Drug Interactions — MCQs

A 54-year-old man, currently taking erythromycin for bronchitis and another medication for gastroesophageal reflux disease (GERD), is brought to the emergency department after experiencing several syncopal episodes. If an electrocardiogram reveals torsades de pointes, what medication is the patient most likely taking for his reflux disease?

Q124Easy

Cimetidine inhibits the metabolism of all of the following drugs except?

Q125Medium

Digoxin toxicity may result from the concurrent administration of digoxin with all of the following drugs EXCEPT?

Q126Easy

Which of the following drugs is contraindicated along with spironolactone?

Q127Medium

Polymorphic ventricular tachycardia can occur when terfenadine (antihistaminic) is:

Q128

A patient with rheumatoid arthritis is taking indomethacin and an ACE inhibitor for hypertension. What potential side effect is likely to be seen?

Q129

Carbamazepine and erythromycin were given to a patient, and he presented with ataxia and dizziness. Which of the following is the reason for the symptoms?

Q130

Imipramine and Diphenhydramine are given together to a patient. Why is this combination considered irrational?

Drug Interactions Indian Medical PG Practice Questions and MCQs

Question 121: Aspirin is not given to a patient who is already on heparin because aspirin causes:

A. Platelet dysfunction (Correct Answer)
B. Aspirin inhibits the action of heparin
C. Enhanced hypersensitivity to heparin
D. Therapy of heparin cannot be monitored

Explanation: **Explanation:** The combination of Aspirin and Heparin is generally avoided (unless specifically indicated in acute coronary syndromes) due to the significantly increased risk of life-threatening hemorrhage. **1. Why Option A is Correct:** Aspirin is an irreversible inhibitor of the enzyme **Cyclooxygenase-1 (COX-1)**. This inhibition prevents the synthesis of **Thromboxane A2 (TXA2)**, a potent mediator required for platelet aggregation. Since Heparin acts on the coagulation cascade (inhibiting Thrombin and Factor Xa), adding Aspirin creates a "dual hit" on the hemostatic system: Heparin impairs fibrin formation while Aspirin causes **platelet dysfunction**. This synergistic effect prevents the formation of a stable clot, leading to bleeding. **2. Why Incorrect Options are Wrong:** * **Option B:** Aspirin does not inhibit Heparin. In fact, they have an additive pharmacological effect on bleeding time and clotting time. * **Option C:** There is no known immunological cross-reactivity or pharmacological mechanism that suggests Aspirin enhances hypersensitivity (allergic) reactions to Heparin. * **Option D:** Heparin therapy can still be monitored using **aPTT** (Activated Partial Thromboplastin Time) or Anti-Xa levels, regardless of Aspirin intake. However, the clinical risk of bleeding increases even if aPTT is within the therapeutic range. **High-Yield Clinical Pearls for NEET-PG:** * **Aspirin's effect is permanent:** Because platelets are anuclear, they cannot synthesize new COX-1. The antiplatelet effect lasts for the life of the platelet (**7–10 days**). * **NSAIDs vs. Aspirin:** Reversible NSAIDs (like Ibuprofen) can interfere with Aspirin’s binding site on COX-1; therefore, Aspirin should be taken at least 30 minutes before or 8 hours after other NSAIDs. * **Antidotes:** Remember that **Protamine Sulfate** is the antidote for Heparin, but there is no specific pharmacological antidote for Aspirin; management of Aspirin-induced bleeding requires platelet transfusion.

Question 122: Rifampicin failure of oral contraceptive pills (OCPs) is due to which of the following mechanisms?

A. Decreased absorption of OCPs
B. Increased binding of OCPs by rifampicin, leading to reduced free drug concentration
C. Increased metabolism of OCPs (Correct Answer)
D. Increased chances of ovulation due to rifampicin

Explanation: **Explanation:** The correct answer is **C. Increased metabolism of OCPs.** **Mechanism of Action:** Rifampicin is one of the most potent **microsomal enzyme inducers** known in clinical pharmacology. It significantly induces the **Cytochrome P450 (CYP3A4)** enzyme system in the liver. Oral Contraceptive Pills (OCPs), which contain estrogen and progesterone, are primary substrates for CYP3A4. When Rifampicin is co-administered, it accelerates the hepatic metabolism of these hormones, leading to a rapid decline in their plasma concentrations. This reduces the drug levels below the therapeutic threshold required to suppress ovulation, resulting in contraceptive failure and unintended pregnancy. **Analysis of Incorrect Options:** * **Option A:** Rifampicin does not interfere with the gastrointestinal absorption of OCPs. (Note: Certain broad-spectrum antibiotics like Tetracycline may reduce OCP efficacy by disrupting enterohepatic circulation, but not Rifampicin). * **Option B:** Rifampicin does not significantly alter the protein binding of OCPs; its primary interaction is metabolic. * **Option D:** Ovulation is a *consequence* of the interaction (due to low hormone levels), not the *mechanism* itself. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin** is a "Universal Inducer"—it also reduces the efficacy of Warfarin, Corticosteroids, and Sulfonylureas. * **Management:** Patients on Rifampicin should be advised to use **barrier methods** (condoms) or non-hormonal contraception (like Copper-T) during and for 4 weeks after stopping the drug. * **Exception:** Unlike other antibiotics, Rifampicin is the only one with a proven, clinically significant metabolic interaction with OCPs.

Question 123: A 54-year-old man, currently taking erythromycin for bronchitis and another medication for gastroesophageal reflux disease (GERD), is brought to the emergency department after experiencing several syncopal episodes. If an electrocardiogram reveals torsades de pointes, what medication is the patient most likely taking for his reflux disease?

A. Cisapride (Correct Answer)
B. Famotidine
C. Lansoprazole
D. Metoclopramide

Explanation: ### Explanation **Correct Option: A. Cisapride** The clinical presentation of syncopal episodes and **Torsades de Pointes (TdP)** on ECG indicates a drug-induced prolongation of the QT interval. **Mechanism:** Cisapride, a prokinetic agent used for GERD, is metabolized by the hepatic enzyme **CYP3A4**. Erythromycin is a potent **CYP3A4 inhibitor**. When taken together, erythromycin inhibits the metabolism of cisapride, leading to toxic plasma levels. Elevated cisapride levels block the delayed rectifier potassium channels ($I_{Kr}$) in the heart, resulting in delayed repolarization, QT prolongation, and life-threatening ventricular arrhythmias like Torsades de Pointes. Due to this high risk of cardiotoxicity, cisapride has been withdrawn or strictly restricted in many markets. **Why other options are incorrect:** * **B. Famotidine:** An $H_2$ receptor antagonist. It does not significantly inhibit CYP enzymes nor is it associated with QT prolongation. * **C. Lansoprazole:** A Proton Pump Inhibitor (PPI). While PPIs can affect the absorption of some drugs, they do not cause TdP when combined with erythromycin. * **D. Metoclopramide:** A D2 receptor antagonist used as a prokinetic. Its primary side effects are extrapyramidal (e.g., dystonia, parkinsonism) rather than cardiac arrhythmias. **High-Yield NEET-PG Pearls:** * **QT Prolonging Drugs (Mnemonic: ABCDE):** **A**ntiarrhythmics (Class IA, III), **B**iotics (Macrolides, Fluoroquinolones), **C**ant Psychotics (Haloperidol), **D**epressants (TCAs), **E**metics (Ondansetron). * **CYP3A4 Inhibitors:** Macrolides (except Azithromycin), Ketoconazole, Grapefruit juice, Ritonavir, and Cimetidine. * **Management of TdP:** The immediate drug of choice for hemodynamically stable Torsades de Pointes is **Intravenous Magnesium Sulfate**.

Question 124: Cimetidine inhibits the metabolism of all of the following drugs except?

A. Phenytoin
B. Warfarin
C. Ketoconazole (Correct Answer)
D. Diazepam

Explanation: **Explanation:** The core concept tested here is the difference between **Enzyme Inhibition** and **Drug-Drug Interactions (DDI) based on pH changes.** **Why Ketoconazole is the correct answer:** Cimetidine is a potent **Microsomal Enzyme Inhibitor** (specifically Cytochrome P450). However, its interaction with Ketoconazole is not mediated by enzyme inhibition. Ketoconazole requires an **acidic gastric environment** for its dissolution and systemic absorption. Cimetidine, being an H2-receptor antagonist, increases gastric pH (making it more basic), which significantly **decreases the absorption** of Ketoconazole rather than inhibiting its metabolism. Therefore, Ketoconazole is the "except" because the interaction is pharmacokinetic at the level of absorption, not metabolism. **Why the other options are incorrect:** * **A, B, and D (Phenytoin, Warfarin, Diazepam):** These drugs are classic substrates of the hepatic CYP450 system (specifically isoforms like CYP2C9 and CYP3A4). Cimetidine binds to the heme iron of the CYP450 oxidase system, reducing the metabolic clearance of these drugs. This leads to increased plasma levels and potential toxicity (e.g., increased bleeding risk with Warfarin or sedation with Diazepam). **High-Yield Clinical Pearls for NEET-PG:** * **Cimetidine Mnemonic:** Remember **"VITAMIN G"** for Cimetidine side effects/interactions: **V**itamin B12 deficiency, **I**nhibits P450, **T**achyphylaxis, **A**nti-androgenic (Gynecomastia), **M**etabolism decrease, **I**ncreases Prolactin, **N**ephrotoxicity, **G**I upset. * **Other Enzyme Inhibitors:** Valproate, Ketoconazole (itself an inhibitor), Isoniazid, Ciprofloxacin, Erythromycin (Mnemonic: **VICK'S G**roup). * **Ketoconazole Interaction:** Apart from H2 blockers, Proton Pump Inhibitors (PPIs) and Antacids also decrease Ketoconazole absorption.

Question 125: Digoxin toxicity may result from the concurrent administration of digoxin with all of the following drugs EXCEPT?

A. Quinidine
B. Hydrochlorothiazide
C. Triamterene (Correct Answer)
D. Furosemide

Explanation: ### Explanation The core concept behind this question is the relationship between **serum potassium levels** and **digoxin toxicity**. Digoxin competes with potassium for binding sites on the Na+/K+-ATPase pump. Therefore, **hypokalemia** (low potassium) increases digoxin binding, leading to toxicity. **1. Why Triamterene is the Correct Answer:** Triamterene is a **potassium-sparing diuretic**. It inhibits sodium reabsorption in the distal tubule and collecting duct without causing potassium loss. By maintaining or slightly increasing serum potassium levels, it prevents the increased binding of digoxin to the ATPase pump, thereby **reducing** the risk of toxicity rather than causing it. **2. Why the Other Options are Incorrect:** * **Quinidine (Option A):** This is a classic drug interaction. Quinidine reduces the renal and extra-renal clearance of digoxin and displaces it from tissue binding sites, significantly increasing plasma digoxin levels. * **Furosemide (Option D) & Hydrochlorothiazide (Option B):** These are loop and thiazide diuretics, respectively. Both are **potassium-wasting diuretics**. They cause hypokalemia, which sensitizes the myocardium to digoxin, precipitating toxicity even if digoxin levels are within the "normal" therapeutic range. **3. Clinical Pearls for NEET-PG:** * **Electrolyte Triad of Digoxin Toxicity:** Hypokalemia, Hypomagnesemia, and Hypercalcemia all predispose a patient to toxicity. * **Most Common Arrhythmia:** Atrial tachycardia with AV block. * **Most Specific Arrhythmia:** Bidirectional Ventricular Tachycardia. * **Management:** Digibind (Digoxin-specific Fab fragments) is the antidote of choice for severe toxicity. * **Other drugs increasing Digoxin levels:** Verapamil, Amiodarone, and Erythromycin.

Question 126: Which of the following drugs is contraindicated along with spironolactone?

A. Chlorothiazide
B. Beta blockers
C. ACE inhibitors (Correct Answer)
D. Amlodipine

Explanation: **Explanation:** The correct answer is **ACE inhibitors** (Option C). **Mechanism of Interaction:** Spironolactone is a **potassium-sparing diuretic** that acts as a competitive antagonist of aldosterone in the distal convoluted tubule and collecting duct. ACE inhibitors (e.g., Enalapril, Ramipril) decrease the production of Angiotensin II, which in turn leads to a **reduction in aldosterone secretion** from the adrenal cortex. When these two drugs are combined, the additive effect of inhibiting aldosterone leads to a significant decrease in urinary potassium excretion, posing a high risk of **life-threatening hyperkalemia**. **Analysis of Incorrect Options:** * **A. Chlorothiazide:** This is a thiazide diuretic. Thiazides cause potassium depletion (hypokalemia). Combining them with spironolactone is often done intentionally to maintain potassium balance (neutralize the potassium-losing effect). * **B. Beta blockers:** While beta-blockers can slightly increase potassium levels by inhibiting renin release, they are not strictly contraindicated. They are frequently used alongside spironolactone in the management of Heart Failure (GDMT). * **C. Amlodipine:** This is a Calcium Channel Blocker (CCB). It does not significantly affect potassium levels and has no major adverse interaction with spironolactone. **High-Yield Clinical Pearls for NEET-PG:** * **The "K-Sparing" Rule:** Avoid combining Spironolactone with ACE inhibitors, ARBs (e.g., Losartan), or Potassium supplements unless under strict monitoring (e.g., in severe heart failure where the benefit may outweigh the risk). * **Eplerenone:** A more selective aldosterone antagonist with fewer anti-androgenic side effects (like gynecomastia) compared to spironolactone. * **Monitoring:** Always check serum creatinine and potassium levels within one week of initiating this combination. Hyperkalemia is defined as serum $K^+ > 5.5$ mEq/L.

Question 127: Polymorphic ventricular tachycardia can occur when terfenadine (antihistaminic) is:

A. Coadministered with azithromycin
B. Coadministered with fluconazole
C. Given in higher doses (Correct Answer)
D. Used to reduce QT interval

Explanation: Polymorphic ventricular tachycardia can occur when terfenadine (antihistaminic) is given in higher doses. **Terfenadine** is a second-generation H1-antihistamine that was withdrawn from the market due to its potential to cause **Torsades de Pointes (TdP)**, a life-threatening polymorphic ventricular tachycardia [2]. **1. Why "Given in higher doses" is correct:** Terfenadine is a prodrug that is rapidly converted by the hepatic enzyme **CYP3A4** into its active metabolite, fexofenadine. While fexofenadine is safe, parent terfenadine is cardiotoxic; it blocks the delayed rectifier potassium channels ($I_{Kr}$) in the heart [2]. This blockade delays repolarization, leading to **QT interval prolongation** [1]. When terfenadine is given in higher doses, the metabolic capacity of CYP3A4 is overwhelmed, leading to high systemic levels of the parent drug, which triggers TdP [2]. **2. Analysis of Incorrect Options:** * **A & B (Azithromycin/Fluconazole):** While CYP3A4 inhibitors like **Erythromycin** (Macrolide) and **Ketoconazole** (Azole) are classic causes of terfenadine toxicity, Azithromycin is unique among macrolides as it does *not* significantly inhibit CYP3A4. Similarly, Fluconazole is a weaker inhibitor compared to Ketoconazole or Itraconazole. While drug interactions are a major cause, the question asks for a direct cause of tachycardia; high doses of the drug itself are inherently toxic. * **D (Used to reduce QT interval):** This is factually incorrect. Terfenadine **prolongs** the QT interval; it does not reduce it [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Fexofenadine** is the active metabolite of terfenadine and is the "safe" alternative because it does not block cardiac $K^+$ channels. * **Astemizole** is another antihistamine withdrawn for the same reason (QT prolongation). * **Common CYP3A4 Inhibitors to remember:** "VITAMIN K" (Verapamil, Itraconazole, Telithromycin, Amiodarone, Macrolides (except Azithromycin), Indinavir, Nefazodone, Ketoconazole) and Grapefruit juice.

Question 128: A patient with rheumatoid arthritis is taking indomethacin and an ACE inhibitor for hypertension. What potential side effect is likely to be seen?

A. Hyperkalemia (Correct Answer)
B. Hypercalcemia
C. Hypernatremia
D. Hyperphosphatemia

Explanation: ***Hyperkalemia*** - The co-administration of an **ACE inhibitor** and an **NSAID (indomethacin)** significantly increases the risk of **hyperkalemia** due to synergistic effects on renal potassium balance. - **ACE inhibitors** directly block aldosterone synthesis (leading to reduced potassium excretion), while **NSAIDs** reduce prostaglandin synthesis, impairing renin release, which also suppresses aldosterone effect. *Hypercalcemia* - This scenario is not associated with hypercalcemia; drugs notorious for causing hypercalcemia include **thiazide diuretics**, or conditions like **primary hyperparathyroidism**. - Neither ACE inhibitors nor NSAIDs directly impair calcium homeostasis leading to clinically significant increases. *Hypernatremia* - **ACE inhibitors** decrease aldosterone levels, which promotes sodium excretion, typically resulting in **hyponatremia** or normonatremia, not hypernatremia. - Although NSAIDs can cause fluid retention, this is largely due to increased water reabsorption leading to expansion of extracellular fluid, not an increase in serum sodium concentration. *Hyperphosphatemia* - **Hyperphosphatemia** is primarily seen in end-stage **chronic kidney disease (CKD)** because the kidneys cannot excrete phosphate effectively. - The combination of indomethacin and an ACE inhibitor does not directly impair phosphate excretion via the mechanisms needed to cause clinically significant hyperphosphatemia.

Question 129: Carbamazepine and erythromycin were given to a patient, and he presented with ataxia and dizziness. Which of the following is the reason for the symptoms?

A. Toxicity of carbamazepine (Correct Answer)
B. Toxicity of erythromycin
C. Erythromycin speeds up carbamazepine metabolism
D. Sub-therapeutic carbamazepine levels causing seizures

Explanation: ***Toxicity of carbamazepine*** - Erythromycin is a potent inhibitor of the hepatic **CYP3A4 enzyme**, which is primarily responsible for the metabolism and subsequent clearance of carbamazepine. - Inhibition of carbamazepine metabolism leads to increased plasma concentration, resulting in **CNS side effects** such as **ataxia, dizziness**, nystagmus, and drowsiness. *Toxicity of erythromycin* - Erythromycin toxicity typically presents with **gastrointestinal symptoms** (e.g., nausea, vomiting, diarrhea) or cardiac issues like **QT prolongation**. - The described symptoms, ataxia and dizziness, are classic manifestations of **anticonvulsant toxicity**, not macrolide toxicity. *Erythromycin speeds up carbamazepine metabolism* - This statement is incorrect; erythromycin **inhibits** CYP3A4, thus slowing down carbamazepine metabolism and resulting in drug accumulation. - If metabolism were sped up (i.e., enzyme induction), the patient would likely experience sub-therapeutic carbamazepine levels, increasing the risk of **seizure recurrence**. *Sub-therapeutic carbamazepine levels causing seizures* - Recurrent seizures are caused by **sub-therapeutic levels** of carbamazepine, often due to enzyme induction (e.g., by phenytoin, carbamazepine itself) or non-compliance. - The symptoms of ataxia and dizziness indicate **supratherapeutic levels** (toxicity), which is the opposite of the low levels that cause breakthrough seizures.

Question 130: Imipramine and Diphenhydramine are given together to a patient. Why is this combination considered irrational?

A. Both have anticholinergic action (Correct Answer)
B. Both cause serotonin syndrome
C. Both cause hypotension
D. Both cause increased sedation

Explanation: ***Both have anticholinergic action*** - Imipramine, being a **Tricyclic Antidepressant (TCA)**, possesses significant **anticholinergic properties** (Muscarinic receptor blockade). - Diphenhydramine, a first-generation antihistamine, is also a highly effective **anticholinergic agent**; their co-administration leads to severe, potentially fatal, additive anticholinergic effects (e.g., acute confusion, severe urinary retention, paralytic ileus). *Both cause serotonin syndrome* - While Imipramine is a weak **Serotonin-Norepinephrine Reuptake Inhibitor (SNRI)**, Diphenhydramine does not directly contribute significantly to **serotonin toxicity**. - The primary and most hazardous interaction is the severe risk of **anticholinergic crisis**, not Serotonin Syndrome. *Both cause hypotension* - Imipramine can cause **orthostatic hypotension** due to its **alpha-1 adrenergic blockade** effects. - Although side effects include hypotension, the combined risk of severe **anticholinergic side effects** (delirium, ileus) is the overwhelming reason this combination is irrational. *Both cause increased sedation* - Both drugs are highly sedating, which is a valid concern for driving and daily function, mediated by **H1 receptor antagonism**. - However, while increased sedation is a risk, the combination is specifically deemed irrational because of the risk of life-threatening **anticholinergic toxicity**, which is a more critical pharmacological interaction than simple additive sedation.

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