Which EKG finding is most characteristic of hypokalemia?

• A. Prominent U wave
• B. Tall peaked T wave
• C. Shortened QT interval
• D. Increased R wave amplitude

Answer: (A)

Potassium levels shape how the heart muscle repolarizes. When potassium is low, the late phase of ventricular repolarization is altered in a way that creates an extra deflection after the T wave—the prominent U wave. This U wave becomes more conspicuous as potassium falls, especially in the context of bradycardia or after exercise, and it is the hallmark feature that clinicians associate with hypokalemia on an ECG.

In addition to the prominent U wave, you may see ST-segment depression and flattened or inverted T waves as potassium remains low, and the QT interval can be prolonged because repolarization is delayed. By contrast, tall peaked T waves are typical of hyperkalemia, not hypokalemia, and a shortened QT interval is not characteristic of hypokalemia. Increased R wave amplitude is not a characteristic finding either.