**Core Concept**
A short P interval and delta waves on an ECG suggest Wolff-Parkinson-White (WPW) syndrome, a condition characterized by the presence of an accessory electrical pathway between the atria and ventricles, leading to abnormal heart rhythms.

**Why the Correct Answer is Right**
WPW syndrome is caused by an accessory electrical pathway, known as a bypass tract, that connects the atria to the ventricles. This pathway allows electrical signals to bypass the normal atrioventricular (AV) node, resulting in a shortened P-R interval and the appearance of delta waves on the ECG. The least effective measure for managing WPW syndrome would be **beta-blockers**, as they can slow the heart rate but do not address the underlying cause of the condition.

**Why Each Wrong Option is Incorrect**
**Option A:** **Digoxin** is not effective in WPW syndrome as it can worsen the condition by increasing the conduction velocity through the bypass tract.
**Option B:** **Calcium channel blockers** can be effective in WPW syndrome by slowing conduction through the AV node and reducing the risk of tachyarrhythmias.
**Option C:** **Radiofrequency ablation** is a highly effective treatment for WPW syndrome, with a success rate of over 90%, and is considered the definitive treatment for the condition.

**Clinical Pearl / High-Yield Fact**
WPW syndrome is a congenital condition, and patients with a family history of the condition are at increased risk. It's essential to perform a thorough family history and physical examination when diagnosing WPW syndrome.

**Correct Answer: A. Digoxin.**