Drug Interactions Practice Questions

Q: Sparfloxacin and astemizole can cause which of the following?

Ventricular arrhythmia. **Explanation:**The correct answer is **Ventricular arrhythmia**.**Mechanism of Action:**Both **Sparfloxacin** (a fluoroquinolone) and **Astemizole** (a second-generation antihistamine) share a common side effect: they cause **prolongation of the QT interval** on an ECG. They achieve this by blocking the delayed rectifier potassium channels ($I_{Kr}$) in the cardiac myocytes [3]. When two or more drugs that prolong the QT interval are administered together, their effects are additive. This significantly increases the risk of a specific, life-threatening polymorphic ventricular tachycardia known as **Torsades de Pointes (TdP)** [1, 2].**Analysis of Incorrect Options:** * **B. Myopathy:** This is a classic side effect of Statins (HMG-CoA reductase inhibitors) or Daptomycin, but it is not a characteristic interaction between fluoroquinolones and antihistamines.* **C. Electrolyte imbalance:** While hypokalemia or hypomagnesemia can *predispose* a patient to QT prolongation [1], these drugs themselves do not typically cause electrolyte shifts as their primary interaction.* **D. Nephropathy:** While some fluoroquinolones require dose adjustment in renal failure, they are not primarily known for causing synergistic nephrotoxicity when combined with antihistamines.**High-Yield Clinical Pearls for NEET-PG:** * **Astemizole and Terfenadine:** These antihistamines were largely withdrawn from the market because they caused TdP, especially when combined with CYP3A4 inhibitors (like Erythromycin or Ketoconazole) which increased their plasma levels.* **Fluoroquinolones:** Among this class, Sparfloxacin and Moxifloxacin have the highest propensity for QT prolongation.* **Other QT-prolonging drugs to remember:** Macrolides (Erythromycin), Antipsychotics (Haloperidol, Ziprasidone), Class IA and III Antiarrhythmics (Sotalol, Amiodarone), and Chloroquine.

Q: Which of the following drugs can have an increased effect when given with heparin?

NSAIDS. **Explanation:** The correct answer is **B. NSAIDs**. **Mechanism of Interaction:** Heparin is an anticoagulant that acts by activating Antithrombin III, which inhibits thrombin (Factor IIa) and Factor Xa. NSAIDs (Non-Steroidal Anti-inflammatory Drugs), such as aspirin or ibuprofen, interfere with primary hemostasis by inhibiting cyclooxygenase (COX) enzymes, thereby reducing **platelet aggregation**. When these two classes are combined, there is a **pharmacodynamic synergism**. While NSAIDs do not directly increase heparin’s plasma concentration, they significantly increase the **risk of bleeding** (the "effect" of anticoagulation) by impairing platelet function and potentially causing gastric mucosal erosions. **Analysis of Incorrect Options:** * **A. Narcotics (Opioids):** There is no significant direct interaction between narcotics and heparin. Narcotics primarily affect the CNS and GI motility. * **C. OCPs (Oral Contraceptive Pills):** Estrogen-containing OCPs are **pro-thrombotic** (increase clotting factors). They generally antagonize the therapeutic goal of heparin rather than increasing its effect. * **D. Salbutamol:** A beta-2 agonist used in asthma; it does not have a known clinical interaction with the coagulation cascade or heparin. **High-Yield Clinical Pearls for NEET-PG:** * **Antidote:** The specific antagonist for Heparin overdose is **Protamine Sulfate** (1 mg neutralizes ~100 units of heparin). * **Monitoring:** Unfractionated Heparin (UFH) is monitored using **aPTT**, whereas Low Molecular Weight Heparin (LMWH) usually does not require monitoring (except in pregnancy/obesity via Anti-Xa levels). * **HIT:** Watch for **Heparin-Induced Thrombocytopenia**, a paradoxical pro-thrombotic state caused by antibodies against Heparin-Platelet Factor 4 complexes.

Q: Phenobarbitone and warfarin interaction results in:

Increased metabolism of warfarin. **Explanation:** The interaction between Phenobarbitone and Warfarin is a classic example of **Pharmacokinetic Drug Interaction** involving **Enzyme Induction**. **1. Why Option B is Correct:** Phenobarbitone is a potent inducer of the hepatic microsomal enzyme system, specifically the **Cytochrome P450 (CYP2C9)** isoenzyme. Warfarin is primarily metabolized by CYP2C9. When Phenobarbitone is co-administered, it increases the synthesis of these enzymes, leading to the **increased metabolism** of Warfarin. This results in decreased plasma concentrations of Warfarin, reducing its anticoagulant efficacy and lowering the International Normalized Ratio (INR). **2. Why Other Options are Incorrect:** * **Option A:** Phenobarbitone does not significantly affect the gastrointestinal absorption of Warfarin. * **Option C:** Displacement from binding sites (Protein Binding Displacement) is a mechanism associated with drugs like Sulfonamides or Salicylates, which can displace Warfarin from albumin, initially increasing its free fraction. Phenobarbitone does not act via this mechanism. **Clinical Pearls for NEET-PG:** * **Dose Adjustment:** If a patient on Warfarin starts Phenobarbitone, the dose of Warfarin must be **increased** to maintain the therapeutic INR. Conversely, if Phenobarbitone is stopped, the Warfarin dose must be **decreased** to prevent life-threatening bleeding. * **Other Potent Enzyme Inducers (GPRS Cell):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine. * **Enzyme Inhibitors (VITAMINS K):** **V**erapamil, **I**soniazid, **T**amoxifen, **A**miodarone, **M**etronidazole, **I**traconazole, **N**elnavir, **S**ulfonamides, **K**etoconazole (these would *increase* Warfarin toxicity).

Q: Carbamazepine has drug interaction with all of the following except?

Doxycycline. **Explanation:** The core concept behind this question is the **Cytochrome P450 (CYP450) enzyme system**, specifically the role of Carbamazepine as a potent **enzyme inducer**. **Why Doxycycline is the correct answer:** Carbamazepine induces CYP3A4, which increases the metabolism of several drugs. While Carbamazepine *does* decrease the half-life of Doxycycline (reducing its efficacy), the question asks which drug Carbamazepine has an interaction *with*—implying a clinically significant bidirectional or inhibitory interaction that alters Carbamazepine levels. In standard pharmacological teaching for NEET-PG, Doxycycline is the "odd one out" because it does not significantly alter the plasma concentration of Carbamazepine. **Analysis of Incorrect Options:** * **Erythromycin:** This is a potent **CYP3A4 inhibitor**. It inhibits the metabolism of Carbamazepine, leading to toxic levels of the antiepileptic drug (ataxia, drowsiness, diplopia). * **Phenytoin & Barbiturates:** Both are potent **enzyme inducers**. They induce the metabolism of Carbamazepine (and vice versa), leading to decreased plasma concentrations of the drugs. This necessitates dosage adjustments to maintain therapeutic efficacy. **High-Yield Clinical Pearls for NEET-PG:** 1. **Auto-induction:** Carbamazepine is unique because it induces its own metabolism (auto-induction), typically starting after 3–5 days of therapy and completing within 3–4 weeks. 2. **The "G-PACMAN" Mnemonic (Inducers):** **G**riseofulvin, **P**henytoin, **A**lcohol (chronic), **C**arbamazepine, **M**odafinil, **A**minoglutethimide, **N**evirapine. 3. **Drug of Choice:** Carbamazepine remains the drug of choice for **Trigeminal Neuralgia**. 4. **Side Effects:** Watch for SIADH (hyponatremia) and Stevens-Johnson Syndrome (especially in patients with HLA-B*1502 allele).

Q: A patient with Alzheimer's disease is on rivastigmine therapy and develops symptoms of depression. Which of the following drug classes, if used concurrently, would decrease the efficacy of rivastigmine?

Tricyclic antidepressants (TCAs). **Explanation:**1. Why Tricyclic Antidepressants (TCAs) are correct:Rivastigmine is a **cholinesterase inhibitor** used in Alzheimer’s disease to increase synaptic levels of acetylcholine, thereby improving cognitive function [3]. Many TCAs (e.g., Amitriptyline, Imipramine) possess significant **anticholinergic (antimuscarinic) properties** [2]. When used concurrently, TCAs exert a pharmacological antagonism against rivastigmine. By blocking the muscarinic receptors that rivastigmine is trying to stimulate via increased acetylcholine, TCAs directly neutralize the therapeutic efficacy of the Alzheimer's medication and may worsen cognitive decline.2. Why other options are incorrect:* **SSRIs (e.g., Sertraline, Escitalopram):** These are the preferred antidepressants in Alzheimer’s patients because they lack significant anticholinergic activity and do not interfere with the mechanism of cholinesterase inhibitors.* **MAOIs and RIMAs (e.g., Moclobemide):** These drugs act on the metabolism of monoamines (Norepinephrine, Serotonin, Dopamine). They do not possess anticholinergic properties and therefore do not directly decrease the efficacy of rivastigmine.3. High-Yield Clinical Pearls for NEET-PG:* **Rivastigmine Unique Feature:** It is a "pseudo-irreversible" inhibitor of both **Acetylcholinesterase (AChE)** and **Butyrylcholinesterase (BuChE)** [1]. It is also available as a transdermal patch to reduce GI side effects.* **Drugs to Avoid in Dementia:** Always avoid drugs with "atropine-like" effects, including first-generation antihistamines (Diphenhydramine), bladder antispasmodics (Oxybutynin), and TCAs.* **Preferred Antidepressant:** SSRIs are the first-line treatment for depression in patients with dementia due to their favorable side-effect profile.

Q: A 26-year-old female, currently taking phenytoin for idiopathic grand mal epilepsy, wishes to avoid pregnancy and is using oral contraceptive pills (OCPs). She is concerned that phenytoin may decrease the effectiveness of her OCPs. Which of the following is a suitable alternative anticonvulsant that is less likely to interact with OCPs?

Lamotrigine. ### Explanation **Core Concept: Enzyme Induction and OCP Failure** The primary concern in this clinical scenario is the induction of hepatic microsomal enzymes (specifically **CYP3A4**). Most traditional anticonvulsants are potent enzyme inducers that accelerate the metabolism of estrogen and progesterone components in OCPs, leading to decreased plasma levels and potential contraceptive failure [1]. **Why Lamotrigine is Correct:** **Lamotrigine** is the preferred choice here because it is **not a clinically significant hepatic enzyme inducer**. Unlike phenytoin, it does not accelerate the breakdown of OCPs [2]. * *Note:* While OCPs can actually decrease the plasma levels of Lamotrigine (requiring dose adjustment), Lamotrigine itself does not compromise the efficacy of the OCP. Other safe alternatives for women on OCPs include **Levetiracetam, Valproate, and Gabapentin.** **Why Other Options are Incorrect:** * **A. Carbamazepine:** A potent inducer of CYP1A2, 2C9, and 3A4. It significantly increases OCP metabolism. * **C. Topiramate:** While it has multiple mechanisms, at doses >200 mg/day, it acts as a CYP3A4 inducer and can reduce OCP effectiveness. * **D. Phenobarbitone:** A classic, broad-spectrum enzyme inducer that significantly lowers the serum concentration of contraceptive steroids. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Inducers:** "**G**uiness **P**is **S**strong **C**ombat **R**ifle" (**G**riseofulvin, **P**henytoin, **S**moking, **C**arbamazepine, **R**ifampicin, **P**henobarbitone). * **Teratogenicity:** Phenytoin is associated with **Fetal Hydantoin Syndrome** (cleft lip/palate, digital hypoplasia). Valproate is the most teratogenic (neural tube defects). * **Drug of Choice:** Lamotrigine and Levetiracetam are often preferred in pregnancy and women of childbearing age due to a better safety profile and fewer drug interactions [1].

Q: Which antibiotic can accentuate the neuromuscular blockade produced by d-tubocurarine?

Streptomycin. **Explanation:** The correct answer is **Streptomycin**. **Mechanism of Interaction:** Aminoglycoside antibiotics (like Streptomycin, Gentamicin, and Neomycin) possess a known side effect of neuromuscular blockade. They interfere with neuromuscular transmission by two primary mechanisms: 1. **Inhibition of Acetylcholine (ACh) release:** They compete with Calcium ions at the pre-junctional nerve terminal, reducing the amount of ACh released. 2. **Post-junctional sensitivity:** They decrease the sensitivity of the motor end-plate to ACh. When administered with non-depolarizing neuromuscular blockers like **d-tubocurarine**, these effects are additive, leading to prolonged respiratory paralysis or "apnea." **Analysis of Incorrect Options:** * **A. Penicillin G:** A cell-wall synthesis inhibitor (Beta-lactam) that does not interfere with calcium channels or ACh release; it has no effect on the neuromuscular junction. * **B. Erythromycin:** A Macrolide that inhibits protein synthesis (50S subunit). While it can interact with drugs via CYP3A4 inhibition, it does not potentiate neuromuscular blockade. * **D. Chloramphenicol:** Another protein synthesis inhibitor (50S subunit) primarily associated with bone marrow suppression and Gray Baby Syndrome, but not neuromuscular interference. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Potency:** Among aminoglycosides, the neuromuscular blocking potency is: **Neomycin > Streptomycin > Amikacin > Gentamicin.** * **Management:** Aminoglycoside-induced blockade can be partially reversed by **Intravenous Calcium Gluconate** (which overcomes the pre-junctional calcium competition) and Neostigmine. * **Contraindication:** Aminoglycosides should be used with extreme caution in patients with **Myasthenia Gravis** as they can precipitate a myasthenic crisis.

Question 1

Sparfloxacin and astemizole can cause which of the following?

Accepted Answer

Ventricular arrhythmia

Answer

Myopathy

Answer

Electrolyte imbalance

Answer

Nephropathy

Question 2

Which of the following drugs can have an increased effect when given with heparin?

Accepted Answer

NSAIDS

Answer

Narcotics

Answer

OCPS

Answer

Salbutamol

Question 3

Phenobarbitone and warfarin interaction results in:

Accepted Answer

Increased metabolism of warfarin

Answer

Increased absorption of warfarin

Answer

Displacement of warfarin from the binding site

Answer

All of the above

Question 4

A patient is on long-term ketoconazole therapy and has developed gastroesophageal reflux disease (GERD). Which of the following drugs should not be used for the treatment of GERD in this patient?

Accepted Answer

Cisapride

Answer

Itopride

Answer

Metoclopramide

Answer

Domperidone

Question 5

A diabetic female on isoniazid and rifampicin for tuberculosis suffers from deep vein thrombosis. She is started on warfarin but her prothrombin time is not raised. What is the next best step in management?

Accepted Answer

Use low molecular weight heparin

Answer

Long term heparin therapy

Answer

Switch ethambutol for rifampicin

Answer

Replace warfarin with acenocoumarol

Question 6

MAO inhibitors are contraindicated in all the following conditions EXCEPT:

Accepted Answer

Aspirin

Answer

With tricyclic antidepressants

Answer

With indirectly acting sympathomimetics

Answer

Cheese

Question 7

Carbamazepine has drug interaction with all of the following except?

Accepted Answer

Doxycycline

Answer

Erythromycin

Answer

Phenytoin

Answer

Barbiturates

Question 8

A patient with Alzheimer's disease is on rivastigmine therapy and develops symptoms of depression. Which of the following drug classes, if used concurrently, would decrease the efficacy of rivastigmine?

Accepted Answer

Tricyclic antidepressants (TCAs)

Answer

Selective serotonin reuptake inhibitors (SSRIs)

Answer

Monoamine oxidase inhibitors (MAOIs)

Answer

Reversible inhibitors of monoamine oxidase A (RIMAs)

Question 9

A 26-year-old female, currently taking phenytoin for idiopathic grand mal epilepsy, wishes to avoid pregnancy and is using oral contraceptive pills (OCPs). She is concerned that phenytoin may decrease the effectiveness of her OCPs. Which of the following is a suitable alternative anticonvulsant that is less likely to interact with OCPs?

Accepted Answer

Lamotrigine

Answer

Carbamazepine

Answer

Topiramate

Answer

Phenobarbitone

Question 10

Which antibiotic can accentuate the neuromuscular blockade produced by d-tubocurarine?

Accepted Answer

Streptomycin

Answer

Penicillin G

Answer

Erythromycin

Answer

Chloramphenicol

Drug Interactions — MCQs

Drug Interactions — MCQs

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