**Core Concept**
Hypokalemia is a condition characterized by low potassium levels in the blood, which can lead to various ECG changes due to the disruption of normal cardiac electrical activity. The heart's repolarization process is particularly affected, resulting in characteristic ECG alterations.

**Why the Correct Answer is Right**
U waves are indeed an ECG change associated with hypokalemia. They are a result of the slowed repolarization of the ventricles, which is a direct consequence of low potassium levels. Potassium plays a crucial role in the repolarization process, and its deficiency can lead to the appearance of U waves on the ECG. The U wave is thought to represent the final phase of ventricular repolarization, which is delayed in hypokalemia.

**Why Each Wrong Option is Incorrect**
**Option A:** While not a primary ECG change in hypokalemia, flattened T waves can occur in severe cases, but it's not the best distractor for this question.
**Option B:** Prolonged QT interval is indeed associated with hypokalemia, as the low potassium levels can affect the repolarization process, leading to a prolongation of the QT interval.
**Option C:** ST segment depression is a common ECG change in hypokalemia, which is a result of the altered repolarization process.

**Clinical Pearl / High-Yield Fact**
In hypokalemia, ECG changes can be a sensitive indicator of the severity of potassium deficiency, and close monitoring of these changes can guide potassium replacement therapy.

**Correct Answer: U wave**