**Core Concept**
A pacemaker is a medical device that uses electrical impulses to regulate the beating of the heart. It is indicated in conditions where the heart's natural pacemaker, the sinoatrial (SA) node, fails to generate adequate impulses or conducts them normally to the ventricles.

**Why the Correct Answer is Right**
Pacemakers are used to treat bradycardia, which is a condition characterized by a slow heart rate. The device helps to increase the heart rate by generating electrical impulses that stimulate the ventricles to contract. Pacemakers are particularly useful in conditions such as complete heart block (third-degree atrioventricular block) and sick sinus syndrome, where the SA node fails to generate adequate impulses.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a pacemaker is not indicated in conditions where the heart rate is normal or increased, such as tachycardia.
**Option B:** This option is incorrect because a pacemaker is not indicated in conditions where the heart's natural pacemaker is functioning normally, such as in a healthy individual.
**Option C:** This option is incorrect because a pacemaker is not indicated in conditions where the heart's natural pacemaker is functioning normally, such as in a healthy individual.

**Clinical Pearl / High-Yield Fact**
One of the key indications for a pacemaker is the presence of symptoms such as fatigue, weakness, and shortness of breath due to bradycardia. A pacemaker can significantly improve the quality of life for individuals with these symptoms by increasing the heart rate and improving cardiac output.

**Correct Answer: A. Bradycardia**