Drug Interactions Practice Questions

Q: If carbamazepine is added to valproate, which side effect of valproate can increase?

Hepatotoxicity. **Explanation:** The correct answer is **Hepatotoxicity**. **Mechanism of Interaction:** Valproate is primarily metabolized in the liver. When **Carbamazepine** (a potent **Cytochrome P450 enzyme inducer**) is added, it accelerates the metabolism of valproate. This induction shifts the metabolic pathway toward the production of **4-pentenoic acid** (4-en-VPA) and other reactive unsaturated metabolites. These metabolites are highly hepatotoxic and are responsible for the microvesicular steatosis and liver injury associated with valproate therapy. Therefore, enzyme-inducing drugs like carbamazepine or phenytoin significantly increase the risk of valproate-induced liver damage. **Analysis of Incorrect Options:** * **A. Thrombocytopenia:** This is a dose-related side effect of valproate due to bone marrow suppression or peripheral destruction of platelets. It is not specifically potentiated by enzyme induction. * **C. Hyperammonemia:** Valproate causes hyperammonemia by interfering with the urea cycle (inhibiting carbamoyl phosphate synthetase I). While common, its risk is more closely linked to valproate dosage and carnitine deficiency rather than carbamazepine co-administration. * **D. Pancreatitis:** This is an idiosyncratic (unpredictable) reaction to valproate. It is not known to be exacerbated by the metabolic induction caused by carbamazepine. **High-Yield Clinical Pearls for NEET-PG:** * **Valproate's "Black Box" Warnings:** Hepatotoxicity (especially in children <2 years), Pancreatitis, and Teratogenicity (Neural tube defects). * **Enzyme Status:** Valproate is an **Enzyme Inhibitor**, whereas Carbamazepine is an **Enzyme Inducer** (and an auto-inducer). * **Drug of Choice:** Valproate remains the drug of choice for generalized seizures (Myoclonic, Absence, and Tonic-Clonic). * **Monitoring:** Liver Function Tests (LFTs) are mandatory during the first 6 months of valproate therapy, especially if used in polytherapy.

Q: All of the following can result in serotonin syndrome when combined with MAO inhibitors, except?

Tyramine containing foods. The core of this question lies in distinguishing between **Serotonin Syndrome** and the **Cheese Reaction (Hypertensive Crisis)**.1. Why Tyramine containing foods is the correct answer:When MAO inhibitors (especially non-selective ones) are combined with tyramine-rich foods (e.g., aged cheese, red wine), it results in the **"Cheese Reaction."** Tyramine acts as an indirect sympathomimetic, displacing stored norepinephrine from nerve endings. Since MAO is inhibited, norepinephrine is not degraded, leading to massive sympathetic overactivity and a **hypertensive crisis**, not serotonin syndrome [1].2. Why the other options are incorrect:* **SSRIs (Option B):** These are the most common triggers for Serotonin Syndrome when combined with MAOIs. They prevent the reuptake of serotonin, while MAOIs prevent its breakdown, leading to toxic levels of synaptic serotonin [2].* **TCAs (Option A):** Most TCAs (especially Clomipramine) inhibit serotonin reuptake. Combining them with MAOIs significantly increases the risk of serotonin syndrome.* **Carbamazepine (Option C):** Although primarily an anticonvulsant, Carbamazepine has a tricyclic structure and weak serotonin reuptake inhibitory properties. It is clinically contraindicated with MAOIs due to the risk of serotonin toxicity.Clinical Pearls for NEET-PG:* **Serotonin Syndrome Triad:** Cognitive effects (confusion, hypomania), Autonomic hyperactivity (diaphoresis, tachycardia), and Somatic effects (hyperreflexia, **clonus**, tremor).* **Management:** The specific antidote for Serotonin Syndrome is **Cyproheptadine** (5-HT2A antagonist).* **Washout Period:** Always maintain a 14-day gap when switching between MAOIs and other antidepressants (5 weeks for Fluoxetine due to its long half-life).

Q: Theophylline levels are increased by all of the following, except?

Smoking. ### Explanation Theophylline is a methylxanthine with a narrow therapeutic index (10–20 µg/mL), primarily metabolized by the hepatic cytochrome P450 enzyme system, specifically **CYP1A2**. **Why Smoking is the Correct Answer:** Cigarette smoking (specifically polycyclic aromatic hydrocarbons) acts as a potent **enzyme inducer** of CYP1A2. This increases the metabolism of theophylline, leading to **decreased** serum levels. Therefore, smokers often require higher doses of theophylline to achieve therapeutic effects. **Why the Other Options are Incorrect:** * **Erythromycin:** This is a macrolide antibiotic that acts as a **CYP450 inhibitor**. It reduces the clearance of theophylline, thereby increasing its plasma concentration and risk of toxicity. * **Cimetidine:** A classic H2-receptor antagonist known for being a broad **enzyme inhibitor**. It significantly inhibits theophylline metabolism, leading to increased levels. * **Congestive Heart Failure (CHF):** In CHF, hepatic congestion and reduced liver blood flow impair the liver's ability to metabolize drugs. This decreased clearance results in **increased** theophylline levels. **High-Yield Clinical Pearls for NEET-PG:** 1. **Enzyme Inducers (Decrease Theophylline):** Rifampicin, Phenytoin, Phenobarbitone, and Smoking. 2. **Enzyme Inhibitors (Increase Theophylline):** Ciprofloxacin (potent), Erythromycin, Cimetidine, and Allopurinol. 3. **Disease States:** Liver cirrhosis and CHF increase theophylline levels due to reduced metabolism. 4. **Toxicity:** Watch for seizures and cardiac arrhythmias if levels exceed 20–30 µg/mL.

Q: Aspirin is contraindicated in a patient who is on treatment with which of the following medications?

Warfarin. ### Explanation The correct answer is **Warfarin**. **1. Why Warfarin is the correct answer:** The interaction between Aspirin and Warfarin is a classic example of both **pharmacokinetic** and **pharmacodynamic** synergism, leading to a significantly increased risk of life-threatening hemorrhage [1, 2]. * **Pharmacokinetic:** Aspirin is highly plasma protein-bound. It displaces Warfarin from albumin binding sites, increasing the free (active) fraction of Warfarin in the blood [2]. * **Pharmacodynamic:** Aspirin inhibits platelet aggregation (via COX-1 inhibition) [3], while Warfarin inhibits the synthesis of clotting factors (II, VII, IX, X). Combining an antiplatelet with an anticoagulant creates a "double hit" on the hemostatic system [2]. Additionally, Aspirin can cause gastric mucosal erosions, providing a potential site for bleeding. **2. Why other options are incorrect:** * **Prednisolone:** While both drugs are ulcerogenic and their combination increases the risk of peptic ulcer disease, it is not an absolute contraindication. They are often used together in rheumatological conditions with PPI prophylaxis. * **Theophylline:** Aspirin does not significantly alter the metabolism or clearance of theophylline. * **Oral Contraceptives:** There is no major direct contraindication. In fact, OCPs are pro-thrombotic, whereas Aspirin is anti-thrombotic. **3. NEET-PG High-Yield Pearls:** * **NSAIDs and Lithium:** NSAIDs (except Aspirin and Sulindac) decrease the renal clearance of Lithium, leading to toxicity. * **Aspirin and Methotrexate:** Aspirin inhibits the renal tubular secretion of Methotrexate, increasing its toxicity. * **Triple Whammy:** The dangerous combination of an **ACE inhibitor/ARB + Diuretic + NSAID** which can precipitate acute renal failure. * **Reye’s Syndrome:** Aspirin is contraindicated in children with viral infections (Influenza/Varicella) due to the risk of fulminant hepatic failure and encephalopathy.

Q: Which of the following drug groups reacts unfavourably with lithium?

ACE inhibitors. **Explanation:** **Lithium** has a narrow therapeutic index (0.6–1.2 mEq/L) and is primarily excreted by the kidneys. Its handling in the proximal tubule mimics that of sodium. **1. Why ACE Inhibitors are correct:** ACE inhibitors (e.g., Enalapril, Lisinopril) decrease the production of Aldosterone. This leads to increased sodium excretion (natriuresis) and a subsequent decrease in total body sodium. To compensate, the proximal tubule increases the reabsorption of sodium—and because the kidney cannot distinguish between the two ions—it also increases the **reabsorption of Lithium**. This leads to a significant rise in serum lithium levels, potentially causing **Lithium Toxicity**. **2. Analysis of Incorrect Options:** * **Morphine:** There is no significant pharmacokinetic interaction between opioids and lithium. * **Valproate:** Valproate is often used *with* lithium in the management of Bipolar Affective Disorder (BPAD) as a mood stabilizer. While they have additive side effects (like weight gain or tremors), they do not typically cause a toxic rise in lithium levels. * **Antipsychotics:** While combining lithium with high-dose antipsychotics (like Haloperidol) can rarely cause an encephalopathic syndrome, they do not directly interfere with lithium excretion in the same predictable, unfavorable manner as ACE inhibitors. **High-Yield Clinical Pearls for NEET-PG:** * **Drugs that INCREASE Lithium levels:** "The **D-A-N-T** mnemonic" — **D**iuretics (specifically Thiazides), **A**CE inhibitors/ARBs, **N**SAIDs (except Aspirin/Sulindac), and **T**etracyclines. * **Drugs that DECREASE Lithium levels:** Acetazolamide, Theophylline, Caffeine, and Sodium Bicarbonate (they increase lithium excretion). * **Thiazides** are the most common cause of drug-induced lithium toxicity in clinical practice.

Q: Which drugs are known to increase the level of theophylline?

All of these. **Explanation:** Theophylline is a methylxanthine bronchodilator with a **narrow therapeutic index** (10–20 µg/mL). It is primarily metabolized in the liver by the **Cytochrome P450 (CYP1A2 and CYP3A4)** enzyme systems. Any drug that inhibits these enzymes will decrease the clearance of theophylline, leading to increased plasma levels and potential toxicity (seizures, arrhythmias). * **Ciprofloxacin:** A fluoroquinolone that is a potent inhibitor of CYP1A2. It significantly reduces theophylline clearance, often requiring a dose reduction of 30-50%. * **Cimetidine:** An H2-receptor antagonist known as a "pan-enzyme inhibitor." It inhibits multiple CYP isoforms, including those responsible for theophylline metabolism. * **Allopurinol:** Used in gout management, high doses of allopurinol can inhibit the hepatic metabolism of theophylline, leading to elevated serum concentrations. **Why "All of these" is correct:** All three drugs listed are **Enzyme Inhibitors**. Since theophylline relies heavily on hepatic metabolism for elimination, the co-administration of any of these agents increases the risk of theophylline toxicity. **High-Yield Clinical Pearls for NEET-PG:** 1. **Enzyme Inducers (The "Decreasers"):** Drugs like **Rifampicin, Phenytoin, Phenobarbitone, and Smoking** induce CYP enzymes, thereby *decreasing* theophylline levels and necessitating a dose increase. 2. **Toxicity Profile:** Early signs of toxicity include GI upset and tachycardia; severe toxicity manifests as **persistent vomiting, cardiac arrhythmias, and intractable seizures**. 3. **Erythromycin & Clarithromycin:** These macrolides are also frequent "distractors" in exams as they are potent CYP inhibitors that increase theophylline levels (Note: Azithromycin does not significantly inhibit CYP enzymes).

Q: A 40-year-old female with a history of myasthenia gravis on treatment with azathioprine presents with arthritis involving the knee joint and a serum uric acid of 12 mg%. Which of the following is the best drug for lowering uric acid in this patient?

febuxostat. **Explanation:** The core concept in this question is the **clinically significant drug-drug interaction** between Xanthine Oxidase (XO) inhibitors and 6-mercaptopurine (6-MP). 1. **Why Febuxostat is correct:** The patient is taking **Azathioprine** for myasthenia gravis. Azathioprine is a prodrug that is converted into **6-mercaptopurine (6-MP)**. 6-MP is primarily metabolized and inactivated by the enzyme **Xanthine Oxidase**. * **Allopurinol** is a purine-analog inhibitor of XO. It significantly inhibits the metabolism of 6-MP, leading to toxic levels of the drug, which causes life-threatening bone marrow suppression (pancytopenia). * **Febuxostat** is a non-purine selective inhibitor of XO. While it also inhibits XO, clinical guidelines and pharmacological principles dictate that if an XO inhibitor must be used, allopurinol is strictly contraindicated with azathioprine. However, in the context of this specific MCQ, Febuxostat is the "best" choice because it avoids the hypersensitivity risks of allopurinol, though in practice, both require extreme caution or dose reduction of azathioprine (usually by 75%). 2. **Why other options are incorrect:** * **Allopurinol:** Contraindicated due to the high risk of severe bone marrow toxicity when combined with azathioprine. * **Penicillamine:** This is a chelating agent used in Wilson’s disease and sometimes in rheumatoid arthritis; it has no role in lowering serum uric acid levels. **Clinical Pearls for NEET-PG:** * **High-Yield Interaction:** Always check for Azathioprine or 6-MP before prescribing Allopurinol. * **Alternative:** If a patient on Azathioprine develops gout, **Uricosuric agents** (like Probenecid) are safer alternatives as they do not interfere with 6-MP metabolism. * **Febuxostat Advantage:** It is safer in patients with mild-to-moderate renal impairment compared to allopurinol.

Question 1

If carbamazepine is added to valproate, which side effect of valproate can increase?

Accepted Answer

Hepatotoxicity

Answer

Thrombocytopenia

Answer

Hyperammonemia

Answer

Pancreatitis

Question 2

All of the following can result in serotonin syndrome when combined with MAO inhibitors, except?

Accepted Answer

Tyramine containing foods

Answer

Tricyclic antidepressants

Answer

Selective serotonin reuptake inhibitors

Answer

Carbamazepine

Question 3

Which among the following classes of antibiotics are relatively safer to use in patients on Warfarin therapy?

Accepted Answer

Penicillins

Answer

Cephalosporins

Answer

Imidazoles

Answer

Macrolides

Question 4

Theophylline levels are increased by all of the following, except?

Accepted Answer

Smoking

Answer

Erythromycin

Answer

Cimetidine

Answer

Congestive Heart Failure (CHF)

Question 5

Aspirin is contraindicated in a patient who is on treatment with which of the following medications?

Accepted Answer

Warfarin

Answer

Prednisolone

Answer

Theophylline

Answer

Oral contraceptives

Question 6

Which of the following drug groups reacts unfavourably with lithium?

Accepted Answer

ACE inhibitors

Answer

Morphine

Answer

Valproate

Answer

Antipsychotics

Question 7

Which of the following diuretics should not be administered along with aminoglycosides?

Accepted Answer

Furosemide

Answer

Chlorthiazide

Answer

Dorzolamide

Answer

Canrenone

Question 8

Which drugs are known to increase the level of theophylline?

Accepted Answer

All of these

Answer

Ciprofloxacin

Answer

Cimetidine

Answer

Allopurinol

Question 9

A 40-year-old female with a history of myasthenia gravis on treatment with azathioprine presents with arthritis involving the knee joint and a serum uric acid of 12 mg%. Which of the following is the best drug for lowering uric acid in this patient?

Accepted Answer

febuxostat

Answer

allopurinol

Answer

both allopurinol and febuxostat

Answer

penicillamine

Question 10

A diabetic female on isoniazid and rifampicin for tuberculosis developed deep vein thrombosis. She was started on warfarin. Her prothrombin time is not raised. What should be the next step in management?

Accepted Answer

Use low molecular weight heparin

Answer

Increase the dose of warfarin

Answer

Replace warfarin with acenocoumarol

Answer

Switch ethambutol for rifampicin

Drug Interactions — MCQs

Drug Interactions — MCQs

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