**Question:** In a patient with chronic atrial fibrillation with a regular beat of 60/min, the most probable cause is:

A. Structural heart disease
B. Hypothyroidism
C. Hyperthyroidism
D. Anemia

**Core Concept:** Atrial fibrillation is a type of arrhythmia characterized by the rapid, irregular contractions of the atria. In chronic atrial fibrillation, the heart rate is often regular, which differentiates it from other types of arrhythmias.

**Why the Correct Answer is Right:** The correct answer is A (Structural heart disease) because structural heart disease can lead to a regular rhythm due to the presence of a source of re-entry circuit in the heart, which results in a regular atrial contraction rate. In this case, the regular rate of 60/min indicates a slow ventricular response to the rapid atrial contractions.

**Why Each Wrong Option is Incorrect:**

Option B (Hypothyroidism) is incorrect because hypothyroidism typically causes bradycardia (slow heart rate) rather than increased heart rate. Hypothyroidism affects the autonomic nervous system and impairs the heart's ability to respond to the stimuli from the sympathetic nervous system, leading to a reduced heart rate.

Option C (Hyperthyroidism) is incorrect because hyperthyroidism results in tachycardia (increased heart rate) due to the enhanced response of the sympathetic nervous system. The increased production of thyroid hormones leads to an increased heart rate.

Option D (Anemia) is incorrect because anemia does not specifically affect the heart rate in a consistent manner. Anemia can lead to compensatory mechanisms in the cardiovascular system, but the precise relationship between anemia and heart rate is not straightforward and may vary depending on the cause and severity of anemia.

**Clinical Pearl:** When assessing a patient with irregular heart rates, it is essential to look for the underlying cause, as different conditions may lead to similar presentations. In chronic atrial fibrillation, the regular rhythm indicates a slow ventricular response to the rapid atrial contractions, which helps differentiate it from other causes of regular heart rates.