**Core Concept**
Torsades de pointes is a specific type of abnormal heart rhythm, also known as a polymorphic ventricular tachycardia, characterized by a rapid, irregular heart rate. It is a form of long QT syndrome, where the QT interval is prolonged, leading to an increased risk of arrhythmias.

**Why the Correct Answer is Right**
Torsades de pointes is caused by a prolonged QT interval, which can be due to various factors such as genetic mutations, certain medications, electrolyte imbalances (e.g., hypokalemia, hypomagnesemia), or acquired conditions like heart failure or myocardial infarction. The prolonged QT interval disrupts the normal repolarization of the cardiac action potential, leading to a predisposition to re-entrant arrhythmias. This results in a rapid, irregular heart rate with a characteristic "twisting" of the ECG waveform.

**Why Each Wrong Option is Incorrect**
* **Option A:** Torsades de pointes is not typically caused by a prolonged PR interval, which is more commonly associated with first-degree AV block.
* **Option B:** Although hypokalemia can contribute to the development of torsades de pointes, it is not the primary cause. Other electrolyte imbalances, medications, and genetic factors also play a role.
* **Option C:** Atrial fibrillation is a different type of arrhythmia, characterized by rapid, irregular atrial contractions. It is not directly related to torsades de pointes.

**Clinical Pearl / High-Yield Fact**
Torsades de pointes can be life-threatening, and prompt recognition and treatment are essential. The "U-wave" on the ECG is often associated with torsades de pointes, and the "J-point" is a critical point where the T-wave meets the baseline.

**Correct Answer: D. Prolonged QT interval.**