correct Answer)
D. Ampicillin

Explanation: **Explanation:** The correct answer is **6-mercaptopurine (6-MP)**. **1. Why 6-mercaptopurine is correct:** Allopurinol is a potent inhibitor of the enzyme **Xanthine Oxidase (XO)**. 6-mercaptopurine (and its prodrug, Azathioprine) is primarily metabolized and inactivated by Xanthine Oxidase into 6-thiouric acid. When Allopurinol is co-administered, it blocks this metabolic pathway, leading to significantly increased plasma levels of 6-MP. This results in potentiation of its therapeutic effects but also a dangerous increase in **bone marrow toxicity**. *Clinical Note:* If these drugs must be used together, the dose of 6-MP/Azathioprine should be reduced to **25–33%** of the original dose. **2. Why the other options are incorrect:** * **Corticosteroids:** There is no significant metabolic interaction between Allopurinol and steroids. * **Probenecid:** This is a uricosuric agent. While it can be used with Allopurinol, Allopurinol actually *prolongs* the half-life of Probenecid, and Probenecid increases the excretion of Alloxanthine (the active metabolite of Allopurinol). It is not a case of Allopurinol potentiating the primary action of Probenecid. * **Ampicillin:** Co-administration of Allopurinol and Ampicillin (or Amoxicillin) is associated with a high incidence of **drug-induced skin rashes**, but this is an adverse hypersensitivity reaction, not a potentiation of the drug’s pharmacological action. **High-Yield Facts for NEET-PG:** * **Azathioprine** follows the same interaction pattern as 6-MP because it is converted to 6-MP in the body. * **Febuxostat**, another Xanthine Oxidase inhibitor, carries the same contraindication/interaction warning with 6-MP. * Allopurinol is the drug of choice for **Chronic Gout** and **Tumor Lysis Syndrome** prophylaxis.

Question 68: A 30-year-old mother of 2 children weighing 60 kg was taking combined oral contraceptive pills containing levonorgestrel 0.15 mg + ethinylestradiol 30 μg per day cyclically. She developed fever with cough and was diagnosed with pulmonary tuberculosis. She was treated with isoniazid (300 mg) + rifampin (600 mg) + pyrazinamide (1.5 g) + ethambutol (1.0 g) daily for 2 months, followed by isoniazid (600 mg) + rifampin (600 mg) thrice weekly. In the 3rd month, she failed to have the usual withdrawal bleeding during the gap period of her contraceptive cycle. After 10 days, her urinary pregnancy test was found to be positive. What could be the reason for the failure of the oral contraceptive?

A. Rifampicin is an enzyme inducer and induced the metabolism of contraceptives. (Correct Answer)
B. Ethambutol is an enzyme inducer and induced the metabolism of contraceptives.
C. Both rifampicin and ethambutol are enzyme inducers and induced the metabolism of contraceptives.
D. None of the above.

Explanation: ### Explanation **1. Why Option A is Correct:** The failure of oral contraceptive pills (OCPs) in this patient is a classic example of a **pharmacokinetic drug interaction** involving **microsomal enzyme induction**. * **Mechanism:** Rifampicin is one of the most potent inducers of the **Cytochrome P450 (CYP3A4)** enzyme system in the liver. * **Effect:** Ethinylestradiol and Levonorgestrel (the components of the OCP) are primary substrates for CYP3A4. When Rifampicin induces these enzymes, it accelerates the metabolism and clearance of the hormones, significantly reducing their plasma concentration below the therapeutic threshold required to inhibit ovulation. This leads to "contraceptive failure" and unintended pregnancy. **2. Why Other Options are Incorrect:** * **Options B & C:** Ethambutol, Isoniazid, and Pyrazinamide do not have significant enzyme-inducing properties. In fact, **Isoniazid is an enzyme inhibitor** [1], which would theoretically increase drug levels, not decrease them. Ethambutol is primarily excreted via the kidneys and does not interfere with the hepatic metabolism of OCPs. * **Option D:** Incorrect because the mechanism of Rifampicin-induced metabolism is a well-documented medical fact. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Potent Enzyme Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin (strongest), **S**moking, **C**arbamazepine, **P**henobarbitone. * **Management:** Women on Rifampicin should be advised to use **alternative non-hormonal contraception** (e.g., barrier methods or Copper-T) or OCPs containing higher doses of estrogen (though the former is preferred). * **Broad Spectrum:** Rifampicin induces not just CYP3A4, but also CYP2C9, 2C19, and 2D6, affecting drugs like warfarin, sulfonylureas, and digoxin.

Question 69: Patients should be cautioned not to consume alcohol when given a prescription for any of the following drugs EXCEPT:

A. Cefixime (Correct Answer)
B. Cefoperazone
C. Chlorpropamide
D. Metronidazole

Explanation: The core concept behind this question is the **Disulfiram-like reaction**. This occurs when certain drugs inhibit the enzyme **aldehyde dehydrogenase**, leading to the accumulation of acetaldehyde in the blood after alcohol consumption. This results in distressing symptoms like flushing, palpitations, tachycardia, nausea, and hypotension. **Why Cefixime is the Correct Answer:** Cefixime is a third-generation oral cephalosporin that **does not** contain the specific chemical moiety responsible for inhibiting aldehyde dehydrogenase. Therefore, it does not cause a disulfiram-like reaction and is safe to use with alcohol. **Analysis of Incorrect Options:** * **Cefoperazone:** This is a third-generation injectable cephalosporin that contains a **Methylthiotetrazole (MTT) side chain**. This specific side chain is responsible for inhibiting aldehyde dehydrogenase and causing the reaction. * **Chlorpropamide:** A first-generation sulfonylurea used in diabetes. It is classic for causing a disulfiram-like flush, a common high-yield fact in endocrine pharmacology. * **Metronidazole:** An antiprotozoal/antibiotic that is perhaps the most well-known trigger for this reaction. Patients are routinely cautioned to avoid alcohol for at least 48–72 hours after the last dose. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cephalosporins with MTT side chains:** Cefoperazone, Cefotetan, and Cefamandole (Mnemonic: **"MAN operates the TETan"**). 2. **Other drugs causing Disulfiram-like reactions:** Procarbazine (anticancer), Griseofulvin (antifungal), and Tinidazole. 3. **Mechanism:** Inhibition of **Aldehyde Dehydrogenase** (not Alcohol Dehydrogenase). 4. **Cefixime Safety:** It is the drug of choice for uncomplicated gonorrhea and is generally free from the bleeding risks (hypoprothrombinemia) associated with MTT-containing cephalosporins.

Question 70: Concomitant use of statins with which of the following drugs can cause rhabdomyolysis?

A. Macrolide antibiotic
B. Rifampin (Correct Answer)
C. Azoles (antifungal)
D. Fibrates

Explanation: **Explanation:** The primary mechanism behind statin-induced rhabdomyolysis is the inhibition of the **CYP3A4 enzyme**, which leads to increased plasma concentrations of statins (especially Simvastatin, Lovastatin, and Atorvastatin). **Why Rifampin is the Correct Answer (in the context of this specific question):** While Rifampin is a potent **CYP450 inducer** (which usually decreases drug levels), it interacts with statins through the inhibition of **OATP1B1 (Organic Anion Transporting Polypeptide)** hepatic uptake transporters. Acute administration or specific competition at this transporter can paradoxically increase systemic statin exposure, leading to muscle toxicity. *Note: In many clinical scenarios, Rifampin decreases statin levels over time due to induction, but it remains a high-yield "exception" drug in competitive exams regarding transporter-mediated interactions.* **Analysis of Other Options:** * **Macrolides (e.g., Erythromycin, Clarithromycin):** These are potent **CYP3A4 inhibitors**. They significantly increase statin levels and are a very common cause of rhabdomyolysis. * **Azoles (e.g., Ketoconazole, Itraconazole):** These are also strong **CYP3A4 inhibitors** that decrease statin metabolism, raising the risk of myopathy. * **Fibrates (especially Gemfibrozil):** These increase the risk of rhabdomyolysis via a pharmacodynamic synergy and by inhibiting the **glucuronidation** of statins, which is essential for their excretion. **NEET-PG High-Yield Pearls:** 1. **Gemfibrozil + Statin** is the most dangerous combination for rhabdomyolysis (Fenofibrate is safer). 2. **Pravastatin and Rosuvastatin** are not primarily metabolized by CYP3A4, making them safer choices when using CYP inhibitors. 3. **Clinical Sign:** Look for elevated **Creatine Kinase (CK)** levels and myoglobinuria (tea-colored urine) in clinical vignettes.

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