**Question:** Therapeutic plasma level of the digoxin is

A. 0.5-2 ng/mL
B. 0.5-2.5 ng/mL
C. 0.5-4 ng/mL
D. 0.5-5 ng/mL

**Core Concept:** Digoxin is a cardiac glycoside used for the management of heart failure and atrial fibrillation. It is a potent inhibitor of Na+/K+-ATPase, leading to increased intracellular calcium levels and ultimately exerting its effects on the heart.

**Why the Correct Answer is Right:** The therapeutic plasma level of digoxin is crucial for its optimal efficacy and minimal side effects. The correct answer, A (0.5-2 ng/mL), represents the narrow therapeutic range that ensures therapeutic effects without causing toxicity. A higher range (B, C, D) would increase the risk of adverse events due to increased intracellular calcium levels and cardiac irritability.

**Why Each Wrong Option is Incorrect:**

- Option B (0.5-2.5 ng/mL): While a higher range is better than option A, the upper limit (2.5 ng/mL) is too high, increasing the risk of toxicity.
- Option C (0.5-4 ng/mL): The lower limit is too low, making it less effective in achieving the desired therapeutic effects.
- Option D (0.5-5 ng/mL): Similar to option B, the upper limit is too high, causing increased risk of toxicity.

**Why the Correct Answer is Right:** The appropriate therapeutic plasma level (0.5-2 ng/mL) ensures that digoxin achieves its desired effects without causing significant toxicity. The narrow range minimizes the risk of side effects and allows for optimal cardiac glycoside action.

**Clinical Pearl:** Regular monitoring of digoxin levels is essential to maintain the therapeutic range and avoid toxicity. Rapid fluctuations in digoxin levels can lead to digoxin-induced toxicity, which presents with symptoms like nausea, vomiting, diarrhea, abdominal pain, and in severe cases, bradycardia, low blood pressure, and cardiac arrest.

Digoxin is essential to treat heart failure and atrial fibrillation, but it requires careful monitoring to prevent toxicity that can be life-threatening.