Drug Interactions Practice Questions

Q: Warfarin therapy is prolonged by all except:

Rifampicin. The core concept behind this question is the modulation of **Cytochrome P450 (CYP450)** enzymes and the role of intestinal flora in Vitamin K synthesis. **1. Why Rifampicin is the correct answer:** Warfarin is metabolized primarily by the CYP2C9 enzyme. **Rifampicin** is a potent **microsomal enzyme inducer** [2]. By inducing these enzymes, it accelerates the metabolism of Warfarin, leading to decreased plasma levels and a **reduced** anticoagulant effect (shortened prothrombin time). Therefore, it does *not* prolong Warfarin therapy; it antagonizes it. **2. Why the other options are incorrect:** * **Erythromycin:** This is a known **enzyme inhibitor** [1]. It inhibits CYP450 enzymes, slowing down Warfarin metabolism, which increases its plasma concentration and **prolongs** its effect (increasing the risk of bleeding). * **Amoxicillin & Tetracycline:** Broad-spectrum antibiotics prolong Warfarin therapy through a different mechanism [3]. They eliminate **intestinal bacterial flora** that normally synthesize Vitamin K2 [3]. Since Warfarin works by inhibiting Vitamin K epoxide reductase, a reduction in endogenous Vitamin K levels potentiates Warfarin’s anticoagulant action. **Clinical Pearls for NEET-PG:** * **S-Warfarin** is 3-5 times more potent than R-Warfarin and is metabolized by **CYP2C9** [2]. * **Enzyme Inducers (Reduce Warfarin effect):** Phenytoin, Carbamazepine, Phenobarbitone, Rifampicin, Griseofulvin (Mnemonic: **GPRS Cell Phone**). * **Enzyme Inhibitors (Increase Warfarin effect):** Valproate, Ketoconazole, Cimetidine, Erythromycin, Isoniazid (Mnemonic: **Vitamins K, C, E, I**). * **Monitoring:** Warfarin efficacy is monitored using **PT/INR** (Extrinsic pathway) [3].

Q: Which of the following prostaglandin analogues is used in the management of glaucoma?

Latanoprost. **Explanation:** **Latanoprost** is the correct answer because it is a **PGF2α analogue** specifically designed for ophthalmic use. It functions by increasing the **uveoscleral outflow** of aqueous humor, thereby reducing intraocular pressure (IOP). It is considered a first-line treatment for Open-Angle Glaucoma due to its high efficacy and once-daily dosing profile. **Analysis of Incorrect Options:** * **Misoprostol (Option A):** A PGE1 analogue used primarily for the prevention of NSAID-induced gastric ulcers and for medical abortion (in combination with mifepristone). It is not used in ophthalmology. * **Enprostil (Option B) & Rioprostil (Option D):** These are PGE2 and PGE1 analogues, respectively. They were developed as anti-secretory agents to treat peptic ulcer disease by inhibiting gastric acid secretion, but they have no role in glaucoma management. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Prostaglandin analogues (Latanoprost, Bimatoprost, Travoprost) lower IOP by increasing uveoscleral outflow, unlike Beta-blockers which decrease aqueous production. * **Specific Side Effects:** A classic "image-based" or clinical question favorite is the triad of side effects: **Increased pigmentation of the iris** (permanent), **thickening/darkening of eyelashes** (trichomegaly), and **periorbital fat atrophy**. * **Prodrug Status:** Latanoprost is administered as a prodrug to enhance corneal penetration and is hydrolyzed by esterases in the cornea to its active form.

Q: A patient is taking ketoconazole for fungal infection develops a cold for which he is prescribed terfenadine. What is the likely drug interaction between terfenadine and ketoconazole?

Ketoconazole decreases the metabolism of terfenadine.. ### Explanation **1. Why Option A is Correct:** The interaction between ketoconazole and terfenadine is a classic example of **enzyme inhibition**. Ketoconazole is a potent inhibitor of the hepatic microsomal enzyme **CYP3A4**. Terfenadine is a pro-drug that is normally rapidly metabolized by CYP3A4 into its active, non-toxic metabolite (fexofenadine). When ketoconazole is co-administered, it blocks the metabolism of terfenadine, leading to significantly elevated plasma levels of the parent drug. **2. Why the Other Options are Incorrect:** * **Option B:** Terfenadine does not significantly affect the metabolic enzymes responsible for ketoconazole clearance; the interaction is unidirectional. * **Option C:** This is the opposite of the actual effect. Ketoconazole *increases* terfenadine levels by preventing its breakdown. * **Option D:** There is a major, life-threatening interaction between these two drugs, which led to the withdrawal of terfenadine from many markets. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mechanism of Toxicity:** High levels of terfenadine (and astemizole) block the **delayed rectifier potassium channels (IKr)** in the heart. * **Clinical Outcome:** This leads to **QT interval prolongation**, which can progress to a life-threatening polymorphic ventricular tachycardia known as **Torsades de Pointes**. * **The "Safe" Alternative:** **Fexofenadine** (the active metabolite of terfenadine) is used clinically because it does not block potassium channels and does not cause cardiotoxicity, even in the presence of CYP3A4 inhibitors. * **Other CYP3A4 Inhibitors:** Be wary of similar interactions with Erythromycin, Clarithromycin, and Grapefruit juice.

Q: Which of the following drugs does NOT cause curare-like effects?

Chloramphenicol. ### Explanation The question tests your knowledge of **drug-induced neuromuscular blockade (NMB)**. Certain antibiotics can potentiate the effects of skeletal muscle relaxants or cause "curare-like" effects by interfering with the release of acetylcholine (ACh) or reducing the sensitivity of the motor endplate. **1. Why Chloramphenicol is correct:** **Chloramphenicol** is a protein synthesis inhibitor (50S subunit) that does **not** interfere with neuromuscular transmission. It lacks the chemical structure and mechanism required to inhibit calcium entry or ACh release at the motor nerve terminal. Therefore, it does not cause curare-like paralysis. **2. Why the other options are incorrect:** * **Streptomycin (Aminoglycosides):** This is the most notorious group for this effect. Aminoglycosides (like Streptomycin, Neomycin, Gentamicin) inhibit the pre-junctional release of ACh by competing with **Calcium ions** at the voltage-gated channels. * **Polymyxin (B and Colistin):** These exert a potent "curare-like" effect via a non-competitive blockade at the post-junctional membrane. Their effect is often more difficult to reverse than that of aminoglycosides. * **Tetracycline:** These can cause neuromuscular blockade, likely by chelating calcium ions which are essential for ACh release. **NEET-PG High-Yield Pearls:** * **Reversal:** Aminoglycoside-induced blockade can often be reversed by **Intravenous Calcium Gluconate** (which overcomes the competition) or Neostigmine. * **Contraindication:** These antibiotics should be used with extreme caution in patients with **Myasthenia Gravis** or those receiving neuromuscular blockers (like Succinylcholine or Vecuronium) during surgery. * **Mnemonic:** "Antibiotics that block the NMJ" – **A**minoglycosides, **P**olymyxins, **T**etracyclines, and **L**incosamides (Clindamycin).

Q: A patient receiving allopurinol requires dose reduction of which of the following medications?

6-Mercaptopurine. The correct answer is **6-Mercaptopurine (6-MP)**. This is a classic high-yield drug interaction based on the inhibition of drug metabolism. **Mechanism of Interaction:** 6-Mercaptopurine is an antimetabolite (purine analog) [2] primarily metabolized by the enzyme **Xanthine Oxidase (XO)** into inactive metabolites (6-thiouric acid) [1]. **Allopurinol** is a potent inhibitor of Xanthine Oxidase [1]. When administered concurrently, allopurinol prevents the degradation of 6-MP, leading to toxic plasma levels and severe bone marrow suppression. Therefore, the dose of 6-MP must be reduced to **25–33% (one-fourth to one-third)** of the original dose. **Analysis of Incorrect Options:** * **B. Cyclophosphamide:** This is an alkylating agent metabolized by hepatic CYP450 enzymes, not Xanthine Oxidase. While allopurinol may slightly increase its bone marrow toxicity via different mechanisms, it does not require a standard dose reduction. * **C. 6-Thioguanine:** Unlike 6-MP, 6-Thioguanine is primarily metabolized by **deamination** (guanase) and S-methylation, rather than Xanthine Oxidase [1]. Thus, it can be used at standard doses with allopurinol. * **D. Cimetidine:** This is an H2 blocker and a general CYP450 inhibitor. It does not share a metabolic pathway with allopurinol. **NEET-PG High-Yield Pearls:** 1. **Azathioprine:** Since Azathioprine is a prodrug converted into 6-MP, it also requires a similar dose reduction when given with allopurinol. 2. **Febuxostat:** Like allopurinol, this non-purine XO inhibitor also interacts with 6-MP/Azathioprine. 3. **Tumor Lysis Syndrome:** Allopurinol is often used here to prevent hyperuricemia, making this interaction clinically significant during chemotherapy.

Q: Which of the following drugs has the least affinity for CYP3A4?

Rabeprazole. **Explanation:** The correct answer is **Rabeprazole**. **1. Underlying Medical Concept:** Proton Pump Inhibitors (PPIs) are primarily metabolized in the liver by the cytochrome P450 system, specifically **CYP2C19** and **CYP3A4**. The degree of reliance on these enzymes determines the potential for drug-drug interactions (e.g., with Clopidogrel or Warfarin). **Rabeprazole** is unique because it is predominantly metabolized via a **non-enzymatic pathway** (reduction to rabeprazole thioether) with only minor involvement of the CYP system. Consequently, it has the least affinity for CYP3A4 and CYP2C19 among the PPIs, making its pharmacokinetics less affected by genetic polymorphisms or concurrent drug use. **2. Analysis of Incorrect Options:** * **Omeprazole (A):** This is the prototype PPI and has the **highest affinity** for CYP2C19 and significant affinity for CYP3A4. It is a potent inhibitor of these enzymes, leading to many drug interactions. * **Esomeprazole (C):** As the S-isomer of omeprazole, it is also heavily metabolized by CYP2C19 and CYP3A4, though it shows slightly improved bioavailability. * **Lansoprazole (B):** This drug is extensively metabolized by both CYP2C19 and CYP3A4. While it has a shorter half-life, its dependence on the CYP system is much higher than that of Rabeprazole. **3. NEET-PG High-Yield Pearls:** * **Drug of Choice for Drug Interactions:** Rabeprazole and **Pantoprazole** are the preferred PPIs for patients on multiple medications (polypharmacy) due to their lower potential for CYP-mediated interactions. * **Clopidogrel Interaction:** Omeprazole inhibits CYP2C19, which prevents the conversion of Clopidogrel (a prodrug) into its active form, increasing the risk of cardiovascular events. * **Genetic Polymorphism:** The efficacy of Omeprazole varies significantly between "Poor Metabolizers" and "Extensive Metabolizers" of CYP2C19; Rabeprazole avoids this variability.

Question 1

Nitrates are contraindicated with which of the following drug classes?

Accepted Answer

Phosphodiesterase-5 (PDE-5) inhibitors

Answer

Sulphonylureas

Answer

Immunomodulators

Answer

Selective Serotonin Reuptake Inhibitors (SSRIs)

Question 2

Which drug may inhibit the cytochrome P450 metabolism of warfarin?

Accepted Answer

Cimetidine

Answer

Ethanol

Answer

Rifampicin

Answer

Procainamide

Question 3

Warfarin therapy is prolonged by all except:

Accepted Answer

Rifampicin

Answer

Amoxicillin

Answer

Erythromycin

Answer

Tetracycline

Question 4

Which of the following prostaglandin analogues is used in the management of glaucoma?

Accepted Answer

Latanoprost

Answer

Misoprostol

Answer

Enprostil

Answer

Rioprostil

Question 5

A patient is taking ketoconazole for fungal infection develops a cold for which he is prescribed terfenadine. What is the likely drug interaction between terfenadine and ketoconazole?

Accepted Answer

Ketoconazole decreases the metabolism of terfenadine.

Answer

Terfenadine increases the levels of ketoconazole.

Answer

Ketoconazole decreases the levels of terfenadine.

Answer

There is no interaction between these drugs.

Question 6

Which of the following drugs does NOT cause curare-like effects?

Accepted Answer

Chloramphenicol

Answer

Polymyxin

Answer

Tetracycline

Answer

Streptomycin

Question 7

A 50-year-old patient diagnosed with pulmonary tuberculosis is started on standard drug therapy. After 1.5 months, the patient develops fatigue, low-grade fever, muscle aches, bone pains, and body aches. The patient also complains of pleuritic chest pain. Sputum tests are negative for acid-fast bacilli. Further investigations reveal the patient is positive for anti-histone antibodies. Abnormality in which of the following metabolic processes is responsible for the patient's current condition?

Accepted Answer

Acetylation

Answer

Sulfation

Answer

Glucuronide conjugation

Answer

Hydroxylation

Question 8

A patient receiving allopurinol requires dose reduction of which of the following medications?

Accepted Answer

6-Mercaptopurine

Answer

Cyclophosphamide

Answer

6-Thioguanine

Answer

Cimetidine

Question 9

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 10

Which of the following drugs has the least affinity for CYP3A4?

Accepted Answer

Rabeprazole

Answer

Omeprazole

Answer

Lansoprazole

Answer

Esomeprazole

Drug Interactions — MCQs

Drug Interactions — MCQs

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