**Core Concept**
Digitalis is a cardiac glycoside used to increase the force of cardiac contractions and slow the heart rate. In atrial flutter, digitalis can convert the rhythm to sinus rhythm, but it can also produce unwanted effects.

**Why the Correct Answer is Right**
Digitalis works by inhibiting the sodium-potassium ATPase pump, leading to an increase in intracellular calcium and an increase in contractility. In the context of atrial flutter, digitalis can help to convert the rhythm to sinus rhythm by increasing the atrioventricular (AV) node refractory period, which can slow or block the conduction of the abnormal electrical impulses. However, digitalis can also produce unwanted effects such as **Option A:** increased atrioventricular block, which can be a limiting factor in its use.

**Why Each Wrong Option is Incorrect**
* **Option A:** Increased atrioventricular block is a potential side effect of digitalis, but it is not the most common effect when given to a patient with atrial flutter.
* **Option B:** Atrial fibrillation is a different type of arrhythmia that can be caused by digitalis toxicity, but it is not the expected result of giving digitalis to a patient with atrial flutter.
* **Option C:** Bradycardia is a potential side effect of digitalis, but it is not the most common effect when given to a patient with atrial flutter.

**Clinical Pearl / High-Yield Fact**
When using digitalis in patients with atrial flutter, it is essential to monitor the patient closely for signs of increased atrioventricular block, which can be a limiting factor in its use.

**Correct Answer: C. Atrial fibrillation.**