**Question:** Which of the following drugs shows "Therapeutic window" phenomenon?

A. Warfarin
B. Isoniazid
C. Digoxin
D. Morphine

**Core Concept:** The "Therapeutic window" phenomenon refers to a narrow range of serum concentrations of a drug where therapeutic effects are achieved while minimizing side effects. This concept is important for ensuring optimal treatment outcomes and reducing the risk of adverse reactions.

**Why the Correct Answer is Right:**

Correct Answer: C. Digoxin

Digoxin is a cardiac glycoside used in the treatment of heart failure and atrial fibrillation. Its therapeutic window is narrow due to its narrow therapeutic index. A small change in serum digoxin concentration can result in either therapeutic effects or severe toxicity, making it essential to monitor serum digoxin levels closely and adjust the dosage accordingly.

**Why Each Wrong Option is Incorrect:**

A. Warfarin (Option A) - Warfarin is an anticoagulant الدواء المستخدمة لمنع تجلط الدم. Its therapeutic window is broad, as it requires regular monitoring of prothrombin time (PT) to maintain the desired anticoagulant effect without causing bleeding complications.

B. Isoniazid (Option B) - Isoniazid is an antibiotic used in the treatment of tuberculosis. Its therapeutic window is broad as well, as it requires monitoring liver function tests (LFTs) to ensure safe dosage adjustments without hepatotoxicity.

D. Morphine (Option D) - Morphine is an opioid analgesic used for pain relief. Its therapeutic window is broad because it primarily targets pain relief and does not have specific toxicity concerns related to serum concentration.

**Clinical Pearl:** Understanding the therapeutic windows of various medications is crucial for safe and effective drug therapy. Monitoring serum concentrations or specific laboratory values allows healthcare providers to adjust dosages accordingly and minimize the risk of adverse reactions (e.g., digoxin toxicity) while ensuring optimal therapeutic effects (e.g., warfarin, isoniazid).