A 24-year-old man comes to the university health center for a pre-participation physical examination before joining the varsity soccer team. He has been playing soccer recreationally for 10 years and exercises 5-6 days per week without limitation. He has no history of major medical illness and takes no medications. He has never experienced palpitations, chest pain, or episodes of dizziness. His temperature is 36.8°C (98.2°F), pulse is 68/min, respirations are 14/min, and blood pressure is 118/76 mm Hg. Physical examination shows a well-developed, athletic man in no distress. Cardiovascular examination shows regular rate and rhythm with no murmurs, rubs, or gallops. The lungs are clear to auscultation bilaterally. The remainder of the examination shows no abnormalities. An electrocardiogram is performed as part of the screening evaluation. The ECG shows a regular rhythm at 68 bpm. The PR interval is notably short at 90 ms and the QRS duration is widened at 130 ms. There is an initial slurred, slow-rising deflection at the beginning of the QRS complex seen in leads I, II, and V2 through V6.
Which of the following is the most likely diagnosis?