## **Core Concept**
Sinus bradycardia is a type of abnormal heart rhythm that originates from the sinoatrial (SA) node, characterized by a slower than normal heart rate. The SA node acts as the heart's natural pacemaker. Sinus bradycardia is diagnosed based on specific electrocardiogram (ECG) findings and a heart rate below the normal range.

## **Why the Correct Answer is Right**
The correct answer defines sinus bradycardia as a heart rate of less than 60 beats per minute (bpm). This is because a normal adult heart rate ranges from 60 to 100 bpm. A heart rate below 60 bpm, in the absence of other complicating factors, is considered bradycardic if it originates from the SA node. This condition can be physiological (e.g., in athletes) or pathological (e.g., due to heart block, inflammation, or ischemia).

## **Why Each Wrong Option is Incorrect**
- **Option A:** A heart rate of 60-100 bpm is considered normal; therefore, any option suggesting a range within this normal limit cannot define bradycardia.
- **Option B:** This option might suggest a range that overlaps with normal heart rates or is not specific enough to define bradycardia accurately.
- **Option D:** A heart rate of more than 100 bpm is considered tachycardia; thus, an option stating a rate above this threshold cannot define bradycardia.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that sinus bradycardia can be a normal finding in well-conditioned athletes or during sleep. However, in other contexts, it may indicate underlying pathology such as hypothyroidism, increased intracranial pressure, or myocardial ischemia. Always consider the clinical context when evaluating bradycardia.

## **Correct Answer:** C. Less than 60 bpm.