Drug Interactions Practice Questions

Q: Which of the following interacts with Digitalis maximally?

Furosemide. The interaction between **Digitalis (Digoxin)** and diuretics is primarily mediated by changes in serum potassium levels. Digoxin works by inhibiting the **Na+/K+-ATPase pump** on cardiac myocytes [2]. Since potassium competes with Digoxin for the same binding site on this pump, **hypokalemia** (low potassium) increases Digoxin binding, leading to digitalis toxicity [2]. **Why Furosemide is the correct answer:** Furosemide is a potent **Loop Diuretic**. It inhibits the Na+-K+-2Cl- symporter in the thick ascending limb of the Loop of Henle, leading to significant excretion of potassium in the urine [1]. This profound hypokalemia sensitizes the myocardium to Digoxin, "maximally" increasing the risk of life-threatening arrhythmias [1]. **Analysis of Incorrect Options:** * **B, C, and D (Triamterene, Amiloride, Spironolactone):** These are all **Potassium-Sparing Diuretics**. Unlike Furosemide, they increase serum potassium levels. While hyperkalemia can actually *decrease* the effectiveness of Digoxin (by outcompeting it for the pump) [2], it does not carry the same immediate risk of acute toxicity as the hypokalemia induced by Furosemide. In fact, Spironolactone is often used alongside Digoxin in heart failure management to prevent hypokalemia. **NEET-PG High-Yield Pearls:** * **Electrolyte Triad of Digoxin Toxicity:** Hypokalemia, Hypomagnesemia, and Hypercalcemia all increase the risk of toxicity. * **ECG Hallmark:** The most common ECG finding in Digoxin toxicity is **PVCs (Premature Ventricular Contractions)**, while the most characteristic/specific is **Atrial Tachycardia with AV block**. * **Antidote:** Digoxin-specific antibody fragments (**DigiFab/Digibind**). * **Drug of Choice:** For Digoxin-induced arrhythmias, the drug of choice is **Lidocaine** or **Phenytoin**.

Q: A 50-year-old woman with a history of hypothyroidism, currently on treatment, was recently diagnosed with endometrial fibroids and started on iron supplements. She reports excessive fatigue and constipation during her follow-up visit. Her blood pressure and cholesterol levels have remained stable. A TSH level is checked and found to be elevated. What is the most likely cause of her elevated TSH?

Poor absorption of levothyroxine due to ferrous sulfate. **Explanation:** The correct answer is **D. Poor absorption of levothyroxine due to ferrous sulfate.** **Mechanism:** Levothyroxine is a drug with a narrow therapeutic index and its absorption is highly sensitive to the gastric environment. Ferrous sulfate (iron supplements) acts as a **cationic binder**. When taken concurrently, iron forms an insoluble complex (chelation) with levothyroxine in the gastrointestinal tract, significantly reducing its bioavailability. This leads to subtherapeutic serum levels of T4, causing a compensatory rise in **Thyroid Stimulating Hormone (TSH)** and the return of hypothyroid symptoms like fatigue and constipation. **Analysis of Incorrect Options:** * **A & B (Celiac disease/Colon cancer):** While malabsorption (Celiac) or occult blood loss (Colon cancer) can affect thyroid management or iron status, the temporal relationship between starting iron supplements and the rise in TSH strongly points toward a drug-drug interaction. * **C (Medication noncompliance):** While common, the clinical vignette specifically mentions the initiation of a new medication (iron), which is a classic "trigger" for board questions regarding altered drug absorption. **NEET-PG High-Yield Pearls:** * **The "4-Hour Rule":** Patients should be advised to take levothyroxine on an empty stomach, at least 30–60 minutes before breakfast, and separate it from interfering drugs (Iron, Calcium carbonate, Aluminum antacids, Sucralfate, and Proton Pump Inhibitors) by at least **4 hours**. * **Other Chelators:** Besides Iron, **Calcium** and **Bile acid sequestrants** (Cholestyramine) are frequent culprits in reducing levothyroxine absorption. * **Estrogen Connection:** In patients with fibroids, if they were started on Oral Contraceptive Pills (OCPs), TSH might rise because estrogen increases **Thyroid Binding Globulin (TBG)**, reducing the free (active) fraction of T4.

Q: 6-Mercaptopurine (6-MP), frequently used in drug regimens for neoplastic disease, is metabolized by xanthine oxidase. Major dose reductions are advised in patients who are being treated with which of the following drugs that inhibit xanthine oxidase?

Allopurinol. The correct answer is **Allopurinol**. **Mechanism of Interaction:** 6-Mercaptopurine (6-MP) is a purine analog used in the treatment of leukemia [3]. Its primary metabolic pathway involves oxidation by the enzyme **xanthine oxidase (XO)** into inactive 6-thiouric acid [1]. **Allopurinol** is a potent inhibitor of xanthine oxidase (clinically used for gout) [2]. When administered concurrently, Allopurinol prevents the degradation of 6-MP, leading to toxic systemic levels of the chemotherapy drug [1]. This results in severe, life-threatening bone marrow suppression. Therefore, if a patient requires both drugs, the dose of 6-MP must be reduced to **25–33% (1/3rd to 1/4th)** of the original dose. **Analysis of Incorrect Options:** * **A. Cimetidine:** An H2-receptor antagonist and a known microsomal enzyme (Cytochrome P450) inhibitor, but it does not inhibit xanthine oxidase. * **B. Sulfinpyrazone:** A uricosuric agent that inhibits the reabsorption of uric acid in the proximal tubule; it does not inhibit the synthesis of uric acid via xanthine oxidase . * **D. Indomethacin:** A non-steroidal anti-inflammatory drug (NSAID) used for acute gouty arthritis; it inhibits cyclooxygenase (COX) but has no effect on purine metabolism. **NEET-PG High-Yield Pearls:** * **Azathioprine**, a prodrug of 6-MP, follows the same metabolic rule. Its dose must also be reduced when given with Allopurinol or Febuxostat. * **Febuxostat** is a newer, non-purine selective inhibitor of xanthine oxidase that carries the same interaction risk. * **Thiopurine Methyltransferase (TPMT):** This is the other major enzyme that metabolizes 6-MP. Genetic deficiency of TPMT also leads to 6-MP toxicity. * **Drug of Choice:** Allopurinol is the drug of choice for preventing **Tumor Lysis Syndrome** during chemotherapy [1].

Q: In HIV patients on Stavudine, Lamivudine, and Didanosine, which antitubercular therapy (ATT) drug should not be given concurrently?

Rifampicin. ### Explanation The correct answer is **Rifampicin (Option B)**. **Why Rifampicin is the correct answer:** Rifampicin is a potent **inducer of the Cytochrome P450 (CYP3A4)** enzyme system in the liver. In patients on Antiretroviral Therapy (ART), Rifampicin significantly accelerates the metabolism of various HIV drugs, particularly Protease Inhibitors (PIs) and Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs). While the drugs mentioned in the question (Stavudine, Lamivudine, and Didanosine) are **NRTIs** (which are primarily renally excreted and less affected by CYP induction), clinical guidelines generally advise against using Rifampicin-based regimens in complex HIV cases due to the high risk of drug-drug interactions and the potential for sub-therapeutic levels of other ART components. In practice, **Rifabutin** is often used as a substitute for Rifampicin because it is a much weaker enzyme inducer. **Why other options are incorrect:** * **Ethambutol (A), Isoniazid (C), and Pyrazinamide (D):** These drugs do not significantly induce or inhibit the hepatic microsomal enzymes. They do not have major pharmacokinetic interactions with NRTIs like Stavudine or Lamivudine and can be safely administered in a standard ATT regimen alongside ART. **High-Yield Clinical Pearls for NEET-PG:** 1. **Rifampicin vs. Rifabutin:** Rifampicin is a *universal inducer*; Rifabutin is preferred in HIV patients on PIs. 2. **Overlapping Toxicities:** Both **Stavudine** and **Isoniazid** can cause peripheral neuropathy. If used together, pyridoxine (Vit B6) supplementation is mandatory. 3. **Didanosine + Tenofovir:** This combination is avoided as it increases Didanosine levels, leading to toxicity (pancreatitis). 4. **Efavirenz:** This is the NNRTI of choice when Rifampicin must be used, though the dose may need adjustment.

Q: Which of the following inhibits CYP-P450?

Grapefruit juice. **Explanation:** The correct answer is **Grapefruit juice**. **1. Why Grapefruit juice is correct:** Grapefruit juice contains compounds called **furanocoumarins** (e.g., bergamottin), which are potent **inhibitors** of the **CYP3A4** enzyme, primarily in the intestinal wall. By inhibiting this enzyme, grapefruit juice reduces the first-pass metabolism of many drugs, leading to increased plasma concentrations and potential toxicity. Common drugs affected include statins (Atorvastatin), calcium channel blockers (Amlodipine), and immunosuppressants (Cyclosporine). **2. Why the other options are incorrect:** * **Ethanol:** Chronic alcohol consumption is a well-known **enzyme inducer** (specifically CYP2E1). However, acute large doses can act as an inhibitor. In the context of standard pharmacology exams, ethanol is typically categorized as an inducer. * **Rifampicin:** This is one of the most potent **broad-spectrum enzyme inducers**. It increases the synthesis of multiple CYP enzymes (CYP3A4, 2C9, 2C19), leading to decreased efficacy of drugs like oral contraceptives and warfarin. * **Procainamide:** This is a Class 1A antiarrhythmic drug. It is metabolized via **acetylation** (by the NAT2 enzyme), not primarily through the CYP-P450 system, and it does not act as a significant inducer or inhibitor. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Inhibitors (VITAMIN K):** **V**alproate, **I**soniazid, **T**ame (Cimetidine), **A**miodarone, **M**acrolides (except Azithromycin), **I**ndinavir, **N**on-DHP CCBs (Verapamil/Diltiazem), **K**etoconazole (and Grapefruit juice). * **Mnemonic for Enzyme Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Note:** Erythromycin inhibits CYP enzymes, but **Azithromycin** does not, making it a safer choice to avoid drug interactions.

Q: Which of the following drugs does NOT affect warfarin activity?

Propylthiouracil. Warfarin is a narrow-therapeutic-index anticoagulant metabolized primarily by the hepatic cytochrome P450 (CYP) enzyme system (specifically **CYP2C9**) [2], [3]. Its activity is highly sensitive to drugs that induce or inhibit these enzymes. **Why Propylthiouracil (PTU) is the correct answer:** Propylthiouracil is an antithyroid drug used to treat hyperthyroidism. While thyroid status itself can influence the turnover of clotting factors (hyperthyroidism increases turnover, enhancing warfarin effect), **PTU does not have a direct pharmacokinetic interaction** with warfarin via the CYP450 system. Therefore, it does not inherently "affect" warfarin activity in the same manner as enzyme inducers or inhibitors. **Analysis of Incorrect Options:** * **Rifampicin:** A potent **CYP450 inducer**. It increases the metabolism of warfarin, leading to decreased plasma levels and a **reduced** anticoagulant effect (decreased INR) [2]. * **Phenytoin:** Another strong **CYP450 inducer**. Similar to rifampicin, it accelerates warfarin clearance, necessitating a higher dose of warfarin to maintain therapeutic anticoagulation. * **Amiodarone:** A potent **CYP450 inhibitor** (specifically CYP2C9 and 3A4) [1]. It slows down warfarin metabolism, leading to increased plasma levels and an **enhanced** anticoagulant effect, significantly increasing the risk of bleeding [1], [2]. **High-Yield Clinical Pearls for NEET-PG:** * **S-Warfarin** is 3-5 times more potent than R-warfarin and is metabolized by **CYP2C9** [3]. * **Common Inhibitors (Increase INR):** Valproate, Erythromycin, Azoles, Cimetidine, Amiodarone (**VECA**). * **Common Inducers (Decrease INR):** Phenytoin, Carbamazepine, Rifampicin, Griseofulvin, Barbiturates (**PCR-GB**). * **Broad-spectrum antibiotics** can also enhance warfarin activity by killing gut flora that synthesize Vitamin K.

Q: Tricyclic antidepressants can alter the oral absorption of many drugs by which of the following mechanisms?

Altering gut motility. **Explanation:** **Mechanism of Action (Correct Answer):** Tricyclic antidepressants (TCAs), such as Amitriptyline and Imipramine, possess significant **antimuscarinic (anticholinergic) properties**. By blocking muscarinic receptors in the gastrointestinal tract, they decrease parasympathetic stimulation, leading to **delayed gastric emptying and reduced intestinal motility**. Since the rate of drug absorption is highly dependent on gastric emptying time and intestinal transit, TCAs can significantly alter the pharmacokinetics of co-administered drugs. For instance, delayed emptying may slow the absorption of drugs like paracetamol, or conversely, increase the total absorption of drugs that are poorly soluble by allowing them more time to dissolve in the gut. **Analysis of Incorrect Options:** * **Option A (Complexing):** This refers to chemical interactions (chelation) in the lumen, typical of drugs like **Cholestyramine** or **Antacids** (which bind to Tetracyclines/Fluoroquinolones), not TCAs. * **Option C (Altering gut flora):** This is a mechanism associated with **broad-spectrum antibiotics**, which can interfere with the enterohepatic circulation of drugs like oral contraceptives. * **Option D (Damaging gut mucosa):** This is characteristic of cytotoxic **chemotherapeutic agents** (e.g., Methotrexate) or chronic NSAID use, leading to malabsorption syndromes. **NEET-PG High-Yield Pearls:** * **Anticholinergic Toxidrome:** Remember the mnemonic "Blind as a bat, Mad as a hatter, Red as a beet, Hot as a hare, **Bowel and bladder lose their tone**." * **Other drugs causing decreased motility:** Opioids, Atropine, and Phenothiazines. * **Prokinetic agents:** Drugs like Metoclopramide *increase* motility and can speed up the absorption of other drugs.

Question 1

Which of the following interacts with Digitalis maximally?

Accepted Answer

Furosemide

Answer

Triamterene

Answer

Amiloride

Answer

Spironolactone

Question 2

Indomethacin can antagonize the diuretic action of loop diuretics by which mechanism?

Accepted Answer

Preventing prostaglandin-mediated intrarenal-hemodynamic actions

Answer

Blocking the action in the ascending limb of the loop of Henle

Answer

Enhancing salt and water reabsorption in distal tubules

Answer

Increasing aldosterone secretion

Question 3

A 50-year-old woman with a history of hypothyroidism, currently on treatment, was recently diagnosed with endometrial fibroids and started on iron supplements. She reports excessive fatigue and constipation during her follow-up visit. Her blood pressure and cholesterol levels have remained stable. A TSH level is checked and found to be elevated. What is the most likely cause of her elevated TSH?

Accepted Answer

Poor absorption of levothyroxine due to ferrous sulfate

Answer

Celiac disease

Answer

Colon cancer

Answer

Medication noncompliance

Question 4

6-Mercaptopurine (6-MP), frequently used in drug regimens for neoplastic disease, is metabolized by xanthine oxidase. Major dose reductions are advised in patients who are being treated with which of the following drugs that inhibit xanthine oxidase?

Accepted Answer

Allopurinol

Answer

Cimetidine

Answer

Sulfinpyrazone

Answer

Indomethacin

Question 5

In HIV patients on Stavudine, Lamivudine, and Didanosine, which antitubercular therapy (ATT) drug should not be given concurrently?

Accepted Answer

Rifampicin

Answer

Ethambutol

Answer

Isoniazid (INH)

Answer

Pyrazinamide

Question 6

Which of the following inhibits CYP-P450?

Accepted Answer

Grapefruit juice

Answer

Ethanol

Answer

Rifampicin

Answer

Procainamide

Question 7

A 35-year-old male with a history of schizophrenia is on aripiprazole and metoclopramide for 3 months. He now presents with complaints of gait disturbance (akinesia while walking) and tremors when sitting. What is the drug of choice for this condition?

Accepted Answer

Benzhexol

Answer

Amantadine

Answer

Bromocriptine

Answer

Entacapone

Question 8

The anticoagulant action of warfarin is potentiated by all of the following EXCEPT:

Accepted Answer

Both 2 and 3

Answer

Phenylbutazone

Answer

Vitamin K

Answer

Rifampicin

Question 9

Which of the following drugs does NOT affect warfarin activity?

Accepted Answer

Propylthiouracil

Answer

Rifampicin

Answer

Phenytoin

Answer

Amiodarone

Question 10

Tricyclic antidepressants can alter the oral absorption of many drugs by which of the following mechanisms?

Accepted Answer

Altering gut motility

Answer

Complexing with other drugs in the intestinal lumen

Answer

Altering gut flora

Answer

Damaging gut mucosa

Drug Interactions — MCQs

Drug Interactions — MCQs

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