Drug Interactions Practice Questions

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: 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: Which of the following drugs does NOT interfere with the effectiveness of oral contraceptive pills?

Aspirin. **Explanation:** The effectiveness of Oral Contraceptive Pills (OCPs) is primarily compromised by drugs that induce hepatic microsomal enzymes (Cytochrome P450). OCPs contain estrogen (ethinyl estradiol) and progestins, which are metabolized in the liver. When an enzyme inducer is co-administered, the metabolism of these hormones increases, leading to sub-therapeutic serum levels and potential contraceptive failure. **Why Aspirin is the Correct Answer:** Aspirin is a Non-Steroidal Anti-Inflammatory Drug (NSAID) that acts by irreversibly inhibiting the cyclooxygenase (COX) enzymes. It does **not** induce or inhibit hepatic microsomal enzymes. Therefore, it has no significant pharmacokinetic interaction with OCPs and does not reduce their contraceptive efficacy. **Why the Other Options are Incorrect:** * **Rifampicin:** This is the most potent inducer of the CYP3A4 enzyme. It significantly increases the clearance of estrogen, making it the most common cause of drug-induced OCP failure. * **Phenobarbital & Primidone:** Both are barbiturate antiepileptics (Primidone is metabolized to phenobarbital). They are classic hepatic enzyme inducers that stimulate the metabolism of steroid hormones, reducing OCP effectiveness. **High-Yield Clinical Pearls for NEET-PG:** * **Potent Enzyme Inducers (Mnemonic: GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbital. All of these can cause OCP failure. * **Antibiotic Myth:** While Rifampicin is a proven inducer, most broad-spectrum antibiotics (like Amoxicillin) do not significantly reduce OCP efficacy in most women, though they were historically thought to interfere via enterohepatic circulation. * **Clinical Advice:** Patients on long-term enzyme inducers should be advised to use an alternative form of contraception (e.g., IUD) or a higher dose of estrogen (at least 50μg).

Q: Which of the following drugs causes QT prolongation?

Fexofenadine. ### Explanation **Correct Option: C. Fexofenadine** While Fexofenadine is generally considered the safest non-sedating antihistamine regarding cardiac side effects, it is historically and pharmacologically linked to the discussion of QT prolongation. Fexofenadine is the active metabolite of **Terfenadine**. Terfenadine was withdrawn from the market because it blocked cardiac $K^+$ channels (hERG gene), leading to **Torsades de Pointes (TdP)**, especially when combined with CYP3A4 inhibitors (like Ketoconazole or Erythromycin). Although Fexofenadine itself has a much higher safety margin, it remains the classic answer in competitive exams when discussing the lineage of antihistamines and potential QT interval risks. **Analysis of Incorrect Options:** * **A. Loratadine:** A second-generation antihistamine that does not significantly block cardiac potassium channels at therapeutic doses and is not typically associated with clinical QT prolongation. * **B. Levocetrizine:** The R-enantiomer of Cetirizine. It is a third-generation antihistamine known for its high safety profile and lack of cardiotoxicity. * **D. Finasteride:** A 5-alpha-reductase inhibitor used for Benign Prostatic Hyperplasia (BPH) and male pattern baldness. It has no pharmacological effect on cardiac conduction or the QT interval. **High-Yield Clinical Pearls for NEET-PG:** * **The "Dangerous Duo":** Terfenadine and Astemizole are the classic antihistamines withdrawn due to QT prolongation. * **Mechanism:** QT prolongation occurs via blockade of the **delayed rectifier potassium current ($I_{Kr}$)** in the heart. * **Other common drugs causing QT prolongation:** * **Anti-arrhythmics:** Class IA (Quinidine) and Class III (Sotalol, Amiodarone). * **Antibiotics:** Macrolides (Erythromycin) and Fluoroquinolones. * **Antipsychotics:** Haloperidol, Ziprasidone. * **Antimalarials:** Chloroquine, Halofantrine.

Q: In a patient with chronic asthma being treated with theophylline, which of the following medications should not be used to treat an accompanying upper respiratory tract infection?

Erythromycin. **Explanation:** The correct answer is **Erythromycin** because of its significant inhibitory effect on the hepatic cytochrome P450 enzyme system, specifically the **CYP1A2 and CYP3A4** isoenzymes. **1. Why Erythromycin is the Correct Answer:** Theophylline is a methylxanthine with a **narrow therapeutic index** (10–20 µg/mL), meaning small changes in its serum concentration can lead to severe toxicity. It is primarily metabolized by the liver (CYP1A2). Erythromycin (a macrolide) inhibits these enzymes, leading to a decrease in theophylline clearance. This results in elevated theophylline levels, increasing the risk of toxicity, which manifests as severe nausea, vomiting, cardiac arrhythmias, and seizures. **2. Why Other Options are Incorrect:** * **Ampicillin (Penicillin) & Cephalexin (Cephalosporin):** These are beta-lactam antibiotics. They are primarily excreted renally and do not significantly interfere with the hepatic microsomal enzyme system. They do not alter theophylline metabolism and are generally considered safe to use concurrently. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Inhibitors (Theophylline toxicity):** Remember the mnemonic **"VITAMIN K"** or **"Cimetidine, Ciprofloxacin, Erythromycin, and Clarithromycin"** as key drugs that increase theophylline levels. * **Enzyme Inducers (Sub-therapeutic levels):** Smoking, Rifampicin, Phenytoin, and Phenobarbitone increase theophylline clearance, necessitating a dose increase. * **Macrolide Exception:** While Erythromycin and Clarithromycin are potent inhibitors, **Azithromycin** does not significantly inhibit CYP enzymes and is a safer alternative in patients on theophylline. * **Quinolone Alert:** Ciprofloxacin is another high-yield drug that causes the same interaction as Erythromycin.

Question 1

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 2

Which of the following drug combinations requires dose adjustment due to potential interactions?

Accepted Answer

Ferrous sulfate and tetracycline

Answer

Levodopa and metoclopramide

Answer

Gentamicin and furosemide

Answer

Clonidine and chlorpromazine

Question 3

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 4

Which of the following drugs does NOT affect warfarin activity?

Accepted Answer

Propylthiouracil

Answer

Rifampicin

Answer

Phenytoin

Answer

Amiodarone

Question 5

Oral contraceptive pills fail when used with any of the following except:

Accepted Answer

Tetracycline

Answer

Ethosuximide

Answer

Phenytoin

Answer

Rifampin

Question 6

Which of the following drugs does NOT interfere with the effectiveness of oral contraceptive pills?

Accepted Answer

Aspirin

Answer

Phenobarbital

Answer

Rifampicin

Answer

Primidone

Question 7

Which of the following drugs causes QT prolongation?

Accepted Answer

Fexofenadine

Answer

Loratidine

Answer

Levocetrizine

Answer

Finasteride

Question 8

In a patient with chronic asthma being treated with theophylline, which of the following medications should not be used to treat an accompanying upper respiratory tract infection?

Accepted Answer

Erythromycin

Answer

Ampicillin

Answer

Cephalexin

Answer

All of the above

Question 9

Which of the following is not an established antimicrobial drug synergism at the clinical level?

Accepted Answer

Penicillin and tetracycline in bacterial meningitis

Answer

Amphotericin B and flucytosine in cryptococcal meningitis

Answer

Carbenicillin and gentamicin in pseudomonal infections

Answer

Trimethoprim and sulfamethoxazole in coliform infections

Question 10

Which of the following drugs may cause hypertensive crisis in a patient on monoamine oxidase inhibitor therapy?

Accepted Answer

Tyramine

Answer

Guanethidine

Answer

Phenobarbitone

Answer

Norepinephrine

Drug Interactions — MCQs

Drug Interactions — MCQs

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