MSFinals-0239

MSFinals-0239

A 25-year-old woman attends a new patient health check at the General Practice surgery she has recently joined. She mentions she occasionally gets episodes of palpitations and light-headedness and has done so for several years. Her pulse is currently regular, with a rate of 70 bpm, and her blood pressure is 110/76 mmHg. A full blood count is sent, which comes back as normal. The general practitioner requests an electrocardiogram (ECG), which shows a widened QRS complex with a slurred upstroke and a shortened PR interval.
Which of the following is the most likely diagnosis?