Which ECG finding is a sign of hypokalemia?

• A. Tall, peaked T waves
• B. ST depression
• C. Flattened T waves
• D. Prominent U waves

Answer: (D)

Low potassium slows ventricular repolarization, and while it can dull the T wave and cause ST-segment changes, the most distinctive sign of hypokalemia on the ECG is a prominent U wave. This small deflection after the T wave becomes more noticeable as potassium levels drop and reflects late repolarization of the ventricles (often involving Purkinje fibers and subendocardial regions). Among the given choices, the prominent U wave is the best indicator of low potassium. Tall, peaked T waves point to high potassium; ST depression and flattened T waves can occur with hypokalemia but are less specific, whereas the U wave is the classic sign.