**Core Concept**
Atrial fibrillation is a type of cardiac arrhythmia characterized by rapid and irregular heartbeats due to abnormal electrical activity in the atria. The underlying cause can be due to various factors including heart disease, high blood pressure, or other underlying medical conditions.

**Why the Correct Answer is Right**
In a patient with chronic atrial fibrillation, a regular beat of 60/min suggests that the ventricular response is being controlled by a fixed rate, likely due to the presence of a junctional or ventricular escape rhythm. This can occur when the atria are not contributing to the heart rhythm, and the ventricles are taking over, resulting in a regular but abnormal heart rhythm.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option may be incorrect as it doesn't directly relate to the underlying cause of a regular beat in a patient with atrial fibrillation.
* **Option B:** This option is incorrect as it doesn't provide a plausible explanation for a regular beat in a patient with atrial fibrillation.
* **Option D:** This option is incorrect as it doesn't take into account the underlying cause of a regular beat in a patient with atrial fibrillation.

**Clinical Pearl / High-Yield Fact**
In patients with atrial fibrillation, a regular beat can be a sign of a junctional or ventricular escape rhythm, which can occur when the atria are not contributing to the heart rhythm.

**Correct Answer: C. Atrioventricular (AV) block or junctional rhythm.**