**Core Concept**
Hypokalemia is a condition characterized by low serum potassium levels, which can lead to various cardiac conduction abnormalities. One of the key ECG changes in hypokalemia is the appearance of U waves, which are often seen in conjunction with other changes such as T-wave flattening or inversion. However, the relationship between hypokalemia and QT interval prolongation is more complex and not directly related to potassium levels.

**Why the Correct Answer is Right**
The QT interval represents the time from the beginning of the Q wave to the end of the T wave in the heart's electrical cycle. Prolongation of the QT interval is typically associated with conditions that affect the repolarization of the myocardium, such as hypomagnesemia, bradycardia, or the use of certain medications like quinidine or sotalol. In contrast, hypokalemia primarily affects the depolarization phase of the cardiac cycle, leading to changes in the ST segment and T wave. While there may be some indirect effects of hypokalemia on the QT interval, these are not as direct or significant as the changes seen in the ST segment and T wave.

**Why Each Wrong Option is Incorrect**
**Option A:** U waves are indeed a characteristic ECG change in hypokalemia, often seen as a low-amplitude wave following the T wave. They are thought to represent the late repolarization of the Purkinje fibers.
**Option B:** ST segment sagging is a common ECG finding in hypokalemia, reflecting the depression of the ST segment. This is often seen in conjunction with T-wave flattening or inversion.
**Option C:** T-wave flattening or inversion is another hallmark of hypokalemia, resulting from the decreased repolarization of the ventricular myocardium.
**Option D:** (This is the correct answer, so we won't discuss it further.)

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with suspected hypokalemia, it's essential to consider the clinical context and other potential causes of ECG changes. U waves, ST segment sagging, and T-wave flattening or inversion are all characteristic findings in hypokalemia, but these changes can also be seen in other conditions. A thorough history, physical examination, and laboratory evaluation are necessary to establish the correct diagnosis.

**✓ Correct Answer: D. QT interval prolongation**