A 75-year-old male on metoprolol presents with U waves on ECG. What is the most likely cause of the U waves?

The presence of U waves on an electrocardiogram (ECG) is often associated with underlying electrolyte disturbances, particularly hypokalemia. U waves are small deflections that appear following the T wave and can be indicative of changes in repolarization in the heart. Hypokalemia, or low potassium levels, can lead to changes in the myocardial action potentials, resulting in the appearance of U waves.

In this scenario, although metoprolol is a beta-blocker that can influence heart rate and rhythm, it is not directly responsible for the development of U waves. Instead, the primary consideration in a patient presenting with U waves is to evaluate the electrolyte status, specifically potassium levels. Low potassium can disrupt normal cardiac conduction, leading to the manifestation of U waves on the ECG.

While other electrolyte imbalances, such as hypocalcemia, can affect the QT interval and overall cardiac function, they are less directly associated with U wave formation. Additionally, medication noncompliance could lead to variations in drug effectiveness but does not specifically explain the presence of U waves. Therefore, hypokalemia is the most directly related cause of U waves in this context.