A 26-year-old woman presented to the emergency room with palpitations and shortness of breath that started suddenly while she was eating breakfast.Although the health-screening examination performed three weeks ago showed delta waves in her ECG, echocardiography taken at a nearby hospital showed no abnormal findings.

At presentation, she was slightly hypotensive with a blood pressure of 96/68 mmHg. Her ECG on admission showed a narrow QRS-complex tachycardia at a rate of 180/min. Neither ST elevation nor T wave abnormality was present.

What is the most probable diagnosis of the arrhythmia?