Differential Diagnosis of U Wave Abnormalities
Prominent upright U waves

Sinus bradycardia accentuates the U wave

Hypokalemia (remember the triad of ST segment depression, low amplitude T waves, and prominent U waves)

Quinidine and other type 1A antiarrhythmics

CNS disease with long QT intervals (often the T and U fuse to form a giant "T-U fusion wave")
LVH (right precordial leads with deep S waves)

Mitral valve prolapse (some cases)

Hyperthyroidism