Digoxin is contraindicated in
**Core Concept**
Digoxin is a cardiac glycoside used to treat heart failure and atrial fibrillation by increasing the force of cardiac contractions. Its efficacy is limited by its narrow therapeutic index and potential toxicity, which can be life-threatening.
**Why the Correct Answer is Right**
Digoxin toxicity is often precipitated by hypokalemia, which increases the binding of digoxin to the Na+/K+-ATPase pump in cardiac cells, leading to an increase in intracellular calcium and, consequently, an increase in cardiac contractility. Hypokalemia also prolongs the action potential duration and refractory period, making the heart more susceptible to arrhythmias. Therefore, digoxin is contraindicated in conditions that may lead to hypokalemia, such as potassium-wasting diuretics.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because digoxin is not contraindicated in heart failure; in fact, it is often used to treat this condition.
**Option B:** This option is incorrect because digoxin is not contraindicated in atrial fibrillation; it is often used to control ventricular rate in this condition.
**Option C:** This option is incorrect because digoxin is not contraindicated in hyperthyroidism; in fact, it is often used to treat atrial fibrillation and heart failure in patients with hyperthyroidism.
**Clinical Pearl / High-Yield Fact**
It is essential to monitor potassium levels regularly in patients taking digoxin, as even mild hypokalemia can increase the risk of digoxin toxicity.
**Correct Answer: C. Hyperthyroidism.**