July 26th, 2020
An 18-year-old man is seen at a university clinic in South Florida for a pre-participation sports physical. He was on the cross-country track team in high school. He has been in good health, reports no symptoms, and denies recent travel out of the area. The physical examination is within normal limits. A routine electrocardiogram (ECG) is obtained and shows a heart rate of 58 and a PR interval of 300 milliseconds. The ECG is otherwise within normal limits.
Which of the following is the most likely cause of the prolonged P-R interval?