Which electrolyte disturbance is associated with U waves on ECG?

• A. Hyperkalemia
• B. Hypokalemia
• C. Hypomagnesemia
• D. Hypermagnesemia

Answer: (B)

U waves after the T wave are most characteristically seen when potassium is low. Potassium helps regulate ventricular repolarization, and when extracellular potassium falls, repolarization becomes slower and the small residual repolarization of the Purkinje fibers becomes more visible, producing a distinct U wave on the ECG, especially in the chest leads. This is the classic sign of hypokalemia. In contrast, hyperkalemia tends to produce tall, peaked T waves and can widen the QRS, not produce U waves. Hypomagnesemia is linked more with QT prolongation and torsades de pointes rather than a prominent U wave. So the presence of U waves points toward hypokalemia.