The answer is A: hypokalemia. Although subtle, this electrocardiogram (ECG) demonstrates a flattening of the T waves with development of U waves (see arrows) associated with hypokalemia. The ECG patterns seen in patients with hypokalemia range from slight T-wave flattening alone to the appearance of prominent U waves, occasionally with ST depressions or T-wave inversions. This is usually a progressive pattern. When the U wave is greater than the T wave, the potassium level is usually less than 2.7 mEq per L (2.7 mmol per L). The most common causes of hypokalemia are medications (especially diuretics) or renal loss related to metabolic alkalosis or potassium loss in the stool secondary to diarrhea.1 Hypokalemia, although tolerated well in most patients, can be life-threatening in those with underlying cardiac disease. The potassium level in this patient was 2.3 mEq per L (2.3 mmol per L). Treatment includes potassium replacement and correction of the underlying cause when possible. Some patients require daily potassium supplementation. Interestingly, the hypokalemia occurring in this patient was a result of Bartter's syndrome, a rare hereditary disorder of renal potassium wasting secondary to impaired electrolyte transport in the ascending loop of Henle.2 It is treated mainly by liberal potassium supplementation. The use of spironolactone (Aldactone) can also help prevent potassium wasting.
As for the remaining multiple-choice answers, hypocalcemia manifests itself on ECG as a prolonged QT interval, which is not present in this tracing. There are no ST elevations or other changes suggestive of acute myocardial infarction in this tracing. First-degree atrioventricular block should also be ruled out because the PR interval on the ECG shown here is less than 0.20 mm. Finally, Wollf-Parkinson-White syndrome is a preexcitation syndrome characterized on ECG by wide QRS complexes, short PR intervals and delta waves, none of which are present on the ECG shown here.