A patient with a potassium level of 3.2 mEq/L is most likely to show which feature on an electrocardiogram?

• A. Atrial fibrillation
• B. U waves
• C. ST segment elevation
• D. Wide QRS

Answer: (B)

Low potassium alters how heart muscle cells repolarize. When potassium is low, the return to resting voltage slows and a small, extra deflection after the T wave called a U wave appears on the ECG. That U wave is the classic sign of hypokalemia, which fits with a potassium level of 3.2 mEq/L. Other changes you might see with low potassium include flattened T waves and ST-segment depression, but the distinctive feature here is the U wave.

The other options don’t fit as well: ST segment elevation is more typical of acute injury or inflammation, wide QRS is usually associated with high potassium or other conduction problems, and atrial fibrillation can occur with electrolyte disturbances but is not the hallmark finding for hypokalemia on ECG.